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For a sample of Swift and Fermi gamma-ray bursts, we show that the minimum variability timescale and the spectral lag of the prompt emission is related to the bulk Lorentz factor in a complex manner. For small Γ's, the variability timescale exhibits a shallow (plateau) region. For large Γ's, the variability timescale declines steeply as a function of Γ ($\delta T\propto {{{\Gamma }}^{-4.05\pm 0.64}}$). Evidence is also presented for an intriguing correlation between the peak times, t_p, of the afterglow emission and the prompt emission variability timescale.

We have performed a statistical study of the properties of 110 bright X-ray outbursts in 36 low-mass X-ray binary transients (LMXBTs) seen with the All-Sky Monitor (2–12 keV) on board the Rossi X-ray Timing Explorer (RXTE) in 1996–2011. We have measured a number of outburst properties, including peak X-ray luminosity, rate of change of luminosity on a daily timescale, e-folding rise and decay timescales, outburst duration, and total radiated energy. We found that the average properties, such as peak X-ray luminosity, rise and decay timescales, outburst duration, and total radiated energy of black hole LMXBTs, are at least two times larger than those of neutron star LMXBTs, implying that the measurements of these properties may provide preliminary clues to the nature of the compact object of a newly discovered LMXBT. We also found that the outburst peak X-ray luminosity is correlated with the rate of change of X-ray luminosity in both the rise and decay phases, which is consistent with our previous studies. Positive correlations between total radiated energy and peak X-ray luminosity, and between total radiated energy and the e-folding rise or decay timescale, are also found in the outbursts. These correlations suggest that the mass stored in the disk before an outburst is the primary initial condition that sets up the outburst properties seen later. We also found that the outbursts of two transient stellar-mass ultraluminous X-ray sources in M31 also roughly follow the correlations, which indicate that the same outburst mechanism works for the brighter outbursts of these two sources in M31 that reached the Eddington luminosity.

In recent years, more and more gamma-ray bursts (GRBs) with late rebrightenings in their multi-band afterglows have revealed the late-time activity of their central engines. GRB 100814A is a special case among the well-sampled events, with complex temporal and spectral evolution. The single power-law shallow decay index of the optical light curve observed by GROND between 640 s and 10 ks is ${{\alpha }_{{\rm opt}}}=0.57\pm 0.02$, which apparently conflicts with expectations from the simple external shock model. In particular, there is remarkable rebrightening in the optical to near-infrared bands at late times, challenging the external shock model with synchrotron emission coming from the interaction of the blast wave with the surrounding interstellar medium. In this paper, we invoke a magnetar with spin evolution to explain the complex multi-band afterglow emission of GRB 100814A. The initial shallow decay phase in the optical bands and the plateau in the X-ray can be explained as being due to energy injection from a spin-down magnetar. At late times, with materials from the fall-back disk falling onto the central object of the burster, the angular momentum of the accreted materials is transferred to the magnetar, which leads to a spin up process. As a result, the magnetic dipole radiation luminosity will increase, resulting in significant rebrightening of the optical afterglow. We show that the model can well reproduce the observed multi-band afterglow emission.

One favored progenitor model for short duration gamma-ray bursts (GRBs) is the coalescence of two neutron stars (NS–NS). One possible outcome of such a merger would be a rapidly spinning, strongly magnetized neutron star (known as a millisecond magnetar). These magnetars may be "supra-massive," implying that they would collapse to black holes after losing centrifugal support due to magnetic dipole spin down. By systematically analyzing the Burst Alert Telescope (BAT)-XRT light curves of all short GRBs detected by Swift, we test how consistent the data are with this central engine model of short GRBs. We find that the so-called "extended emission" feature observed with BAT in some short GRBs is fundamentally the same component as the "internal X-ray plateau" observed in many short GRBs, which is defined as a plateau in the light curve followed by a very rapid decay. Based on how likely a short GRB is to host a magnetar, we characterize the entire Swift short GRB sample into three categories: the "internal plateau" sample, the "external plateau" sample, and the "no plateau" sample. Based on the dipole spin-down model, we derive the physical parameters of the putative magnetars and check whether these parameters are consistent with expectations from the magnetar central engine model. The derived magnetar surface magnetic field ${{B}_{p}}$ and the initial spin period P₀ fall into a reasonable range. No GRBs in the internal plateau sample have a total energy exceeding the maximum energy budget of a millisecond magnetar. Assuming that the beginning of the rapid fall phase at the end of the internal plateau is the collapse time of a supra-massive magnetar to a black hole, and applying the measured mass distribution of NS–NS systems in our Galaxy, we constrain the neutron star equation of state (EOS). The data suggest that the NS EOS is close to the GM1 model, which has a maximum non-rotating NS mass of ${{M}_{{\rm TOV}}}\sim 2.37\;{{M}_{\odot }}$.

We present 20 Wide-field Infrared Survey Explorer (WISE)-selected galaxies with bolometric luminosities L_bol > 10¹⁴L_☉, including five with infrared luminosities L_IR ≡ L_{(rest 8–1000 μm)} > 10¹⁴L_☉. These "extremely luminous infrared galaxies," or ELIRGs, were discovered using the "W1W2-dropout" selection criteria which requires marginal or non-detections at 3.4 and 4.6 μm (W1 and W2, respectively) but strong detections at 12 and 22 μm in the WISE survey. Their spectral energy distributions are dominated by emission at rest-frame 4–10 μm, suggesting that hot dust with T_d ∼ 450 K is responsible for the high luminosities. These galaxies are likely powered by highly obscured active galactic nuclei (AGNs), and there is no evidence suggesting these systems are beamed or lensed. We compare this WISE-selected sample with 116 optically selected quasars that reach the same L_bol level, corresponding to the most luminous unobscured quasars in the literature. We find that the rest-frame 5.8 and 7.8 μm luminosities of the WISE-selected ELIRGs can be 30%–80% higher than that of the unobscured quasars. The existence of AGNs with L_bol > 10¹⁴L_☉ at z > 3 suggests that these supermassive black holes are born with large mass, or have very rapid mass assembly. For black hole seed masses ∼10³M_☉, either sustained super-Eddington accretion is needed, or the radiative efficiency must be <15%, implying a black hole with slow spin, possibly due to chaotic accretion.

We present a general relativistic (GR) model of jet variability in active galactic nuclei due to orbiting blobs in helical motion along a funnel or cone-shaped magnetic surface anchored to the accretion disk near the black hole. Considering a radiation pressure driven flow in the inner region, we find that it stabilizes the flow, yielding Lorentz factors ranging between 1.1 and 7 at small radii for reasonable initial conditions. Assuming these as inputs, simulated light curves (LCs) for the funnel model include Doppler and gravitational shifts, aberration, light bending, and time delay. These LCs are studied for quasi-periodic oscillations (QPOs) and the power spectral density (PSD) shape, and yield an increased amplitude (∼12%), a beamed portion and a systematic phase shift with respect to that from a previous special relativistic model. The results strongly justify implementing a realistic magnetic surface geometry in Schwarzschild geometry to describe effects on emission from orbital features in the jet close to the horizon radius. A power-law-shaped PSD with a typical slope of −2 and QPOs with timescales in the range of (1.37–130.7) days consistent with optical variability in blazars, emerges from the simulations for black hole masses ${{M}_{\bullet }}=(0.5-5)\times {{10}^{8}}\;{{M}_{\odot }}$ and initial Lorentz factors ${{\gamma }_{{\rm jet},{\rm i}}}=2-10$. The models presented here can be applied to explain radio, optical, and X-ray variability from a range of jetted sources including active galactic nuclei, X-ray binaries, and neutron stars.

Massive stars can be efficiently ejected from their birth star clusters through encounters with other massive stars. We study how the dynamical ejection fraction of O star systems varies with the masses of very young star clusters, ${{M}_{{\rm ecl}}}$, by means of direct N-body calculations. We include diverse initial conditions by varying the half-mass radius, initial mass segregation, initial binary fraction, and orbital parameters of the massive binaries. The results show robustly that the ejection fraction of O star systems exhibits a maximum at a cluster mass of ${{10}^{3.5}}\;{{M}_{\odot }}$ for all models, even though the number of ejected systems increases with cluster mass. We show that lower mass clusters (${{M}_{{\rm ecl}}}\approx 400\;{{M}_{\odot }}$) are the dominant sources for populating the Galactic field with O stars by dynamical ejections, considering the mass function of embedded clusters. About 15% (up to ≈38%, depending on the cluster models) of O stars of which a significant fraction are binaries, and which would have formed in a ≈10 Myr epoch of star formation in a distribution of embedded clusters, will be dynamically ejected to the field. Individual clusters may eject 100% of their original O star content. A large fraction of such O stars have velocities up to only 10 km s⁻¹. Synthesising a young star cluster mass function, it follows, given the stellar-dynamical results presented here, that the observed fractions of field and runaway O stars, and the binary fractions among them, can be well understood theoretically if all O stars form in embedded clusters.

We have simulated the Expanded Owens Valley Solar Array (EOVSA) radio images generated at multiple frequencies from a model solar active region, embedded in a realistic solar disk model, and explored the resulting data cube for different spectral analysis schemes to evaluate the potential for realizing one of EOVSA's most important scientific goals—coronal magnetography. In this paper, we focus on modeling the gyroresonance and free–free emission from an on-disk solar active region model with realistic complexities in electron density, temperature and magnetic field distribution. We compare the magnetic field parameters extrapolated from the image data cube along each line of sight after folding through the EOVSA instrumental profile with the original (unfolded) parameters used in the model. We find that even the most easily automated, image-based analysis approach (Level-0) provides reasonable quantitative results, although they are affected by systematic effects due to finite sampling in the Fourier (UV) plane. Finally, we note the potential for errors due to misidentified harmonics of the gyrofrequency, and discuss the prospects for applying a more sophisticated spectrally based analysis scheme (Level-1) to resolve the issue in cases where improved UV coverage and spatial resolution are available.

We analyze 3 yr of nearly continuous Kepler spacecraft short cadence observations of the pulsating subdwarf B (sdB) star KIC 3527751. We detect a total of 251 periodicities, most in the g-mode domain, but some where p-modes occur, confirming that KIC 3527751 is a hybrid pulsator. We apply seismic tools to the periodicities to characterize the properties of KIC 3527751. Techniques to identify modes include asymptotic period spacing relationships, frequency multiplets, and the separation of multiplet splittings. These techniques allow for 189 (75%) of the 251 periods to be associated with pulsation modes. Included in these are three sets of ℓ = 4 multiplets and possibly an ℓ = 9 multiplet. Period spacing sequences indicate ℓ = 1 and 2 overtone spacings of 266.4 ± 0.2 and 153.2 ± 0.2 s, respectively. We also calculate reduced periods, from which we find evidence of trapped pulsations. Such mode trappings can be used to constrain the core/atmosphere transition layers. Interestingly, frequency multiplets in the g-mode region, which sample deep into the star, indicate a rotation period of 42.6 ± 3.4 days while p-mode multiplets, which sample the outer envelope, indicate a rotation period of 15.3 ± 0.7 days. We interpret this as differential rotation in the radial direction with the core rotating more slowly. This is the first example of differential rotation for a sdB star.

Star-forming galaxies, due to their high star formation rates, and hence large number of supernova remnants (SNRs) therein, are huge reservoirs of cosmic rays (CRs). These CRs collide with gases in  galaxies and produce high-energy neutrinos through proton–proton collisions. In this paper, we calculate the neutrino production efficiency in star-forming galaxies by considering realistic galaxy properties, such as the gas density and galactic wind in star-forming galaxies. To calculate the accumulated neutrino flux, we use the infrared luminosity function of star-forming galaxies recently obtained by the  Herschel PEP/HerMES survey. The intensity of CRs producing PeV neutrinos in star-forming galaxies is normalized with the observed CR flux at EeV (1 EeV = ${{10}^{18}}$ eV), assuming that SNR or hypernova remnants in star-forming galaxies can accelerate protons to EeV energies. Our calculations show that the accumulated neutrino emission produced by CRs in star-forming galaxies can account for the flux and spectrum of the sub-PeV/PeV neutrinos under reasonable assumptions on the CR confinement time in these galaxies.

We present host stellar velocity dispersion measurements for a sample of 88 broad-line quasars at $0.1\lt z\lt 1$ (46 at $z\gt 0.6$) from the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. High signal-to-noise ratio coadded spectra (average ${\rm S}/{\rm N}\approx 30$ per 69 ${\rm km}\;{{{\rm s}}^{-1}}$ pixel) from SDSS-RM allowed for the decomposition of the host and quasar spectra and for measurements of the host stellar velocity dispersions and black hole (BH) masses using the single-epoch (SE) virial method. The large sample size and dynamic range in luminosity (${{L}_{5100}}={{10}^{43.2-44.7}}\;{\rm erg}\;{{{\rm s}}^{-1}}$) lead to the first clear detection of a correlation between SE virial BH mass and host stellar velocity dispersion far beyond the local universe. However, the observed correlation is significantly flatter than the local relation, suggesting that there are selection biases in high-z luminosity-threshold quasar samples for such studies. Our uniform sample and analysis enable an investigation of the redshift evolution of the ${{M}_{\bullet }}-{{\sigma }_{*}}$ relation relatively free of caveats by comparing different samples/analyses at disjoint redshifts. We do not observe evolution of the ${{M}_{\bullet }}-{{\sigma }_{*}}$ relation in our sample up to $z\sim 1$, but there is an indication that the relation flattens toward higher redshifts. Coupled with the increasing threshold luminosity with redshift in our sample, this again suggests that certain selection biases are at work, and simple simulations demonstrate that a constant ${{M}_{\bullet }}-{{\sigma }_{*}}$ relation is favored to $z\sim 1$. Our results highlight the scientific potential of deep coadded spectroscopy from quasar monitoring programs, and offer a new path to probe the co-evolution of BHs and galaxies at earlier times.

In order to better understand the nature of active region outflows, the electron density was measured by using a density-sensitive line pair, Fe xiv 264.78 Å/274.20 Å. Because coronal line profiles of the outflow region are composed of a major component with a Doppler shift of $\leqslant 10\;{\rm km}\;{{{\rm s}}^{-1}}$ and a minor component (enhanced blue wing, EBW) blueshifted by up to $100\;{\rm km}\;{{{\rm s}}^{-1}}$, we extracted EBW from the line profiles through double-Gaussian fitting. We tried applying the simultaneous fitting to those two Fe xiv lines with several physical restrictions. Electron density for both components (${{n}_{{\rm Major}}}$ and ${{n}_{{\rm EBW}}}$, respectively) was calculated by referring to the theoretical intensity ratio as a function of electron density as per the CHIANTI database. We studied six locations in the outflow regions around NOAA AR10978. The average electron density was ${{n}_{{\rm Major}}}={{10}^{9.16\pm 0.16}}\;{\rm c}{{{\rm m}}^{-3}}$ and ${{n}_{{\rm EBW}}}={{10}^{8.74\pm 0.29}}\;{\rm c}{{{\rm m}}^{-3}}$. The magnitude relationship between ${{n}_{{\rm Major}}}$ and ${{n}_{{\rm EBW}}}$ was the opposite in the eastern and western outflow regions. The column depth was also calculated for each component, which leads to the result that the outflows possess only a small fraction (∼0.1) in the eastern region, whereas they dominate over the major component in the line profiles by a factor of five in the western region. When taking into account the extended coronal structures, the western region can be thought to represent the mass leakage. In contrast, we suggest a possibility that the eastern region actually contributes to mass supply to coronal loops.

Mass loss remains one of the primary uncertainties in stellar evolution. In the most massive stars, mass loss dictates the circumstellar medium and can significantly alter the fate of the star. Mass loss is caused by a variety of wind mechanisms and also through binary interactions. Supernovae (SNe) are excellent probes of this mass loss, both the circumstellar material and the reduced mass of the hydrogen-rich envelope. In this paper, we focus on the effects of reducing the hydrogen-envelope mass on the SN light curve, studying both the shock breakout and peak light-curve emission for a wide variety of mass-loss scenarios. Even though the trends of this mass loss will be masked somewhat by variations caused by different progenitors, explosion energies, and circumstellar media, these trends have significant effects on the SN light curves that should be seen in SN surveys. We conclude with a comparison of our results to a few key observations.

We study the stellar content in the tidal tails of three nearby merging galaxies, NGC 520, NGC 2623, and NGC 3256, using BVI imaging taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. The tidal tails in all three systems contain compact and fairly massive young star clusters, embedded in a sea of diffuse, unresolved stellar light. We compare the measured colors and luminosities with predictions from population synthesis models to estimate cluster ages and find that clusters began forming in tidal tails during or shortly after the formation of the tails themselves. We find a lack of very young clusters (≤10 Myr old), implying that eventually star formation shuts off in the tails as the gas is used up or dispersed. There are a few clusters in each tail with estimated ages that are older than the modeled tails themselves, suggesting that these may have been stripped out from the original galaxy disks. The luminosity function of the tail clusters can be described by a single power-law, dN/dL ∝ L^α, with −2.6 < α < −2.0. We find a stellar age gradient across some of the tidal tails, which we interpret as a superposition of (1) newly formed stars and clusters along the dense center of the tail and (2) a sea of broadly distributed, older stellar material ejected from the progenitor galaxies.

A recent Atacama Large Millimeter/Submillimeter Array image revealed several concentric gaps in the protoplanetary disk surrounding the young star HL Tau. We consider the hypothesis that these gaps are carved by planets, and present a general framework for understanding the dynamical stability of such systems over typical disk lifetimes, providing estimates for the maximum planetary masses. We collect these easily evaluated constraints into a workflow that can help guide the design and interpretation of new observational campaigns and numerical simulations of gap opening in such systems. We argue that the locations of resonances should be significantly shifted in massive disks like HL Tau, and that theoretical uncertainties in the exact offset, together with observational errors, imply a large uncertainty in the dynamical state and stability in such disks. This presents an important barrier to using systems like HL Tau as a proxy for the initial conditions following planet formation. An important observational avenue to breaking this degeneracy is to search for eccentric gaps, which could implicate resonantly interacting planets. Unfortunately, massive disks like HL Tau should induce swift pericenter precession that would smear out any such eccentric features of planetary origin. This motivates pushing toward more typical, less massive disks. For a nominal non-resonant model of the HL Tau system with five planets, we find a maximum mass for the outer three bodies of approximately 2 Neptune masses. In a resonant configuration, these planets can reach at least the mass of Saturn. The inner two planets' masses are unconstrained by dynamical stability arguments.

Here we analyze radio, optical, and X-ray data for the peculiar cluster Abell 578. This cluster is not fully relaxed and consists of two merging sub-systems. The brightest cluster galaxy (BCG), CGPG 0719.8+6704, is a pair of interacting ellipticals with projected separation ∼10 kpc, the brighter of which hosts the radio source 4C+67.13. The Fanaroff–Riley type-II radio morphology of 4C+67.13 is unusual for central radio galaxies in local Abell clusters. Our new optical spectroscopy revealed that both nuclei of the CGPG 0719.8+6704 pair are active, albeit at low accretion rates corresponding to the Eddington ratio $\sim {{10}^{-4}}$ (for the estimated black hole masses of $\sim 3\times {{10}^{8}}\;{{M}_{\odot }}$ and $\sim {{10}^{9}}\;{{M}_{\odot }}$). The gathered X-ray (Chandra) data allowed us to confirm and to quantify robustly the previously noted elongation of the gaseous atmosphere in the dominant sub-cluster, as well as a large spatial offset (∼60 kpc projected) between the position of the BCG and the cluster center inferred from the modeling of the X-ray surface brightness distribution. Detailed analysis of the brightness profiles and temperature revealed also that the cluster gas in the vicinity of 4C+67.13 is compressed (by a factor of about ∼1.4) and heated (from $\simeq 2.0$ keV up to 2.7 keV), consistent with the presence of a weak shock (Mach number ∼1.3) driven by the expanding jet cocoon. This would then require the jet kinetic power of the order of $\sim {{10}^{45}}$ erg s⁻¹, implying either a very high efficiency of the jet production for the current accretion rate, or a highly modulated jet/accretion activity in the system.

We describe a new model for the "stripes" of synchrotron radiation seen in the remnant of Tycho's supernova. In our picture, cosmic rays streaming ahead of the forward shock generate parallel propagating (with respect to the local magnetic field direction) circularly polarized Alfvén waves that are almost free of dissipation, and, due to being circularly polarized, exhibit no spatial variation of magnetic field strength. Following the interaction with the supernova remnant (SNR) shock with nonzero obliquity, these parallel propagating waves become obliquely propagating, due the the wave refraction (which is different, in principle, for the different plane wave components), and dissipation sets in. The magnetosonic polarization decays faster, due to transit time damping, leaving only the Alfvén mode. This surviving mode now exhibits a spatial variation of the magnetic field, leading to local maxima and minima in the synchrotron emission, i.e., the stripes. We attribute the initial wave generation to the Bell instability, which in contrast to the resonant generation of upstream Alfvén waves, gives rise to a preferred wavelength, and hence the single wave period at which the stripes are seen. Based on estimates for damping rates due to turbulent cascade and transit time damping, we estimate the dependence of the visibility of the stripes on the shock obliquity and determine a maximum cosmic ray energy in Tycho's SNR in the range $6\times {{10}^{14}}-1\times {{10}^{15}}$ eV.

Victoria-Regina isochrones for $-0.4{\mkern 1mu} \leqslant$ [α/Fe] $\leqslant +0.4$ and a wide range in [Fe/H], along with complementary zero-age horizontal branch (ZAHB) loci, have been applied to the color–magnitude diagram (CMD) of Carina. The color transformations that we have used have been "calibrated" so that isochrones provide excellent fits to the [${{(B-V)}_{0}},{{M}_{V}}$] diagrams of M3 and M92 when well supported estimates of the globular cluster (GC) reddenings and metallicities are assumed. The adopted distance moduli, for both the GCs and Carina, are based on our ZAHB models, which are able to reproduce the old horizontal branch (HB) component (as well as the luminosity of the HB clump) of the dwarf spheroidal galaxy quite well—even if it spans a range in [Fe/H] of ∼1.5 dex, provided that [α/Fe] varies with [Fe/H] in approximately the way that has been derived spectroscopically. Ages derived here agree reasonably well with those found previously for the old and intermediate-age turnoff (TO) stars, as well as for the period of negligible star formation (SF) activity (∼6–10 Gyr ago). CMD simulations have been carried out for the faintest TO and subgiant stars. They indicate a clear preference for SF that lasted several Gyr instead of a short burst, with some indication that ages decrease with increasing [Fe/H]. In general, stellar models that assume spectroscopic metallicities provide satisfactory fits to the observations, including the thin giant branch of Carina, though higher oxygen abundances than those implied by the adopted values of [α/Fe] would have favorable consequences.

Previous analysis of the fossil-group/cluster RX J1159+5531 with X-ray observations from a central Chandra pointing and an offset-north Suzaku pointing indicate a radial intracluster medium (ICM) entropy profile at the virial radius (R_vir) consistent with predictions from gravity-only cosmological simulations, in contrast to other cool-core clusters. To examine the generality of these results, we present three new Suzaku observations that, in conjunction with the north pointing, provide complete azimuthal coverage out to R_vir. With two new Chandra ACIS-I observations overlapping the north Suzaku pointing, we have resolved ≳50% of the cosmic X-ray background there. We present radial profiles of the ICM density, temperature, entropy, and pressure obtained for each of the four directions. We measure only modest azimuthal scatter in the ICM properties at R₂₀₀ between the Suzaku pointings: 7.6% in temperature and 8.6% in density, while the systematic errors can be significant. The temperature scatter, in particular, is lower than that studied at R₂₀₀ for a small number of other clusters observed with Suzaku. These azimuthal measurements verify that RX J1159+5531 is a regular, highly relaxed system. The well-behaved entropy profiles we have measured for RX J1159+5531 disfavor the weakening of the accretion shock as an explanation of the entropy flattening found in other cool-core clusters but is consistent with other explanations such as gas clumping, electron-ion non-equilibrium, non-thermal pressure support, and cosmic-ray acceleration. Finally, we mention that the large-scale galaxy density distribution of RX J1159+5531 seems to have little impact on its gas properties near R_vir.

The aberrated radiation pressure at the inner edge of the accretion disk around an astrophysical black hole imparts a relative azimuthal velocity on the electrons with respect to the ions which gives rise to a ring electric current that generates large-scale poloidal magnetic field loops. This is the Cosmic Battery established by Contopoulos and Kazanas in 1998. In the present work we perform realistic numerical simulations of this important astrophysical mechanism in advection-dominated accretion flows, ADAFs. We confirm the original prediction that the inner parts of the loops are continuously advected toward the central black hole and contribute to the growth of the large-scale magnetic field, whereas the outer parts of the loops are continuously diffusing outward through the turbulent accretion flow. This process of inward advection of the axial field and outward diffusion of the return field proceeds all the way to equipartition, thus generating astrophysically significant magnetic fields on astrophysically relevant timescales. We confirm that there exists a critical value of the magnetic Prandtl number between unity and 10 in the outer disk above which the Cosmic Battery mechanism is suppressed.

Following the basic principles of a charge-separated pulsar magnetosphere, we consider the magnetosphere to be stationary in space, instead of corotating, and the electric field to be uploaded from the potential distribution on the pulsar surface, set up by the unipolar induction. Consequently, the plasma of the magnetosphere undergoes guiding center drifts of the gyromotion due to the forces transverse to the magnetic field. These forces are the electric force, magnetic gradient force, and field line curvature force. Since these plasma velocities are of drift nature, there is no need to introduce an emf along the field lines, which would contradict the ${{E}_{\parallel }}={\boldsymbol{E}} \cdot {\boldsymbol{B}} =0$ plasma condition. Furthermore, there is also no need to introduce the critical field line separating the electron and ion open field lines. We present a self-consistent description where the magnetosphere is described in terms of electric and magnetic fields and also in terms of plasma velocities. The fields and velocities are then connected through the space-charge densities self-consistently. We solve the pulsar equation analytically for the fields and construct the standard steady-state pulsar magnetosphere. By considering the unipolar induction inside the pulsar and the magnetosphere outside the pulsar as one coupled system, and under the condition that the unipolar pumping rate exceeds the Poynting flux in the open field lines, plasma pressure can build up in the magnetosphere, in particular, in the closed region. This could cause a periodic opening up of the closed region, leading to a pulsating magnetosphere, which could be an alternative to pulsar beacons. The closed region can also be opened periodically by the build up of toroidal magnetic field through a positive feedback cycle.

Recently, ACTPol measured the cosmic microwave background (CMB) B-mode and E-mode polarizations and obtained TE, EE, BB, TB, and EB power spectra in the multipole range 225–8725. In our previous paper (Paper I), we jointly analyzed the results of three experiments on the CMB B-mode polarization—SPTpol, POLARBEAR, and BICEP2—to include in the model, in addition to the gravitational lensing and inflationary gravitational waves components, the fluctuation effects induced by cosmic polarization rotation (CPR) if it exists within the upper limits at the time. In this paper, we fit both the mean CPR angle $\langle \alpha \rangle$ and its fluctuation $\langle \delta {{\alpha }^{2}}\rangle$ from the new ACTPol data, and update our fitting of CPR fluctuations using the BICEP2 data taking the new Planck dust measurement results into consideration. We follow the same method used in Paper I. The mean CPR angle is constrained from the EB correlation power spectra to $|\langle \alpha \rangle |\lt 14$ mrad (08) and the fluctuation (rms) is constrained from the BB correlation power spectra to ${{\langle \delta {{\alpha }^{2}}\rangle }^{1/2}}\lt 29.3$ mrad (168). Assuming that the polarization angle of Tau A does not change from 89.2 to 146 GHz, the ACTPol data give $\langle \alpha \rangle =1.0\pm 0\buildrel{\circ}\over{.} 63.$ These results suggest that the inclusion of the present ACTPol data is consistent with no CPR detection. Using the new Planck dust measurement, we update our fits of the BICEP2 CPR fluctuation constraint to be 32.8 mrad (188). The joint ACTPol-BICEP2-POLARBEAR CPR fluctuation constraint is 23.7 mrad (136).

We show that dust absorption in disk galaxies leads to a color- and orientation-dependent centroid shift, which is expected to be observable in multi-band imaging surveys. This centroid shift is an interesting new probe that contains astrophysically and cosmologically relevant information: it can be used to probe the dust content of a large sample of galaxies and to reduce the shape noise due to inclination of disk galaxies for weak lensing shear. Specifically, we find that data sets comparable to CFHTLenS, the Dark Energy Survey, or the Hyper Suprime-Cam survey should provide a dust measurement for several hundred galaxies per square degree. Conversely, given knowledge of the dust optical depth, this technique will significantly lower the shape noise for the brightest galaxies in the sample (signal to noise greater than a few hundred), thereby increasing their relative importance for the weak lensing shear measurement.

As is usual in dwarf spheroidal galaxies, today the Local Group galaxy Ursa Minor is depleted of its gas content. How this galaxy lost its gas is still a matter of debate. To study the history of gas loss in Ursa Minor, we conducted the first three-dimensional hydrodynamical simulations of this object, assuming that the gas loss was driven by galactic winds powered only by type II supernovae (SNe II). The initial gas setup and supernova (SN) rates used in our simulations are mainly constrained by the inferred star formation history and the observed velocity dispersion of Ursa Minor. After 3 Gyr of evolution, we found that the gas removal efficiency is higher when the SN rate is increased, and also when the initial mean gas density is lowered. The derived mass-loss rates are systematically higher in the central regions ($\lt 300$ pc), even though such a relationship has not been strictly linear in time and in terms of the galactic radius. The filamentary structures induced by Rayleigh–Taylor instabilities and the concentric shells related to the acoustic waves driven by SNe can account for the inferred mass losses from the simulations. Our results suggest that SNe II are able to transfer most of the gas from the central region outward to the galactic halo. However, other physical mechanisms must be considered in order to completely remove the gas at larger radii.

Rich in H ii regions, giant molecular clouds are natural laboratories to study massive stars and sequential star formation. The Galactic star-forming complex W33 is located at $l=\sim 12\buildrel{\circ}\over{.} 8$ and at a distance of 2.4 kpc and has a size of $\approx 10$ pc and a total mass of $\approx$(0.8−8.0) $\times \;{{10}^{5}}$M$_{\odot }$. The integrated radio and IR luminosity of W33—when combined with the direct detection of methanol masers, the protostellar object W33A, and the protocluster embedded within the radio source W33 main—mark the region as a site of vigorous ongoing star formation. In order to assess the long-term star formation history, we performed an infrared spectroscopic search for massive stars, detecting for the first time 14 early-type stars, including one WN6 star and four O4–7 stars. The distribution of spectral types suggests that this population formed during the past ∼2–4 Myr, while the absence of red supergiants precludes extensive star formation at ages 6–30 Myr. This activity appears distributed throughout the region and does not appear to have yielded the dense stellar clusters that characterize other star-forming complexes such as Carina and G305. Instead, we anticipate that W33 will eventually evolve into a loose stellar aggregate, with Cyg OB2 serving as a useful, albeit richer and more massive, comparator. Given recent distance estimates, and despite a remarkably similar stellar population, the rich cluster Cl 1813–178 located on the northwest edge of W33 does not appear to be physically associated with W33.

Energy densities of relativistic electrons and protons in extended galactic and intracluster regions are commonly determined from spectral radio and (rarely) γ-ray measurements. The time-independent particle spectral density distributions are commonly assumed to have a power-law (PL) form over the relevant energy range. A theoretical relation between energy densities of electrons and protons is usually adopted, and energy equipartition is invoked to determine the mean magnetic field strength in the emitting region. We show that for typical conditions, in both star-forming and starburst (SB) galaxies, these estimates need to be scaled down substantially due to significant energy losses that (effectively) flatten the electron spectral density distribution, resulting in a much lower energy density than deduced when the distribution is assumed to have a PL form. The steady-state electron distribution in the nuclear regions of SB galaxies is calculated by accounting for Coulomb, bremsstrahlung, Compton, and synchrotron losses; the corresponding emission spectra of the latter two processes are calculated and compared to the respective PL spectra. We also determine the proton steady-state distribution by taking into account Coulomb and π production losses, and briefly discuss implications of our steady-state particle spectra for estimates of proton energy densities and magnetic fields.

We present results from a very deep (650 ks) Chandra X-ray observation of the galaxy group NGC 5813, the deepest Chandra observation of a galaxy group to date. This system uniquely shows three pairs of collinear cavities, with each pair associated with an unambiguous active galactic nucleus (AGN) outburst shock front. The implied mean kinetic power is roughly the same for each outburst, demonstrating that the average AGN kinetic luminosity can remain stable over long timescales (∼50 Myr). The two older outbursts have larger, roughly equal total energies as compared with the youngest outburst, implying that the youngest outburst is ongoing. We find that the gas radiative cooling rate and mean shock heating rate are well balanced at each shock front, suggesting that shock heating alone is sufficient to offset cooling and establish AGN/intracluster medium (ICM) feedback within at least the central 30 kpc. This heating takes place roughly isotropically and most strongly at small radii, as is required for feedback to operate. We suggest that shock heating may play a significant role in AGN feedback at smaller radii in other systems, where weak shocks are more difficult to detect. We find non-zero shock front widths that are too large to be explained by particle diffusion. Instead, all measured widths are consistent with shock broadening due to propagation through a turbulent ICM with a mean turbulent speed of ∼70 km s⁻¹. Finally, we place lower limits on the temperature of any volume-filling thermal gas within the cavities that would balance the internal cavity pressure with the external ICM.

Height–time plots of the leading edge of coronal mass ejections (CMEs) have often been used to study CME kinematics. We propose a new method to analyze the CME kinematics in more detail by determining the radial mass transport process throughout the entire CME. Thus, our method is able to estimate not only the speed of the CME front but also the radial flow speed inside the CME. We have applied this method to a slow CME with an average leading edge speed of about 480 km s⁻¹. In the Lagrangian frame, the speeds of the individual CME mass elements stay almost constant within 2 and 15 R_S, the range over which we analyzed the CME. Hence, we have no evidence of net radial forces acting on parts of the CME in this range or of a pile up of mass ahead of the CME. We find evidence that the leading edge trajectory obtained by tie-pointing may gradually lag behind the Lagrangian front-side trajectories derived from our analysis. Our results also allow a much more precise estimate of the CME energy. Compared with conventional estimates using the CME total mass and leading edge motion, we find that the latter may overestimate the kinetic energy and the gravitational potential energy.

We numerically study the detailed evolutionary features of the wave-like disturbance and its propagation in the eruption. This work is a follow-up to Wang et al., using significantly upgraded new simulations. We focus on the contribution of the velocity vortices and the fast shock reflection and refraction in the solar corona to the formation of the EUV waves. Following the loss of equilibrium in the coronal magnetic structure, the flux rope exhibits rapid motions and invokes the fast-mode shock at the front of the rope, which then produces a type II radio burst. The expansion of the fast shock, which is associated with outward motion, takes place in various directions, and the downward expansion shows the reflection and the refraction as a result of the non-uniform background plasma. The reflected component of the fast shock propagates upward and the refracted component propagates downward. As the refracted component reaches the boundary surface, a weak echo is excited. The Moreton wave is invoked as the fast shock touches the bottom boundary, so the Moreton wave lags the type II burst. A secondary echo occurs in the area where reflection of the fast shock encounters the slow-mode shock, and the nearby magnetic field lines are further distorted because of the interaction between the secondary echo and the velocity vortices. Our results indicate that the EUV wave may arise from various processes that are revealed in the new simulations.

We present the latest development of the disk gravitational instability and fragmentation model, originally introduced by us to explain episodic accretion bursts in the early stages of star formation. Using our numerical hydrodynamics model with improved disk thermal balance and star-disk interaction, we computed the evolution of protostellar disks formed from the gravitational collapse of prestellar cores. In agreement with our previous studies, we find that cores of higher initial mass and angular momentum produce disks that are more favorable to gravitational instability and fragmentation, while a higher background irradiation and magnetic fields moderate the disk tendency to fragment. The protostellar accretion in our models is time-variable, thanks to the nonlinear interaction between different spiral modes in the gravitationally unstable disk, and can undergo episodic bursts when fragments migrate onto the star owing to the gravitational interaction with other fragments or spiral arms. Most bursts occur in the partly embedded Class I phase, with a smaller fraction taking place in the deeply embedded Class 0 phase and a few possible bursts in the optically visible Class II phase. The average burst duration and mean luminosity are found to be in good agreement with those inferred from observations of FUors. The model predicts the existence of two types of bursts: the isolated ones, showing well-defined luminosity peaks separated with prolonged periods ($\sim {{10}^{4}}$ yr) of quiescent accretion, and clustered ones, demonstrating several bursts occurring one after another during just a few hundred years. Finally, we estimate that 40%–70% of the star-forming cores can display bursts after forming a star-disk system.

Theory and simulations suggest that it is possible to form low-mass hydrogen-burning stars, brown dwarfs (BDs), and planetary-mass objects (PMOs) via disk fragmentation. As disk fragmentation results in the formation of several bodies at comparable distances to the host star, their orbits are generally unstable. Here, we study the dynamical evolution of these objects. We set up the initial conditions based on the outcomes of the smoothed-particle hydrodynamics simulations of Stamatellos & Whitworth, and for comparison we also study the evolution of systems resulting from lower-mass fragmenting disks. We refer to these two sets of simulations as set 1 and set 2, respectively. At 10 Myr, approximately half of the host stars have one companion left, and approximately 22% (set 1) to 9.8% (set 2) of the host stars are single. Systems with multiple secondaries in relatively stable configurations are common (about 30% and 44%, respectively). The majority of the companions are ejected within 1 Myr with velocities mostly below 5 km s⁻¹, with some runaway escapers with velocities over 30 km s⁻¹. Roughly 6% (set 1) and 2% (set 2) of the companions pair up into very low-mass binary systems, resulting in respective binary fractions of 3.2% and 1.2%. The majority of these pairs escape as very low-mass binaries, while others remain bound to the host star in hierarchical configurations (often with retrograde inner orbits). Physical collisions with the host star (0.43 and 0.18 events per host star for set 1 and set 2, respectively) and between companions (0.08 and 0.04 events per host star for set 1 and set 2, respectively) are relatively common and their frequency increases with increasing disk mass. Our study predicts observable properties of very low-mass binaries, low-mass hierarchical systems, the BD desert, and free-floating BDs and PMOs in and near young stellar groupings, which can be used to distinguish between different formation scenarios of very low-mass stars, BDs, and PMOs.

Determining the physical parameters of binary microlenses is hampered by the lack of information about the angular Einstein radius due to the difficulty involved in resolving caustic crossings. In this paper, we present an analysis of the binary microlensing event OGLE-2013-BLG-0578, for which the caustic exit was precisely predicted in advance from real-time analysis, enabling us to densely resolve the caustic crossing and to measure the Einstein radius. From the mass measurement of the lens system based on the Einstein radius, combined with additional information about the lens parallax, we determine that the lens is a binary composed of a late-type M dwarf primary and a substellar brown dwarf companion. This event demonstrates the capability of current real-time microlensing modeling and the usefulness of microlensing for detecting and characterizing faint or dark objects in the Galaxy.

We investigate the nature of the Alfvénic turbulence cascade in two-fluid magnetohydrodynamic (MHD) simulations in order to determine if turbulence is damped once the ion and neutral species become decoupled at a critical scale called the ambipolar diffusion scale (L_AD). Using mode decomposition to separate the three classical MHD modes, we study the second-order structure functions of the Alfvén mode velocity field of both neutrals and ions in the reference frame of the local magnetic field. On scales greater than L_AD we confirm that two-fluid turbulence strongly resembles single-fluid MHD turbulence. Our simulations show that the behavior of two-fluid turbulence becomes more complex on scales less than L_AD. We find that Alfvénic turbulence can exist past L_AD when the turbulence is globally super-Alfvénic, with the ions and neutrals forming separate cascades once decoupling has taken place. When turbulence is globally sub-Alfvénic and hence strongly anisotropic, with a large separation between the parallel and perpendicular decoupling scales, turbulence is damped at L_AD. We also find that the power spectrum of the kinetic energy in the damped regime is consistent with a ${{k}^{-4}}$ scaling (in agreement with the predictions of Lazarian et al.).

The integrated radio spectrum of Cassiopeia A in continuum was analyzed with special emphasis on possible high-frequency spectral curvature. We conclude that the most probable scenario is that Planck's new data reveal the imprint of nonlinear particle acceleration in the case of this young Galactic supernova remnant.

Young supernova remnants (SNRs) show characteristic ejecta-dominated X-ray emission that allows us to probe the products of explosive nucleosynthesis processes and to ascertain important information about the physics of supernova explosions. Hard X-ray observations have recently revealed the presence of the radioactive decay lines of ⁴⁴Ti at ∼67.9 and ∼78.4 keV in Tycho's SNR. Here, we analyze a set of XMM-Newton archive observations of Tycho's SNR. We produce equivalent width (EW) maps of the Fe K and Ca xix emission lines and find indications for a stratification of the abundances of these elements and significant anisotropies. We then perform spatially resolved spectral analysis by identifying five different regions characterized by high/low values of the Fe K EW. We find that the spatial distribution of the Fe K emission is correlated with that of Cr xxii. We also detect the Ti K line complex in the spectra extracted from the two regions with the highest values of Fe and Cr EWs. The Ti line emission remains undetected in regions where Fe and Cr EWs are low. Our results indicate that the post-shock Ti is spatially colocated with other iron-peak nuclei in Tycho's SNR, in agreement with the predictions of multi-D models of SNe Ia.

It is well-known that a galaxy's environment has a fundamental influence in shaping its properties. We study the environmental effects on galaxy evolution, with an emphasis on the environment defined as the local number density of galaxies. The density field is estimated with different estimators (weighted adaptive kernel smoothing, 10th and 5th nearest neighbors, Voronoi and Delaunay tessellation) for a K_s < 24 sample of ∼190,000 galaxies in the COSMOS field at 0.1 < z < 3.1. The performance of each estimator is evaluated with extensive simulations. We show that overall there is a good agreement between the estimated density fields using different methods over ∼2 dex in overdensity values. However, our simulations show that adaptive kernel and Voronoi tessellation outperform other methods. Using the Voronoi tessellation method, we assign surface densities to a mass complete sample of quiescent and star-forming galaxies out to z ∼ 3. We show that at a fixed stellar mass, the median color of quiescent galaxies does not depend on their host environment out to z ∼ 3. We find that the number and stellar mass density of massive (>10¹¹$\;{{M}_{\odot }}$) star-forming galaxies have not significantly changed since z ∼ 3, regardless of their environment. However, for massive quiescent systems at lower redshifts (z ≲ 1.3), we find a significant evolution in the number and stellar mass densities in denser environments compared to lower density regions. Our results suggest that the relation between stellar mass and local density is more fundamental than the color–density relation and that environment plays a significant role in quenching star-formation activity in galaxies at z ≲ 1.

We present an X-ray and multiwavelength study of 33 weak emission-line quasars (WLQs) and 18 quasars that are analogs of the extreme WLQ, PHL 1811, at $z\approx 0.5$–2.9. New Chandra 1.5–9.5 ks exploratory observations were obtained for 32 objects while the others have archival X-ray observations. Significant fractions of these luminous type 1 quasars are distinctly X-ray weak compared to typical quasars, including 16 (48%) of the WLQs and 17 (94%) of the PHL 1811 analogs with average X-ray weakness factors of 17 and 39, respectively. We measure a relatively hard (${\Gamma }=1.16_{-0.32}^{+0.37}$) effective power-law photon index for a stack of the X-ray weak subsample, suggesting X-ray absorption, and spectral analysis of one PHL 1811 analog, J1521+5202, also indicates significant intrinsic X-ray absorption. We compare composite Sloan Digital Sky Survey spectra for the X-ray weak and X-ray normal populations and find several optical–UV tracers of X-ray weakness, e.g., Fe ii rest-frame equivalent width (REW) and relative color. We describe how orientation effects under our previously proposed "shielding-gas" scenario can likely unify the X-ray weak and X-ray normal populations. We suggest that the shielding gas may naturally be understood as a geometrically thick inner accretion disk that shields the broad line region from the ionizing continuum. If WLQs and PHL 1811 analogs have very high Eddington ratios, the inner disk could be significantly puffed up (e.g., a slim disk). Shielding of the broad emission-line region by a geometrically thick disk may have a significant role in setting the broad distributions of C iv REW and blueshift for quasars more generally.

Over the past 15 yr, examples of exotic radio-quiet quasars with intrinsically weak or absent broad emission line regions (BELRs) have emerged from large-scale spectroscopic sky surveys. Here, we present spectroscopy of seven such weak emission line quasars (WLQs) at moderate redshifts (z = 1.4–1.7) using the X-shooter spectrograph, which provides simultaneous optical and near-infrared spectroscopy covering the rest-frame ultraviolet (UV) through optical. These new observations effectively double the number of WLQs with spectroscopy in the optical rest-frame, and they allow us to compare the strengths of (weak) high-ionization emission lines (e.g., C iv) to low-ionization lines (e.g., Mg ii, Hβ, Hα) in individual objects. We detect broad Hβ and Hα emission in all objects, and these lines are generally toward the weaker end of the distribution expected for typical quasars (e.g., Hβ has rest-frame equivalent widths ranging from 15–40 Å). However, these low-ionization lines are not exceptionally weak, as is the case for high-ionization lines in WLQs. The X-shooter spectra also display relatively strong optical Fe ii emission, Hβ FWHM ≲ 4000 km s⁻¹, and significant C iv blueshifts (≈1000–5500 km s⁻¹) relative to the systemic redshift; two spectra also show elevated UV Fe ii emission, and an outflowing component to their (weak) Mg ii emission lines. These properties suggest that WLQs are exotic versions of "wind-dominated" quasars. Their BELRs either have unusual high-ionization components, or their BELRs are in an atypical photoionization state because of an unusually soft continuum.

We investigate the relationship between the rest-frame equivalent width (EW) of the C iv$\lambda 1549$ broad-emission line, monochromatic luminosity at rest-frame 5100 Å, and the Hβ-based Eddington ratio in a sample of 99 ordinary quasars across the widest possible ranges of redshift ($0\lt z\lt 3.5$) and bolometric luminosity (${{10}^{44}}\lesssim L\lesssim {{10}^{48}}\;{\rm erg}\;{{{\rm s}}^{-1}}$). We find that EW(C iv) is primarily anti-correlated with the Eddington ratio, a relation we refer to as a modified Baldwin effect (MBE), an extension of the result previously obtained for quasars at $z\lt 0.5$. Based on the MBE, weak emission line quasars (WLQs), typically showing EW(C iv)≲10 Å, are expected to have extremely high Eddington ratios. By selecting all WLQs with archival Hβ and C iv spectroscopic data, nine sources in total, we find that their Hβ-based Eddington ratios are typical of ordinary quasars with similar redshifts and luminosities. Four of these WLQs can be accommodated by the MBE, but the other five deviate significantly from this relation, at the $\gtrsim 3\sigma$ level, by exhibiting C iv lines much weaker than predicted from their Hβ-based Eddington ratios. Assuming the supermassive black hole masses in all quasars can be determined reliably using the single-epoch Hβ-method, our results indicate that EW(C iv) cannot depend solely on the Eddington ratio. We briefly discuss a strategy for further investigation into the roles that basic physical properties play in controlling the relative strengths of broad-emission lines in quasars.

We present a 1.3 mm dust continuum survey toward nine Class 0 protostars and two Class I protostars in the Perseus molecular cloud, using CARMA with a resolution of ∼03 (70 AU). This sample approximately doubles the number of Class 0 protostars observed with spatial resolutions <100 AU at millimeter wavelengths, enabling the presence of large protostellar disks and proto-binary systems to be probed. We have detected flattened structures with radii >100 AU around two sources (L1448 IRS2 and Per-emb-14), and these sources may be strong disk candidates. Marginally resolved structures within 30° of perpendicular to the outflow are found toward three protostars (L1448 IRS3C, IRAS 03282+3035, L1448C) and are considered disk candidates. Two others (L1448 IRS3B, IRAS 03292+3039) have complex resolved structures, possibly indicative of massive, fragmenting inner envelopes or disks; L1448 IRS3B also has evidence for a companion separated by 09 (∼210 AU). The candidate first hydrostatic core L1451-MMS is marginally resolved on 1'' scales and the Class 0 protostar IC 348-MMS and does not have strong indications of resolved structure at any scale. The strong disk candidate sources were followed up with C¹⁸O ($J=2\to 1$) observations; we detect velocity gradients that are consistent with the expected rotation axis, but without enough sensitivity to determine if it is Keplerian. We compare the observed visibility amplitudes to radiative transfer models of protostellar envelopes and disks. The visibility amplitude ratios show that a compact component (possibly a disk) is necessary for five of nine Class 0 sources. An envelope-only scenario cannot be ruled out for the other four Class 0 sources. We conclude that there is evidence for the formation of large disks in the Class 0 phase, but Class 0 disks likely have a range of radii and masses that depend on the initial conditions of their parent cores.

We demonstrate a new procedure to derive accurate and precise surface gravities from high resolution spectra without the use of external constraints. Our analysis utilizes Spectroscopy Made Easy with robust spectral line constraints and uses an iterative process to mitigate degeneracies in the fitting process. We adopt an updated radiative transfer code, a new treatment for neutral perturber broadening, a line list with multiple gravity constraints and separate fitting for global stellar properties and abundance determinations. To investigate the sources of temperature dependent trends in determining ${\rm log} {\mkern 1mu} g$ noted in previous studies, we obtained Keck HIRES spectra of 42 Kepler asteroseismic stars. In comparison to asteroseismically determined ${\rm log} {\mkern 1mu} g$ our spectroscopic analysis has a constant offset of 0.01 dex with a rms scatter of 0.05 dex. We also analyzed 30 spectra which had published surface gravities determined using the $a/{{R}_{*}}$ technique from planetary transits and found a constant offset of 0.06 dex and rms scatter of 0.07 dex. The two samples covered effective temperatures between 5000 and 6700 K with ${\rm log} {\mkern 1mu} g$ between 3.7 and 4.6.

With recent advances in asteroseismology it is now possible to peer into the cores of red giants, potentially providing a way to study processes such as nuclear burning and mixing through their imprint as sharp structural variations—glitches—in the stellar cores. Here we show how such core glitches can affect the oscillations we observe in red giants. We derive an analytical expression describing the expected frequency pattern in the presence of a glitch. This formulation also accounts for the coupling between acoustic and gravity waves. From an extensive set of canonical stellar models we find glitch-induced variation in the period spacing and inertia of non-radial modes during several phases of red giant evolution. Significant changes are seen in the appearance of mode amplitude and frequency patterns in asteroseismic diagrams such as the power spectrum and the échelle diagram. Interestingly, along the red giant branch glitch-induced variation occurs only at the luminosity bump, potentially providing a direct seismic indicator of stars in that particular evolution stage. Similarly, we find the variation at only certain post-helium-ignition evolution stages, namely, in the early phases of helium core burning and at the beginning of helium shell burning, signifying the asymptotic giant branch bump. Based on our results, we note that assuming stars to be glitch-free, while they are not, can result in an incorrect estimate of the period spacing. We further note that including diffusion and mixing beyond classical Schwarzschild could affect the characteristics of the glitches, potentially providing a way to study these physical processes.

Fluorine nucleosynthesis represents one of the most intriguing open questions in nuclear astrophysics. It has triggered new measurements which may modify the presently accepted paradigm of fluorine production and establish fluorine as an accurate probe of the inner layers of asymptotic giant branch (AGB) stars. Both direct and indirect measurements have attempted to improve the recommended extrapolation to astrophysical energies, showing no resonances. In this work, we will demonstrate that the interplay between direct and indirect techniques represents the most suitable approach to attain the required accuracy for the astrophysical factor at low energies, ${{E}_{{\rm c}.{\rm m}.}}\lesssim 300$ keV, which is of interest for fluorine nucleosynthesis in AGB stars. We will use the recently measured direct $^{19}{\rm F}{{(p,\alpha )}^{16}}{\rm O}$ astrophysical factor in the $600\;{\rm keV}\lesssim {{E}_{{\rm c}.{\rm m}.}}\lesssim 800\;{\rm keV}$ energy interval to renormalize the existing Trojan Horse Method (THM) data spanning the astrophysical energies, accounting for all identified sources of uncertainty. This has a twofold impact on nuclear astrophysics. It shows the robustness of the THM approach even in the case of direct data of questionable quality, as normalization is extended over a broad range, minimizing systematic effects. Moreover, it allows us to obtain more accurate resonance data at astrophysical energies, thanks to the improved $^{19}{\rm F}{{(p,\alpha )}^{16}}{\rm O}$ direct data. Finally, the present work strongly calls for more accurate direct data at low energies, so that we can obtain a better fitting of the direct reaction mechanism contributing to the $^{19}{\rm F}{{(p,\alpha )}^{16}}{\rm O}$ astrophysical factor. Indeed, this work points out that the major source of uncertainty affecting the low-energy S(E) factor is the estimate of the non-resonant contribution, as the dominant role of the 113 keV resonance is now well established.

We have conducted the first parallax and proper motion measurements of 6.7 GHz methanol maser emission using the Australian Long Baseline Array. The parallax of G 339.884–1.259 measured from five epochs of observations is 0.48 ± 0.08 mas, corresponding to a distance of $2.1_{-0.3}^{+0.4}$ kpc, placing it in the Scutum spiral arm. This is consistent (within the combined uncertainty) with the kinematic distance estimate for this source at 2.5 ± 0.5 kpc using the latest Solar and Galactic rotation parameters. We find from the Lyman continuum photon flux that the embedded core of the young star is of spectral type B1, demonstrating that luminous 6.7 GHz methanol masers can be associated with high-mass stars toward the lower end of the mass range.

We have used the publicly released Dark Energy Survey (DES) data to hunt for new satellites of the Milky Way (MW) in the southern hemisphere. Our search yielded a large number of promising candidates. In this paper, we announce the discovery of nine new unambiguous ultra-faint objects, whose authenticity can be established with the DES data alone. Based on the morphological properties, three of the new satellites are dwarf galaxies, one of which is located at the very outskirts of the MW, at a distance of 380 kpc. The remaining six objects have sizes and luminosities comparable to the Segue 1 satellite and cannot be classified straightforwardly without follow-up spectroscopic observations. The satellites we have discovered cluster around the LMC and the SMC. We show that such spatial distribution is unlikely under the assumption of isotropy, and, therefore, conclude that at least some of the new satellites must have been associated with the Magellanic Clouds in the past.

We use N-body/smoothed particle hydrodynamics simulations of encounters between an early-type galaxy (ETG) and a late-type galaxy (LTG) to study the effects of hot halo gas on the evolution for a case with the mass ratio of the ETG to LTG of 2:1 and the closest approach distance of ∼100 kpc. We find that the dynamics of the cold disk gas in the tidal bridge and the amount of the newly formed stars depend strongly on the existence of a gas halo. In the run of interacting galaxies not having a hot gas halo, the gas and stars accreted into the ETG do not include newly formed stars. However, in the run using the ETG with a gas halo and the LTG without a gas halo, a shock forms along the disk gas tidal bridge and induces star formation near the closest approach. The shock front is parallel to a channel along which the cold gas flows toward the center of the ETG. As a result, the ETG can accrete star-forming cold gas and newly born stars at and near its center. When both galaxies have hot gas halos, a shock is formed between the two gas halos somewhat before the closest approach. The shock hinders the growth of the cold gas bridge to the ETG and also ionizes it. Only some of the disk stars transfer through the stellar bridge. We conclude that the hot halo gas can give significant hydrodynamic effects during distant encounters.

HAT-P-20b is a giant metal-rich exoplanet orbiting a metal-rich star. We analyze two secondary eclipses of the planet in each of the 3.6 and 4.5 μm bands of Warm Spitzer. We have developed a simple, powerful, and radically different method to correct the intra-pixel effect for Warm Spitzer data, which we call pixel-level decorrelation (PLD). PLD corrects the intra-pixel effect very effectively, but without explicitly using—or even measuring—the fluctuations in the apparent position of the stellar image. We illustrate and validate PLD using synthetic and real data and comparing the results to previous analyses. PLD can significantly reduce or eliminate red noise in Spitzer secondary eclipse photometry, even for eclipses that have proven to be intractable using other methods. Our successful PLD analysis of four HAT-P-20b eclipses shows a best-fit blackbody temperature of 1134 ± 29 K, indicating inefficient longitudinal transfer of heat, but lacking evidence for strong molecular absorption. We find sufficient evidence for variability in the 4.5 μm band that the eclipses should be monitored at that wavelength by Spitzer, and this planet should be a high priority for James Webb Space Telescope spectroscopy. All four eclipses occur about 35 minutes after orbital phase 0.5, indicating a slightly eccentric orbit. A joint fit of the eclipse and transit times with extant RV data yields $e{\rm cos} \omega =0.01352_{-0.00057}^{+0.00054}$ and establishes the small eccentricity of the orbit to high statistical confidence. HAT-P-20b is another excellent candidate for orbital evolution via Kozai migration or other three-body mechanisms.

Meridional flow is thought to play a very important role in the dynamics of the solar convection zone; however, because of its relatively small amplitude, precisely measuring it poses a significant challenge. Here we present a complete time–distance helioseismic analysis of about 2 years of ground-based Global Oscillation Network Group (GONG) Doppler data to retrieve the meridional circulation profile for modest latitudes in an attempt to corroborate results from other studies. We use an empirical correction to the travel times due to an unknown center-to-limb systematic effect. The helioseismic inversion procedure is first tested and reasonably validated on artificial data from a large-scale numerical simulation followed by a test to broadly recover the solar differential rotation found from global seismology. From GONG data, we measure poleward photospheric flows at all latitudes with properties that are comparable with earlier studies and a shallow equatorward flow about 65 Mm beneath the surface, in agreement with recent findings from Helioseismic and Magnetic Imager (HMI) data. No strong evidence of multiple circulation cells in depth or latitude is found, yet the whole phase space has not yet been explored. Tests of mass flux conservation are then carried out on the inferred GONG and HMI flows and compared to a fiducial numerical baseline from models, and we find that the continuity equation is poorly satisfied. While the two disparate data sets do give similar results for about the outer 15% of the interior radius, the total inverted circulation pattern appears to be unphysical in terms of mass conservation when interpreted over modest time scales. We can likely attribute this to both the influence of realization noise and subtle effects in the data and measurement procedure.

Realistic models of magnetic reconnection in the solar chromosphere must take into account that the plasma is partially ionized and that plasma conditions within any two magnetic flux bundles undergoing reconnection may not be the same. Asymmetric reconnection in the chromosphere may occur when newly emerged flux interacts with pre-existing, overlying flux. We present 2.5D simulations of asymmetric reconnection in weakly ionized, reacting plasmas where the magnetic field strengths, ion and neutral densities, and temperatures are different in each upstream region. The plasma and neutral components are evolved separately to allow non-equilibrium ionization. As in previous simulations of chromospheric reconnection, the current sheet thins to the scale of the neutral–ion mean free path and the ion and neutral outflows are strongly coupled. However, the ion and neutral inflows are asymmetrically decoupled. In cases with magnetic asymmetry, a net flow of neutrals through the current sheet from the weak-field (high-density) upstream region into the strong-field upstream region results from a neutral pressure gradient. Consequently, neutrals dragged along with the outflow are more likely to originate from the weak-field region. The Hall effect leads to the development of a characteristic quadrupole magnetic field modified by asymmetry, but the X-point geometry expected during Hall reconnection does not occur. All simulations show the development of plasmoids after an initial laminar phase.

Solar flares are an explosive phenomenon where super-sonic flows and shocks are expected in and above the post-flare loops. To understand the dynamics of post-flare loops, a two-dimensional magnetohydrodynamic (2D MHD) simulation of a solar flare has been carried out. We found new shock structures in and above the post-flare loops, which were not resolved in the previous work by Yokoyama & Shibata. To study the dynamics of flows along the reconnected magnetic field, the kinematics and energetics of the plasma are investigated along selected field lines. It is found that shocks are crucial to determine the thermal and flow structures in the post-flare loops. On the basis of the 2D MHD simulation, we developed a new post-flare loop model, which we defined as the pseudo-2D MHD model. The model is based on the one-dimensional (1D) MHD equations, where all variables depend on one space dimension, and all the three components of the magnetic and velocity fields are considered. Our pseudo-2D model includes many features of the multi-dimensional MHD processes related to magnetic reconnection (particularly MHD shocks), which the previous 1D hydrodynamic models are not able to include. We compared the shock formation and energetics of a specific field line in the 2D calculation with those in our pseudo-2D MHD model, and found that they give similar results. This model will allow us to study the evolution of the post-flare loops in a wide parameter space without expensive computational cost or neglecting important physics associated with magnetic reconnection.

Determining reliable distances to classical novae is a challenging but crucial step in deriving their ejected masses and explosion energetics. Here we combine radio expansion measurements from the Karl G. Jansky Very Large Array with velocities derived from optical spectra to estimate an expansion parallax for nova V959 Mon, the first nova discovered through its γ-ray emission. We spatially resolve the nova at frequencies of 4.5–36.5 GHz in nine different imaging epochs. The first five epochs cover the expansion of the ejecta from 2012 October to 2013 January, while the final four epochs span 2014 February–May. These observations correspond to days 126 through 199 and days 615 through 703 after the first detection of the nova. The images clearly show a non-spherical ejecta geometry. Utilizing ejecta velocities derived from three-dimensional modeling of optical spectroscopy, the radio expansion implies a distance between 0.9 ± 0.2 and 2.2 ± 0.4 kpc, with a most probable distance of 1.4 ± 0.4 kpc. This distance implies a γ-ray luminosity of $0.6\times {{10}^{35}}$ erg s⁻¹, which is much less than the prototype γ-ray-detected nova, V407 Cyg, possibly due to the lack of a red giant companion in the V959 Mon system. V959 Mon also has a much lower γ-ray luminosity than other classical novae detected in γ-rays to date, indicating a range of at least a factor of 10 in the γ-ray luminosities for these explosions.

There is evidence that long-duration gamma-ray bursts (GRBs) originate from the core collapse of massive stars. When a jet punctures through the progenitor envelope, high-energy neutrinos can be produced by the reverse shock formed at the jet head. It is suggested that low-luminosity GRBs are possible candidates of this high-energy neutrino precursor up to $\sim {\rm PeV}$ scales. Before leaving the progenitor, these high-energy neutrinos must oscillate from one flavor to another with the matter effect in the envelope. Under the assumption of a power-law stellar envelope density profile $\rho \propto {{r}^{-\alpha }}$ with an index α, we study the properties of TeV–PeV neutrino oscillation. We find that adiabatic conversion is violated for these neutrinos so we calibrate the level crossing effect. The resonance condition is reached for different energies at different radii. We note that the effective mixing angles in matter for ${\rm PeV}$ neutrinos are close to zero, so the transition probabilities from one flavor to another are almost invariant for ${\rm PeV}$ neutrinos. We plot all the transition probabilities versus the energy of TeV–PeV neutrinos from the birth place to the surface of the progenitor. With an initial flavor ratio $\phi _{{{\nu }_{e}}}^{0}:\phi _{{{\nu }_{\mu }}}^{0}:\phi _{{{\nu }_{\tau }}}^{0}=1:2:0$, we plot how the flavor ratio evolves with energy and distance when neutrinos are still in the envelope, and we find the ratio when they reach the Earth. For ${\rm PeV}$ neutrinos, the ratio is always ${{\phi }_{{{\nu }_{e}}}}:{{\phi }_{{{\nu }_{\mu }}}}:{{\phi }_{{{\nu }_{\tau }}}}\simeq 0.30:0.37:0.33$ on Earth. In addition, we discuss the dependence of the flavor ratio on energy and α and find a good result. This dependence may provide a promising probe of the progenitor structure.

We calculate the electron acceleration in random superluminal strong waves (SLSWs) and radiation from them using numerical methods in the context of the termination shocks of pulsar wind nebulae. We pursue the orbit of electrons by solving the equation of motion in the analytically expressed electromagnetic turbulences. These consist of a primary SLS and isotropically distributed secondary electromagnetic waves. Under the dominance of the secondary waves, all electrons gain nearly equal energy. On the other hand, when the primary wave is dominant, selective acceleration occurs. The phase of the primary wave for electrons moving nearly along the wavevector changes very slowly compared with the oscillation of the wave, which is "phase-locked," and such electrons are continuously accelerated. This acceleration by SLSWs may play a crucial role in pre-shock acceleration. In general, the radiation from the phase-locked population is different from the synchro-Compton radiation. However, when the amplitude of the secondary waves is not extremely weaker than that of the primary wave, the typical frequency can be estimated from synchro-Compton theory using the secondary waves. The primary wave does not contribute to the radiation because the SLSW accelerates electrons almost linearly. This radiation can be observed as a radio knot at the upstream of the termination shocks of the pulsar wind nebulae without counterparts in higher frequency ranges.

We have studied the evolution of low-frequency quasi-periodic oscillations (LFQPOs) during the rising phase of seven outbursts of the neutron star soft X-ray transient Aql X−1 observed with the RXTE. A frequency correlation between the low-frequency break and the LFQPO sampled on a timescale of ∼2 days was observed. Except for the peculiar 2001 outburst, the frequency of LFQPOs increased with time before the hard-to-soft state transition up to a maximum ${{\nu }_{{\rm max} }}$ at ∼31 Hz, a factor of ∼5 higher than those seen in BH transients such as GX 339−4, making the maximum quasi-periodic oscillation (QPO) frequency a likely indicator of the mass of the central compact object. The characteristic frequencies increased by around 10% per day in the early rising phase and accelerated to nearly 100% per day since ∼2 days before the hard-to-soft state transition. We examined the dependence of the frequency ${{\nu }_{{\rm LF}}}$ on the source flux f and found an anti-correlation between the maximum frequency of the LFQPOs and the corresponding X-ray luminosity of the hard-to-soft transition (or outburst peak luminosity) among the outbursts. We suggest that the X-ray evaporation process cannot be the only mechanism that drives the variation of the inner disk radius if either of the twin kHz QPOs corresponds to the Keplerian frequency at the truncation radius.

We present Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging and grism spectroscopy observations of the Herschel-selected gravitationally lensed starburst galaxy HATLASJ1429-0028. The lensing system consists of an edge-on foreground disk galaxy at z = 0.218 with a nearly complete Einstein ring of the infrared luminous galaxy at z = 1.027. The WFC3 spectroscopy with G102 and G141 grisms, covering the wavelength range of 0.8–1.7 μm, resulted in detections of Hα + [Nii], Hβ, [Sii], and [Oiii] for the background galaxy from which we measure line fluxes and ratios. The Balmer line ratio Hα/Hβ of 7.5 ± 4.4, when corrected for [Nii], results in an extinction for the starburst galaxy of $E(B-V)=0.8\pm 0.5$. The Hα-based star formation rate (SFR), when corrected for extinction, is 60 ± 50 ${{M}_{\odot }}$ yr⁻¹, lower than the instantaneous SFR of 390 ± 90 ${{M}_{\odot }}$ yr⁻¹ from the total IR luminosity. We also compare the nebular line ratios of HATLASJ1429-0028 with other star-forming and sub-millimeter bright galaxies. The nebular line ratios are consistent with an intrinsic ultra-luminous infrared galaxy with no evidence for excitation by an active galactic nucleus (AGN). We estimate the metallicity, 12 + log(O/H), of HATLASJ1429-0028 to be 8.49 ± 0.16. Such a low value is below the average relation for stellar mass versus metallicity of galaxies at $z\sim 1$ for a galaxy with a stellar mass of $\sim 2\times {{10}^{11}}$${{M}_{\odot }}$. The combination of high stellar mass, the lack of AGN indicators, low metallicity, and the high SFR of HATLASJ1429-0028 suggest that this galaxy is currently undergoing a rapid formation.

High-resolution pure rotational spectra of four alkylnaphthalenes were measured in the range of 6–15 GHz using a molecular-beam Fourier-transform microwave spectrometer. Both a- and b-type transitions were observed for 1-methylnaphthalene (1-MN), 1,2-dimethylnaphthalene (1,2-DMN), and 1,3-dimethylnaphthalene (1,3-DMN); only a-type transitions were observed for 2-methylnaphthalene (2-MN). Geometry optimization and vibrational analysis calculations at the B3LYP/6-311++G(d,p) level of theory aided in the assignments of the spectra and the characterization of the structures. Differences between the experimental and predicted rotational constants are small, and they can be attributed in part to low-lying out-of-plane vibrations, which distort the alkylnaphthalenes out of their equilibrium geometries. Splittings of rotational lines due to methyl internal rotation were observed in the spectra of 2-MN, 1,2-DMN, and 1,3-DMN, and allowed for the determination of the barriers to methyl internal rotation, which are compared to values from density functional theory calculations. All four species are moderately polar, so they are candidate species for detection by radio astronomy, by targeting the transition frequencies reported here.

Thermal X-ray emission from young supernova remnants (SNRs) is usually dominated by the emission lines of the supernova ejecta, which are widely believed to be crossed and thus heated by the inward-propagating reverse shock (RS). Previous works using X-ray imaging data have shown that the ejecta are heated by the RS by locating the peak emission region of the most recently ionized matter, which is found to be well separated toward the inside from the outermost boundary. Here we report the discovery of a systematic increase of the Sulfur (S) to Silicon (Si) Kα line flux ratio with radius in Tycho's SNR. This allows us, for the first time, to present continuous radial profiles of the ionization age and, furthermore, the elapsed ionization time since the onset of the ionization, which gives the history of the propagation of the ionization front into the SNR ejecta.

X-ray and radio observations of CIZA J2242.8+5301 suggest that it is a major cluster merger. Despite being well studied in the X-ray and radio, little has been presented on the cluster structure and dynamics inferred from its galaxy population. We carried out a deep ($i\lt 25$) broadband imaging survey of the system with Subaru SuprimeCam (g and i bands) and the Canada–France–Hawaii Telescope (r band), as well as a comprehensive spectroscopic survey of the cluster area (505 redshifts) using Keck DEep Imaging Multi-Object Spectrograph. We use these data to perform a comprehensive galaxy/redshift analysis of the system, which is the first step to a proper understanding of the geometry and dynamics of the merger, as well as using the merger to constrain self-interacting dark matter. We find that the system is dominated by two subclusters of comparable richness with a projected separation of $6\buildrel{\,\prime}\over{.} 9_{-0.5}^{+0.7}$ (1.3$_{-0.10}^{+0.13}\;{\rm Mpc}$). We find that the north and south subclusters have similar redshifts of $z\approx 0.188$ with a relative line-of-sight (LOS) velocity difference of 69 ± 190 ${\rm km}\;{{{\rm s}}^{-1}}$. We also find that north and south subclusters have velocity dispersions of $1160_{-90}^{+100}$ and $1080_{-70}^{+100}\;{\rm km}\;{{{\rm s}}^{-1}}$, respectively. These correspond to masses of $16.1_{-3.3}^{+4.6}\times {{10}^{14}}$ and $13.0_{-2.5}^{+4.0}\times {{10}^{14}}$${{M}_{\odot }}$, respectively. While velocity dispersion measurements of merging clusters can be biased, we believe the bias in this system to be minor due to the large projected separation and nearly plane-of-sky merger configuration. We also find that the cDs of the north and south subclusters are very near their subcluster centers, in both projection (55 and 85 kpc, respectively) and normalized LOS velocity ($|{\Delta }v|/{{\sigma }_{v}}=0.43\pm 0.13$ and 0.21 ± 0.12 for the north and south, respectively). CIZA J2242.8+5301 is a relatively clean dissociative cluster merger with near 1:1 mass ratio, which makes it an ideal merger for studying merger-associated physical phenomena.

The Hubble Space Telescope Advanced Camera for Surveys has been used to determine accurate distances for the spiral galaxy NGC 2683 and 12 other galaxies in a zone of the "local velocity anomaly" from luminosity measurements of the brightest red giant branch stars. These galaxies lie in the Leo Spur, the nearest filament beyond our Local Sheet. The new accurate distance measurements confirm that galaxies along the Leo Spur are more distant than expected from uniform cosmic expansion, and hence have large and peculiar velocities toward us. The motions are generally explained by a previously published model that posits that the Local Sheet is descending at 259 km s⁻¹ toward the south supergalactic pole due to expansion of the Local Void and is being attracted toward the Virgo Cluster at 185 km s⁻¹. With the standard ΛCDM cosmology, an empty void expands at 16 km s⁻¹ Mpc⁻¹, so a motion of 259 km s⁻¹ requires the Local Void to be impressively large and empty. Small residuals from the published model can be attributed to an upward push toward the north supergalactic pole by the expansion of the Gemini–Leo Void below the Leo Spur. The Leo Spur is sparsely populated, but among its constituents there are two associations that contain only dwarf galaxies.

The radial profiles of gas, stars, and far-ultraviolet radiation in 20 dwarf Irregular galaxies are converted to stability parameters and scale heights for a test of the importance of two-dimensional (2D) instabilities in promoting star formation. A detailed model of this instability involving gaseous and stellar fluids with self-consistent thicknesses and energy dissipation on a perturbation crossing time gives the unstable growth rates. We find that all locations are effectively stable to 2D perturbations, mostly because the disks are thick. We then consider the average volume densities in the midplanes, evaluated from the observed H i surface densities and calculated scale heights. The radial profiles of the star-formation rates are equal to about 1% of the H i surface densities divided by the free fall times at the average midplane densities. This 1% resembles the efficiency per unit free fall time commonly found in other cases. There is a further variation of this efficiency with radius in all of our galaxies, following the exponential disk with a scale length equal to about twice the stellar mass scale length. This additional variation is modeled by the molecular fraction in a diffuse medium using radiative transfer solutions for galaxies with the observed dimensions and properties of our sample. We conclude that star formation is activated by a combination of three-dimensional gaseous gravitational processes and molecule formation. Implications for outer disk structure and formation are discussed.

We present an abundance analysis for two newly discovered α-poor stars based on high-resolution spectroscopy. These stars were previously identified in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Data Release 1 as candidate α-poor stars. We observed these candidate α-poor stars with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the 6.5 m Magellan-Clay Telescope located at Las Campanas Observatory. This high-resolution analysis confirms the low [α/Fe] abundance ratios for these stars compared to those of the majority of Galactic stars with comparable metallicities ([Fe/H] ∼ $-0.5$). The large deficiencies of α-elements suggest that these stars possess an anomalous chemical enrichment history. To have found such α-poor stars in our sample indicates that large scatters of the α-element abundance ratios may exist near solar metallicity. These results also demonstrate that our method is capable of selecting α-poor stars from low-resolution stellar spectra.

The outer (>30 AU) regions of the dusty circumstellar disk orbiting the ∼2–5 Myr old, actively accreting solar analog LkCa 15 are known to be chemically rich, and the inner disk may host a young protoplanet within its central cavity. To obtain a complete census of the brightest molecular line emission emanating from the LkCa 15 disk over the 210–270 GHz (1.4–1.1 mm) range, we have conducted an unbiased radio spectroscopic survey with the Institute de Radioastronomie Millimétrique (IRAM) 30 m telescope. The survey demonstrates that in this spectral region, the most readily detectable lines are those of CO and its isotopologues ¹³CO and C¹⁸O, as well as HCO⁺, HCN, CN, C₂H, CS, and H₂CO. All of these species had been previously detected in the LkCa 15 disk; however, the present survey includes the first complete coverage of the CN (2–1) and C₂H (3–2) hyperfine complexes. Modeling of these emission complexes indicates that the CN and C₂H either reside in the coldest regions of the disk or are subthermally excited, and that their abundances are enhanced relative to molecular clouds and young stellar object environments. These results highlight the value of unbiased single-dish line surveys in guiding future high-resolution interferometric imaging of disks.

T Pyxidis is the only recurrent nova known to be surrounded by knots of material ejected in previous outbursts. Following the eruption that began on 2011 April 14.29, we obtained seven epochs (from 4 to 383 days after eruption) of Hubble Space Telescope narrowband Hα images of T Pyx. The ionizing flash of radiation from the nova event had no discernible effect on the surrounding ejecta until at least 55 days after the eruption began. Photoionization of hydrogen located north and south of the central star was seen 132 days after the beginning of the eruption. That photoionized hydrogen recombined in the following 51 days, allowing us to determine a hydrogen atom density of at least $7\times {{10}^{5}}\;{\rm c}{{{\rm m}}^{-3}}$—at least an order of magnitude denser than the previously detected, unresolved [N ii] knots surrounding T Pyx. Material to the northwest and southeast was photoionized, and became bright between 132 and 183 days after the eruption began. Ninety-nine days later that northwest and southeast hydrogen had recombined. Both then (282 days after outburst) and 101 days later, we detected almost no trace of hydrogen emission around T Pyx. We determine that there is a large reservoir of previously unseen, cold diffuse hydrogen overlapping the previously detected, [N ii]-emitting knots of T Pyx ejecta. The mass of this newly detected hydrogen is model-dependent, but is is probably an order of magnitude larger than that of the [N ii] knots. We also determine that there is no significant reservoir of undetected hydrogen-rich ejecta, with density comparable to the flash-ionized ejecta we have detected, from the outer boundaries of the previously detected ejecta out to about twice that distance. The lack of distant ejecta is consistent with the Schaefer et al. scenario for T Pyx, in which the star underwent its first eruption within five years of 1866 after many millennia of quiescence, followed by the six observed recurrent nova eruptions since 1890. The lack of distant ejecta, demonstrated by our observations, is not consistent with scenarios in which T Pyx has been erupting continuously as a recurrent nova for many centuries or millennia.

We analyze three epochs of ultraviolet, optical, and near-infrared (NIR) observations of the Taurus transitional disk GM Aur using the Hubble Space Telescope Imaging Spectrograph (STIS) and the Infrared Telescope Facility SpeX spectrograph. Observations were separated by one week and three months in order to study variability over multiple timescales. We calculate accretion rates for each epoch of observations using the STIS spectra and find that those separated by one week had similar accretion rates ($\sim 1\times {{10}^{-8}}\;{{M}_{\odot }}\;{\rm y}{{{\rm r}}^{-1}}$) while the epoch obtained three months later had a substantially lower accretion rate ($\sim 4\times {{10}^{-9}}\;{{M}_{\odot }}\;{\rm y}{{{\rm r}}^{-1}}$). We find that the decline in accretion rate is caused by lower densities of material in the accretion flows, as opposed to a lower surface coverage of the accretion columns. During the low accretion rate epoch, we also observe lower fluxes at both far-ultraviolet (FUV) and IR wavelengths, which trace molecular gas and dust in the disk, respectively. We find that this can be explained by a lower dust and gas mass in the inner disk. We attribute the observed variability to inhomogeneities in the inner disk, near the corotation radius, where gas and dust may co-exist near the footprints of the magnetospheric flows. These FUV–NIR data offer a new perspective on the structure of the inner disk, the stellar magnetosphere, and their interaction.

Recent observational and theoretical progress has favored merging and helium-accreting sub-Chandrasekhar mass white dwarfs (WDs) in the double-degenerate and the double-detonation channels, respectively, as the most promising progenitors of normal Type Ia supernovae (SNe Ia). Thus the fate of rapidly accreting Chandrasekhar mass WDs in the single-degenerate channel remains more mysterious then ever. In this paper, we clarify the nature of ignition in Chandrasekhar-mass single-degenerate SNe Ia by analytically deriving the existence of a characteristic length scale which establishes a transition from central ignitions to buoyancy-driven ignitions. Using this criterion, combined with data from three-dimensional simulations of convection and ignition, we demonstrate that the overwhelming majority of ignition events within Chandrasekhar-mass WDs in the single-degenerate channel are buoyancy-driven, and consequently lack a vigorous deflagration phase. We thus infer that single-degenerate SNe Ia are generally expected to lead to overluminous 1991T-like SNe Ia events. We establish that the rates predicted from both the population of supersoft X-ray sources (SSSs) and binary population synthesis models of the single-degenerate channel are broadly consistent with the observed rates of overluminous SNe Ia, and suggest that the population of SSSs are the dominant stellar progenitors of SNe 1991T-like events. We further demonstrate that the single-degenerate channel contribution to the normal and failed 2002cx-like rates is not likely to exceed 1% of the total SNe Ia rate. We conclude with a range of observational tests of overluminous SNe Ia which will either support or strongly constrain the single-degenerate scenario.

We have observed a sample of 14 nearby ($z\sim 0.03$) star-forming blue compact galaxies (BCGs) in the rest-frame far-UV (∼1150–2200 Å) using the Cosmic Origins Spectrograph on the Hubble Space Telescope. We have also generated a grid of stellar population synthesis models using the Starburst99 evolutionary synthesis code, allowing us to compare observations and theoretical predictions for the Si iv_1400 and C iv_1550 UV indices; both are comprised of a blend of stellar wind and interstellar lines and have been proposed as metallicity diagnostics in the UV. Our models and observations both demonstrate that there is a positive linear correlation with metallicity for both indices, and we find generally good agreement between our observations and the predictions of the Starburst99 models (with the models slightly under-estimating the value of the indices due to contributions from interstellar lines not simulated by a stellar population synthesis code). By combining the rest-frame UV observations with pre-existing rest-frame optical spectrophotometry of our BCG sample, we also directly compare the predictions of metallicity and extinction diagnostics across both wavelength regimes. This comparison reveals a correlation between the UV absorption and optical strong-line diagnostics, offering the first means of directly comparing interstellar medium (ISM) properties determined across different rest-frame regimes. Finally, using our Starburst99 model grid, we determine theoretical values for the short-wavelength UV continuum slope, ${{\beta }_{18}}$, which can be used for determining extinction in rest-frame UV spectra of star-forming galaxies. We consider the implications of these results and discuss future work aimed at parameterizing these and other environmental diagnostics in the UV (a suite of diagnostics that could offer particular utility in the study of star-forming galaxies at high redshift) as well as the development of robust comparisons between ISM diagnostics across a broad wavelength baseline.

Observations of SN 1006 have shown that ions and electrons in the plasma behind fast supernova remnant shock waves are far from equilibrium, with the electron temperature much lower than the proton temperature and ion temperatures approximately proportional to ion mass. In the ∼360 km s⁻¹shock waves of the Cygnus Loop, on the other hand, electron and ion temperatures are roughly equal, and there is evidence that the oxygen kinetic temperature is not far from the proton temperature. In this paper, we report observations of the He iiλ1640 line and the C ivλ1550 doublet in a 360 km s⁻¹shock in the Cygnus Loop. While the best-fit kinetic temperatures are somewhat higher than the proton temperature, the temperatures of He and C are consistent with the proton temperature and the upper limits are 0.5 and 0.3 times the mass-proportional temperatures, implying efficient thermal equilibration in this collisionless shock. The equilibration of helium and hydrogen affects the conversion between proton temperatures determined from Hα line profiles and shock speeds, and the efficient equilibration found here reduces the shock speed estimates and the distance estimate to the Cygnus Loop of Medina et al. to about 800 pc.

As the local interstellar plasma flows past our heliosphere, it is slowed and deflected around the magnetic obstacle of the heliopause. The interstellar magnetic field, frozen into this plasma, then becomes draped around the heliopause in a characteristic manner. We derive the analytical solution for this draped magnetic field in the limit of weak field intensity, assuming an ideal potential flow around the heliopause, which we model as a Rankine half-body. We compare the structure of the model magnetic field with observed properties of the Interstellar Boundary Explorer (IBEX) ribbon and with in situ observations at the Voyager 1 spacecraft. We find reasonable qualitative agreement, given the idealizations of the model. This agreement lends support to the secondary ENA model of the IBEX ribbon and to the interpretation that Voyager 1 has crossed the heliopause. We also predict that the magnetic field measured by Voyager 2 after it crosses the heliopause will not be significantly rotated away from the direction of the undisturbed interstellar field.

The Chandra X-ray observatory has discovered dozens of resolved, kiloparsec-scale jets associated with powerful quasars in which the X-ray fluxes are observed to be much higher than the expected level based on the radio-optical synchrotron spectrum. The most popular explanation for the anomalously high and hard X-ray fluxes is that these jets do not decelerate significantly by the kiloparsec scale, but rather remain highly relativistic (Lorentz factors ${\Gamma }\sim 10$). By adopting a small angle to the line of sight, the X-rays can thus be explained by inverse Compton upscattering of cosmic microwave background (CMB) photons (IC/CMB), where the observed emission is strongly Doppler boosted. Using over six years of Fermi monitoring data, we show that the expected hard, steady gamma-ray emission implied by the IC/CMB model is not seen in PKS 0637-752, the prototype jet for which this model was first proposed. IC/CMB emission is thus ruled out as the source of the X-rays, joining recent results for the jets in 3C 273 (using the same method) and PKS 1136-135 (using UV polarization). We further show that the Fermi observations give an upper limit of $\delta \lt 6.5$ for the four brightest X-ray knots of PKS 0637-752, and derive an updated limit of $\delta \lt 7.8$ for knots A and B1 of 3C 273 (assuming equipartition). Finally, we discuss the fact that high levels of synchrotron X-ray emission in a slow jet will unavoidably lead to a level of angle-integrated TeV emission which exceeds that of the TeV BL Lac class.

We present the implementation of turbulence transport equations in addition to the Reynolds-averaged magnetohydrodynamic equations within the Cronos framework. The model is validated by comparisons with earlier findings before it is extended to be applicable to regions in the solar wind that are not highly super-Alfvénic. We find that the respective additional terms result in absolute normalized cross-helicity to decline more slowly, while a proper implementation of the mixing terms can even lead to increased cross-helicities in the inner heliosphere. The model extension allows us to place the inner boundary of the simulations closer to the Sun, where we choose its location at 0.1 AU for future application to the Wang–Sheeley–Arge model. Here, we concentrate on effects on the turbulence evolution for transient events by injecting a coronal mass ejection (CME). We find that the steep gradients and shocks associated with these structures result in enhanced turbulence levels and reduced cross-helicity. Our results can now be used straightforwardly for studying the transport of charged energetic particles, where the elements of the diffusion tensor can now benefit from the self-consistently computed solar wind turbulence. Furthermore, we find that there is no strong back-reaction of the turbulence on the large-scale flow so that CME studies concentrating on the latter need not be extended to include turbulence transport effects.

In the summer of 2012, during a Pulsar Search Collaboratory workshop, two high-school students discovered J1930–1852, a pulsar in a double neutron star (DNS) system. Most DNS systems are characterized by short orbital periods, rapid spin periods, and eccentric orbits. However, J1930–1852 has the longest spin period (${{P}_{{\rm spin}}}\;\sim \;$ 185 ms) and orbital period (${{P}_{b}}\;\sim \;$ 45 days) yet measured among known, recycled pulsars in DNS systems, implying a shorter than average and/or inefficient recycling period before its companion went supernova. We measure the relativistic advance of periastron for J1930–1852, $\dot{\omega }=0.00078$ (4) deg yr⁻¹, which implies a total mass (${{M}_{{\rm tot}}}=2.59$ (4) ${{M}_{\odot }}$) consistent with other DNS systems. The $2\sigma$ constraints on ${{M}_{{\rm tot}}}$ place limits on the pulsar and companion masses (${{m}_{p}}\lt 1.32$${{M}_{\odot }}$ and ${{m}_{c}}\gt 1.30$${{M}_{\odot }}$ respectively). J1930–1852's spin and orbital parameters challenge current DNS population models and make J1930–1852 an important system for further investigation.

We use the distance probability density function formalism of Ellsworth-Bowers et al. to derive physical properties for the collection of 1,710 Bolocam Galactic Plane Survey (BGPS) version 2 sources with well-constrained distance estimates. To account for Malmquist bias, we estimate that the present sample of BGPS sources is 90% complete above $400\;{{M}_{\odot }}$ and 50% complete above $70\;{{M}_{\odot }}$. The mass distributions for the entire sample and astrophysically motivated subsets are generally fitted well by a lognormal function, with approximately power-law distributions at high mass. Power-law behavior emerges more clearly when the sample population is narrowed in heliocentric distance (power-law index $\alpha =2.0\pm 0.1$ for sources nearer than 6.5 kpc and $\alpha =1.9\pm 0.1$ for objects between 2 and 10 kpc). The high-mass power-law indices are generally $1.85\leqslant \alpha \leqslant 2.05$ for various subsamples of sources, intermediate between that of giant molecular clouds and the stellar initial mass function. The fit to the entire sample yields a high-mass power-law $\hat{\alpha }=1.94_{-0.10}^{+0.34}$. Physical properties of BGPS sources are consistent with large molecular cloud clumps or small molecular clouds, but the fractal nature of the dense interstellar medium makes it difficult to map observational categories to the dominant physical processes driving the observed structure. The face-on map of the Galactic disk's mass surface density based on BGPS dense molecular cloud structures reveals the high-mass star-forming regions W43, W49, and W51 to be prominent mass concentrations in the first quadrant. Furthermore, we present a 0.25 kpc resolution map of the dense gas mass fraction across the Galactic disk that peaks around 5%.

To better understand the nature of the multiphase material found in outflowing galaxies, we study the evolution of cold clouds embedded in flows of hot and fast material. Using a suite of adaptive mesh refinement simulations that include radiative cooling, we investigate both cloud mass loss and cloud acceleration under the full range of conditions observed in galaxy outflows. The simulations are designed to track the cloud center of mass, enabling us to study the cloud evolution at long disruption times. For supersonic flows, a Mach cone forms around the cloud, which damps the Kelvin–Helmholtz instability but also establishes a streamwise pressure gradient that stretches the cloud apart. If time is expressed in units of the cloud crushing time, both the cloud lifetime and the cloud acceleration rate are independent of cloud radius, and we find simple scalings for these quantities as a function of the Mach number of the external medium. A resolution study suggests that our simulations accurately describe the evolution of cold clouds in the absence of thermal conduction and magnetic fields, physical processes whose roles will be studied in forthcoming papers.

We study the late-time ($t\gt 0.5$ days) X-ray afterglows of nearby ($z\lt 0.5$) long gamma-ray bursts (GRBs) with Swift and identify a population of explosions with slowly decaying, super-soft (photon index ${{{\Gamma }}_{x}}\gt 3$) X-ray emission that is inconsistent with forward shock synchrotron radiation associated with the afterglow. These explosions also show larger-than-average intrinsic absorption (${\rm N}{{{\rm H}}_{x,i}}\gt 6\times {{10}^{21}}\;{\rm c}{{{\rm m}}^{-2}}$) and prompt γ-ray emission with extremely long duration (${{T}_{90}}\gt 1000$ s). The chance association of these three rare properties (i.e., large ${\rm N}{{{\rm H}}_{x,i}}$, super-soft ${{{\Gamma }}_{x}}$, and extreme duration) in the same class of explosions is statistically unlikely. We associate these properties with the turbulent mass-loss history of the progenitor star that enriched and shaped the circumburst medium. We identify a natural connection between ${\rm N}{{{\rm H}}_{x,i}}$, ${{{\Gamma }}_{x}}$, and ${{T}_{90}}$ in these sources by suggesting that the late-time super-soft X-rays originate from radiation reprocessed by material lost to the environment by the stellar progenitor before exploding (either in the form of a dust echo or as reprocessed radiation from a long-lived GRB remnant), and that the interaction of the explosion's shock/jet with the complex medium is the source of the extremely long prompt emission. However, current observations do not allow us to exclude the possibility that super-soft X-ray emitters originate from peculiar stellar progenitors with large radii that only form in very dusty environments.

We present a new photometric catalog of 326 candidate globular clusters (GCs) in the nearby spiral galaxy M101, selected from B, V, and I Hubble Space Telescope Advanced Camera for Surveys images. The luminosity function (LF) of these clusters has an unusually large number of faint sources compared with GCLFs in many other spiral galaxies. Accordingly, we separate and compare the properties of "bright" (${{M}_{V}}\lt -6.5$) versus "faint" (${{M}_{V}}\gt -6.5$; one magnitude fainter than the expected GC peak) clusters within our sample. The LF of the bright clusters is well fit by a peaked distribution similar to those observed in the Milky Way (MW) and other galaxies. These bright clusters also have similar size (r_eff) and spatial distributions as MW GCs. The LF of the faint clusters, on the other hand, is well described by a power law, $dN({{L}_{V}})/d{{L}_{V}}\propto L_{V}^{\alpha }$ with $\alpha =-2.6\pm 0.3$, similar to those observed for young and intermediate-age cluster systems in star forming galaxies. We find that the faint clusters have larger typical r_eff than the bright clusters, and have a flatter surface density profile, being more evenly distributed, as we would expect for clusters associated with the disk. We use the shape of the LF and predictions for mass-loss driven by two-body relaxation to constrain the ages of the faint clusters. Our results are consistent with two populations of old star clusters in M101: a bright population of halo clusters and a fainter, possibly younger, population of old disk clusters.

Recent work has demonstrated the potential of gravitationally lensed quasars to extend measurements of black hole spin out to high redshift with the current generation of X-ray observatories. Here we present an analysis of a large sample of 27 lensed quasars in the redshift range $1.0\lesssim z\lesssim 4.5$ observed with Chandra, utilizing over 1.6 Ms of total observing time, focusing on the rest-frame iron K emission from these sources. Although the X-ray signal-to-noise ratio (S/N) currently available does not permit the detection of iron emission from the inner accretion disk in individual cases in our sample, we find significant structure in the stacked residuals. In addition to the narrow core, seen almost ubiquitously in local active galactic nuclei (AGNs), we find evidence for an additional underlying broad component from the inner accretion disk, with a clear red wing to the emission profile. Based on simulations, we find the detection of this broader component to be significant at greater than the 3σ level. This implies that iron emission from the inner disk is relatively common in the population of lensed quasars, and in turn further demonstrates that, with additional observations, this population represents an opportunity to significantly extend the sample of AGN spin measurements out to high redshift.

We investigate signs of an active galactic nucleus (AGN) in the luminous infrared (IR) galaxy NGC 3256 at both IR and X-ray wavelengths. NGC 3256 has double nuclei: the northern and southern (hereafter, N and S nuclei, respectively). We show that the Spitzer IRAC colors extracted at the S nucleus are AGN-like, and the Spitzer IRS spectrum is bluer at $\lt 6$μm than at the N nucleus. We built for the S nucleus an AGN–starburst composite model with a heavily absorbed AGN to successfully reproduce not only the IRAC and IRS specrophotometries at $\simeq 3$'', but also the very deep silicate 9.7 μm absorption observed at a 0 36 scale by Díaz-Santos et al. We found a 2.2 μm compact source at the S nucleus in an HST NICMOS image and identified its unresolved core (at 0 26 resolution) with the compact core in previous mid-infrared observations at comparable resolution. The flux of the 2.2 μm core is consistent with our AGN spectral energy distribution model. We also analyzed a deeper than ever Chandra X-ray spectrum of the unresolved (at 0 5 resolution) source at the S nucleus. We found that a dual-component power-law model (for primary and scattered ones) fits an apparently very hard spectrum with a moderately large absorption on the primary component. Together with a limit on equivalent width of a fluorescent Fe–K emission line at 6.4 keV, the X-ray spectrum is consistent with a typical Compton-thin Seyfert 2. We therefore suggest that the S nucleus hosts a heavily absorbed low-luminosity AGN.

We perform 3D relativistic ideal magnetohydrodynamics (MHD) simulations to study the collisions between high-σ (Poynting-flux-dominated (PFD)) blobs which contain both poloidal and toroidal magnetic field components. This is meant to mimic the interactions inside a highly variable PFD jet. We discover a significant electromagnetic field (EMF) energy dissipation with an Alfvénic rate with the efficiency around 35%. Detailed analyses show that this dissipation is mostly facilitated by the collision-induced magnetic reconnection. Additional resolution and parameter studies show a robust result that the relative EMF energy dissipation efficiency is nearly independent of the numerical resolution or most physical parameters in the relevant parameter range. The reconnection outflows in our simulation can potentially form the multi-orientation relativistic mini jets as needed for several analytical models. We also find a linear relationship between the σ values before and after the major EMF energy dissipation process. Our results give support to the proposed astrophysical models that invoke significant magnetic energy dissipation in PFD jets, such as the internal collision-induced magnetic reconnection and turbulence model for gamma-ray bursts, and reconnection triggered mini jets model for active galactic nuclei. The simulation movies are shown in http://www.physics.unlv.edu/∼deng/simulation1.html.

We examine the bright radio synchrotron counterparts of low-luminosity gamma-ray bursts and relativistic supernovae (SNe) and find that they can be powered by spherical hypernova (HN) explosions. Our results imply that radio-bright HNe are driven by relativistic jets that are choked deep inside the progenitor stars or quasi-spherical magnetized winds from fast-rotating magnetars. We also consider the optical synchrotron counterparts of radio-bright HNe and show that they can be observed as precursors several days before the SN peak with an r-band absolute magnitude of ${{M}_{r}}\sim -14$ mag. While previous studies suggested that additional trans-relativistic components are required to power the bright radio emission, we find that they overestimated the energy budget of the trans-relativistic component by overlooking some factors related to the minimum energy of non-thermal electrons. If an additional trans-relativistic component exists, then a much brighter optical precursor with ${{M}_{r}}\sim -20$ mag can be expected. Thus, the scenarios of radio-bright HNe can be distinguished by using optical precursors, which can be detectable from $\lesssim 100\;{\rm Mpc}$ by current SN surveys like the Kiso SN Survey, Palomar Transient Factory, and Panoramic Survey Telescope & Rapid Response System.

The solar meridional flow in the deep convection zone can be measured with time–distance analysis. The difference between northward and southward acoustic wave travel times relates to the speed of meridional flow in the solar convection zone. We study the effects of surface magnetic field on the measured travel time difference in time–distance analysis by comparing the results using data with and without removing surface magnetic regions. Two results are significantly different if the field strength threshold used to remove magnetic regions is small enough, such as 50 G. The difference represents the surface magnetic effects. This difference strongly correlates with the sunspot number. The range of travel distance in this study is 7°–75°, corresponding to a depth range of $0.54-0.96\;{{R}_{\odot }}$. The difference depends on the travel time distance, and is greatest for the travel distance of $11{}^\circ -20{}^\circ .$ The study with different field strength thresholds indicates that a threshold of about 50 G can remove most surface magnetic effects. The measured surface magnetic effects can be explained by an effective downflow inside magnetic regions. These signals are not related to the large-scale meridional flow; they need to be considered in the measured travel time difference even when activity is low, if the travel time difference is used to infer meridional flow signals.

Recent work by Mitra et al. (2014) has shown that in strongly stratified forced two-layer turbulence with helicity and corresponding large-scale dynamo action in the lower layer, and nonhelical turbulence in the upper, a magnetic field occurs in the upper layer in the form of sharply bounded bipolar magnetic spots. Here we extend this model to spherical wedge geometry covering the northern hemisphere up to 75° latitude and an azimuthal extent of 180°. The kinetic helicity and therefore also the large-scale magnetic field are strongest at low latitudes. For moderately strong stratification, several bipolar spots form that eventually fill the full longitudinal extent. At early times, the polarity of spots reflects the orientation of the underlying azimuthal field, as expected from Parker's Ω-shaped flux loops. At late times their tilt changes such that there is a radial field of opposite orientation at different latitudes separated by about 10°. Our model demonstrates the spontaneous formation of spots of sizes much larger than the pressure scale height. Their tendency to produce filling factors close to unity is argued to be reminiscent of highly active stars. We confirm that strong stratification and strong scale separation are essential ingredients behind magnetic spot formation, which appears to be associated with downflows at larger depths.

The dynamics of hot chromospheric plasma of solar flares is a key to understanding the mechanisms of flare energy release and particle acceleration. A moderate M1.0 class flare of 2014 June 12, (SOL2014-06-12T21:12) was simultaneously observed by NASA's Interface Region Imaging Spectrograph (IRIS) and other spacecraft, and also by the New Solar Telescope at the BBSO. This paper presents the first part of our investigation focused on analysis of the IRIS data. Our analysis of the IRIS data in different spectral lines reveals a strong redshifted jet-like flow with a speed of ∼100 km s⁻¹ of the chromospheric material before the flare. Strong nonthermal emission of the C ii k 1334.5 Å line, formed in the chromosphere–corona transition region, is observed at the beginning of the impulsive phase in several small (with a size of ∼1'') points. It is also found that the C ii k line is redshifted across the flaring region before, during, and after the impulsive phase. A peak of integrated emission of the hot (1.1 · 10⁷ K) plasma in the Fe xxi 1354.1 Å line is detected approximately five minutes after the integrated emission peak of the lower temperature C ii k. A strong blueshift of the Fe xxi line across the flaring region corresponds to evaporation flows of the hot chromospheric plasma with a speed of 50 km s⁻¹. Additional analysis of the RHESSI data supports the idea that the upper chromospheric dynamics observed by IRIS has features of "gentle" evaporation driven by heating of the solar chromosphere by accelerated electrons and by a heat flux from the flare energy release site.

Accurate space weather forecasting requires knowledge of the trajectory of coronal mass ejections (CMEs), including any deflections close to the Sun or through interplanetary space. Kay et al. introduced ForeCAT, a model of CME deflection resulting from the background solar magnetic field. For a magnetic field solution corresponding to Carrington Rotation (CR) 2029 (declining phase, 2005 April–May), the majority of the CMEs deflected to the Heliospheric Current Sheet, the minimum in magnetic pressure on global scales. Most of the deflection occurred below 4 ${{R}_{\odot }}$. Here we extend ForeCAT to include a three-dimensional description of the deflecting CME. We attempt to answer the following questions: (1) do all CMEs deflect to the magnetic minimum? and (2) does most deflection occur within the first few solar radii (4 ${{R}_{\odot }}$)? Results for solar minimum and declining-phase CMEs show that not every CME deflects to the magnetic minimum and that typically the majority of the deflection occurs below 10 ${{R}_{\odot }}$. Slow, wide, low-mass CMEs in declining-phase solar backgrounds with strong magnetic field and magnetic gradients exhibit the largest deflections. Local gradients related to active regions tend to cause the largest deviations from the deflection predicted by global magnetic gradients, but variations can also be seen for CMEs in the quiet-Sun regions of the declining-phase CR. We show the torques due to differential forces along the CME can cause rotation about the CME's toroidal axis.

Magnetic fields are ubiquitous in active cool stars, but they are in general complex and weak. Current Zeeman Doppler imaging (ZDI) studies of cool star magnetic fields chiefly employ circular polarization observations because linear polarization is difficult to detect and requires a more sophisticated radiative transfer modeling to interpret. But it has been shown in previous theoretical studies, and in the observational analyses of magnetic Ap stars, that including linear polarization in the magnetic inversion process makes it possible to correctly recover many otherwise lost or misinterpreted magnetic features. We have obtained phase-resolved observations in all four Stokes parameters of the RS CVn star II Peg at two separate epochs. Here we present temperature and magnetic field maps reconstructed for this star using all four Stokes parameters. This is the very first such ZDI study of a cool active star. Our magnetic inversions reveal a highly structured magnetic field topology for both epochs. The strength of some surface features is doubled or even quadrupled when linear polarization is taken into account. The total magnetic energy of the reconstructed field map also becomes about 2.1–3.5 times higher. The overall complexity is also increased as the field energy is shifted toward higher harmonic modes when four Stokes parameters are used. As a consequence, the potential field extrapolation of the four Stokes parameter ZDI results indicates that magnetic field becomes weaker at a distance of several stellar radii due to a decrease of the large-scale field component.

We investigate the possible effects of the supernova (SN) ejecta hitting the companion star in iPTF 13bvn, focusing on the observable features when it becomes visible. iPTF 13bvn is a type Ib SN that may become the first case in which its progenitor is identified as a binary (by observations in the near future). According to calculations by Bersten et al. the progenitor should have a mass $\approx 3.5\;{{M}_{\odot }}$ to reproduce the SN light curve, and such compact stars can only be produced via binary evolution. This is one of the reasons that we expect the progenitor to be a binary, but this should be confirmed by observing the remaining companion after the SN. Bersten et al.'s evolutionary calculations suggest that the companion star will be an overluminous OB star at the moment of SN. With a combination of hydrodynamical and evolutionary simulations, we find that the secondary star will be heated by the SN ejecta and expand to have larger luminosities and lower surface effective temperatures. The star will look like a red supergiant and this should be taken into account when searching for the companion star in the SN ejecta in future observations.

To study the fragmentation and gravitational collapse of dense cores in infrared dark clouds (IRDCs), we have obtained submillimeter continuum and spectral line data as well as multiple inversion transitions of NH₃ and H₂O maser data of four massive clumps in IRDC G28.53−0.25. Combining single-dish and interferometer NH₃ data, we derive a rotation temperature of G28.53. We identity 12 dense cores at a 0.1 pc scale based on submillimeter continuum, and obtain their physical properties using NH₃ and continuum data. By comparing the Jeans masses of cores with the core masses, we find that turbulent pressure is important for supporting the gas when 1 pc scale clumps fragment into 0.1 pc scale cores. All cores have a virial parameter that is smaller than 1 if we assume an inverse squared radial density profile, suggesting they are gravitationally bound, and the three most promising star-forming cores have a virial parameter that is smaller than 1 even when taking the magnetic field into account. We also associate the cores with star formation activities revealed by outflows, masers, or infrared sources. Unlike what previous studies have suggested, MM1 turns out to harbor a few star-forming cores and is likely a progenitor of a high-mass star cluster. MM5 is intermediate while MM7/8 are quiescent in terms of star formation, but they also harbor gravitationally bound dense cores and have the potential for forming stars, as in MM1.

The lack of detected pulsars at the Galactic Center (GC) region is a long-standing mystery. We argue that the high stellar density in the central parsec around the GC is likely to result in a pulsar population dominated by millisecond pulsars (MSPs), similar to the situation in globular cluster environments. Earlier GC pulsar searches have been largely insensitive to such an MSP population, accounting for the lack of pulsar detections. We estimate the best search frequency for such an MSP population with present and upcoming broad-band radio telescopes for two possible scattering scenarios, the "weak-scattering" case suggested by the recent detection of a magnetar close to the GC, and the "strong-scattering" case, with the scattering screen located close to the GC. The optimal search frequencies are ≈8 GHz (weak-scattering) and ≈25 GHz (strong-scattering), for pulsars with periods 1–20 ms, assuming that GC pulsars have a luminosity distribution similar to that those in the rest of the Milky Way. We find that 10–30 hr integrations with the Very Large Array and the Green Bank Telescope would be sufficient to detect MSPs at the GC distance in the weak-scattering case. However, if the strong-scattering case is indeed applicable to the GC, observations with the full Square Kilometre Array would be needed to detect the putative MSP population.

An analytical representation of the interstellar magnetic field in the vicinity of the heliosphere is derived. The three-dimensional field structure close to the heliopause is calculated as a solution of the induction equation under the assumption that it is frozen into a prescribed plasma flow resembling the characteristic interaction of the solar wind with the local interstellar medium. The usefulness of this analytical solution as an approximation to self-consistent magnetic field configurations obtained numerically from the full MHD equations is illustrated by quantitative comparisons.

We investigate the possibility of constraining the sin i degeneracy of α Cen B b—with orbital period P = 3.24 days; a = 0.042 AU; msin i = 1.1 ${{M}_{\oplus }}$—to estimate the true mass of the newly reported terrestrial exoplanet in the nearest stellar system to our Sun. We present detailed numerical simulations of the dynamical stability of the exoplanet in the α Cen AB binary system for a range of initial inclinations, eccentricities, and semimajor axes. The system represents a benchmark case for the interplay of the Kozai mechanism with general relativistic and tidal forces. From our simulations, there is only a small boundary in initial inclinations and initial semimajor axes which result in the migration via the Kozai mechanism of α Cen B b to its present location. Inside this boundary, the planet orbit is stable for up to 1 Gyr against the Kozai mechanism, and outside this boundary the planet collides with α Cen B or is ejected. In our three simulations where the planet migrates in toward the star via the Kozai mechanism, the final inclination is 46°–53° relative to the AB orbital plane, lower than the initial inclination of 75° in each case. We discuss inclination constraints from the formation of α Cen B b in situ at its present location, migration in a proto-planetary disk, or migration in resonance with additional planets. We conclude that α Cen B b probably has a mass of less than 2.7 ${{M}_{\oplus }}$, implying a likely terrestrial composition warranting future confirmation.

We report the discovery of two super-Earth-mass planets orbiting the nearby K0.5 dwarf HD 7924, which was previously known to host one small planet. The new companions have masses of 7.9 and 6.4 ${{M}_{\oplus }}$, and orbital periods of 15.3 and 24.5 days. We perform a joint analysis of high-precision radial velocity data from Keck/HIRES and the new Automated Planet Finder Telescope (APF) to robustly detect three total planets in the system. We refine the ephemeris of the previously known planet using 5 yr of new Keck data and high-cadence observations over the last 1.3 yr with the APF. With this new ephemeris, we show that a previous transit search for the inner-most planet would have covered 70% of the predicted ingress or egress times. Photometric data collected over the last eight years using the Automated Photometric Telescope shows no evidence for transits of any of the planets, which would be detectable if the planets transit and their compositions are hydrogen-dominated. We detect a long-period signal that we interpret as the stellar magnetic activity cycle since it is strongly correlated with the Ca ii H and K activity index. We also detect two additional short-period signals that we attribute to rotationally modulated starspots and a one-month alias. The high-cadence APF data help to distinguish between the true orbital periods and aliases caused by the window function of the Keck data. The planets orbiting HD 7924 are a local example of the compact, multi-planet systems that the Kepler Mission found in great abundance.

Using measurements from the WIND spacecraft, here we report the observation of sunward propagating Alfvén waves (AWs) in solar wind that is magnetically disconnected from the Earth's bow shock. In the sunward magnetic field sector, we find a period lasting for more than three days in which there existed (during most time intervals) a negative correlation between the flow velocity and magnetic field fluctuations, thus indicating that the related AWs are mainly propagating sunward. Simultaneous observations of counter-streaming suprathermal electrons suggest that these sunward AWs may not simply be due to the deflection of an open magnetic field line. Moreover, no interplanetary coronal mass ejection appears to be associated with the counter-streaming suprathermal electrons. As the scale goes from the magnetohydrodynamic down to the ion kinetic regime, the wave vector of magnetic fluctuations usually becomes more orthogonal to the mean magnetic field direction, and the fluctuations become increasingly compressible, which are both features consistent with quasi-perpendicular kinetic AWs. However, in the case studied here, we find clear signatures of quasi-parallel sunward propagating ion-cyclotron waves. Concurrently, the solar wind proton velocity distribution reveals a sunward field-aligned beam that drifts at about the local Alfvén speed. This beam is found to run in the opposite direction of the normally observed (anti-sunward) proton beam, and is apparently associated with sunward propagating Alfvén/ion-cyclotron waves. The results and conclusions of this study enrich our knowledge of solar wind turbulence and foster our understanding of proton heating and acceleration within a complex magnetic field geometry.

Brightest cluster galaxies (BCGs) are usually quiescent, but many exhibit star formation. Here we exploit the opportunity provided by rest-frame UV imaging of galaxy clusters in the Cluster Lensing and Supernovae with Hubble (CLASH) Multi-Cycle Treasury Project to reveal the diversity of UV morphologies in BCGs and to compare them with recent simulations of the cool, star-forming gas structures produced by precipitation-driven feedback. All of the CLASH BCGs are detected in the rest-frame UV (280 nm), regardless of their star formation activity, because evolved stellar populations produce a modest amount of UV light that traces the relatively smooth, symmetric, and centrally peaked stellar distribution seen in the near infrared. Ultraviolet morphologies among the BCGs with strong UV excesses exhibit distinctive knots, multiple elongated clumps, and extended filaments of emission that distinctly differ from the smooth profiles of the UV-quiet BCGs. These structures, which are similar to those seen in the few star-forming BCGs observed in the UV at low redshift, are suggestive of bi-polar streams of clumpy star formation, but not of spiral arms or large, kiloparsec-scale disks. Based on the number of streams and lack of culprit companion galaxies, these streams are unlikely to have arisen from multiple collisions with gas-rich galaxies. These star-forming UV structures are morphologically similar to the cold-gas structures produced in simulations of precipitation-driven active galactic nucleus feedback in which jets uplift low-entropy gas to greater altitudes, causing it to condense. Unobscured star formation rates estimated from CLASH UV images using the Kennicutt relation range up to 80 ${{M}_{\odot }}\;{\rm y}{{{\rm r}}^{-1}}$ in the most extended and highly structured systems. The circumgalactic gas-entropy threshold for star formation in CLASH BCGs at $z\;\sim$ 0.2–0.5 is indistinguishable from that for clusters at $z\lt 0.2$.

We have imaged with Hubble Space Telescope WFC3/UVIS the central $2\buildrel{\,\prime}\over{.} 7\times 2\buildrel{\,\prime}\over{.} 7$ region of the giant elliptical galaxy M87, using the ultraviolet filter F275W. In combination with archival ACS/WFC data taken through the F606W and F814W filters, covering the same field, we have constructed integrated-light UV–optical colors and magnitudes for 1460 objects, most of which are believed to be globular clusters (GCs) belonging to M87. The purpose was to ascertain whether the multiple-populations syndrome, ubiquitous among Galactic GCs, also exists among the M87 family of clusters. To achieve this goal, we sought those GCs with exceptionally blue UV-to-optical colors because helium-enriched sub-populations produce a horizontal-branch morphology that is well populated at high effective temperature. For comparison, integrated, synthetic UV–optical and purely optical colors and magnitudes have been constructed for 45 Galactic GCs, starting from individual-star photometry obtained with the same instruments and the same filters. We identify a small group of M87 clusters exhibiting a radial UV–optical color gradient, representing our best candidate GCs hosting multiple populations with extreme helium content. We also find that the central spatial distribution of the bluer GCs is flattened in a direction parallel to the jet, while the distribution of redder GCs is more spherical. We release to the astronomical community our photometric catalog in F275W, F606W, and F814W bands and the high-quality image stacks in the same bands.

The $6\times {{10}^{9}}\;{{M}_{\odot }}$ supermassive black hole at the center of the giant elliptical galaxy M87 powers a relativistic jet. Observations at millimeter wavelengths with the Event Horizon Telescope have localized the emission from the base of this jet to angular scales comparable to the putative black hole horizon. The jet might be powered directly by an accretion disk or by electromagnetic extraction of the rotational energy of the black hole. However, even the latter mechanism requires a confining thick accretion disk to maintain the required magnetic flux near the black hole. Therefore, regardless of the jet mechanism, the observed jet power in M87 implies a certain minimum mass accretion rate. If the central compact object in M87 were not a black hole but had a surface, this accretion would result in considerable thermal near-infrared and optical emission from the surface. Current flux limits on the nucleus of M87 strongly constrain any such surface emission. This rules out the presence of a surface and thereby provides indirect evidence for an event horizon.

At radio wavelengths, scattering in the interstellar medium distorts the appearance of astronomical sources. Averaged over a scattering ensemble, the result is a blurred image of the source. However, Narayan & Goodman and Goodman & Narayan showed that for an incomplete average, scattering introduces refractive substructure in the image of a point source that is both persistent and wideband. We show that this substructure is quenched but not smoothed by an extended source. As a result, when the scatter-broadening is comparable to or exceeds the unscattered source size, the scattering can introduce spurious compact features into images. In addition, we derive efficient strategies to numerically compute realistic scattered images, and we present characteristic examples from simulations. Our results show that refractive substructure is an important consideration for ongoing missions at the highest angular resolutions, and we discuss specific implications for RadioAstron and the Event Horizon Telescope.

At z ≳ 1, the distinction between merging and "normal" star-forming galaxies based on single band morphology is often hampered by the presence of large clumps which result in a disturbed, merger-like appearance even in rotationally supported disks. In this paper we discuss how a classification based on canonical, non-parametric structural indices measured on resolved stellar mass maps, rather than on single-band images, reduces the misclassification of clumpy but not merging galaxies. We calibrate the mass-based selection of mergers using the MIRAGE hydrodynamical numerical simulations of isolated and merging galaxies which span a stellar mass range of 10^9.8–10^10.6M_⊙ and merger ratios between 1:1–1:6.3. These simulations are processed to reproduce the typical depth and spatial resolution of observed Hubble Ultra Deep Field (HUDF) data. We test our approach on a sample of real $z\simeq 2$ galaxies with kinematic classification into disks or mergers and on ∼100 galaxies in the HUDF field with photometric/spectroscopic redshift between 1.5 ≤ z ≤ 3 and M > 10^9.4M_⊙. We find that a combination of the asymmetry A_MASS and M_{20, MASS} indices measured on the stellar mass maps can efficiently identify real (major) mergers with ≲20% contamination from clumpy disks in the merger sample. This mass-based classification cannot be reproduced in star-forming galaxies by H-band measurements alone, which instead result in a contamination from clumpy galaxies which can be as high as 50%. Moreover, we find that the mass-based classification always results in a lower contamination from clumpy galaxies than an H-band classification, regardless of the depth of the imaging used (e.g., CANDELS versus HUDF).

We present a quantitative spectroscopic study of 27 red supergiants (RSGs) in the Sculptor Galaxy NGC 300. J-band spectra were obtained using KMOS on the Very Large Telescope and studied with state of the art synthetic spectra including NLTE corrections for the strongest diagnostic lines. We report a central metallicity of [Z] = −0.03 ± 0.05 with a gradient of −0.083 ± 0.014 [dex/kpc], in agreement with previous studies of blue supergiants and H ii-region auroral line measurements. This result marks the first application of the J-band spectroscopic method to a population of individual RSG stars beyond the Local Group of galaxies and reveals the great potential of this technique.

We measure the recent star formation history (SFH) across M31 using optical images taken with the Hubble Space Telescope as part of the Panchromatic Hubble Andromeda Treasury (PHAT). We fit the color–magnitude diagrams in ∼9000 regions that are ∼100 pc × 100 pc in projected size, covering a 0.5 square degree area (∼380 kpc², deprojected) in the NE quadrant of M31. We show that the SFHs vary significantly on these small spatial scales but that there are also coherent galaxy-wide fluctuations in the SFH back to ∼500 Myr, most notably in M31's 10 kpc star-forming ring. We find that the 10 kpc ring is at least 400 Myr old, showing ongoing star formation (SF) over the past ∼500 Myr. This indicates the presence of molecular gas in the ring over at least 2 dynamical times at this radius. We also find that the ring's position is constant throughout this time, and is stationary at the level of 1 km s⁻¹, although there is evidence for broadening of the ring due to the diffusion of stars into the disk. Based on existing models of M31's ring features, the lack of evolution in the ring's position makes a purely collisional ring origin highly unlikely. Besides the well-known 10 kpc ring, we observe two other ring-like features. There is an outer ring structure at 15 kpc with concentrated SF starting ∼80 Myr ago. The inner ring structure at 5 kpc has a much lower star formation rate (SFR) and therefore lower contrast against the underlying stellar disk. It was most clearly defined ∼200 Myr ago, but is much more diffuse today. We find that the global SFR has been fairly constant over the last ∼500 Myr, though it does show a small increase at 50 Myr that is 1.3 times the average SFR over the past 100 Myr. During the last ∼500 Myr, ∼60% of all SF has occurred in the 10 kpc ring. Finally, we find that in the past 100 Myr, the average SFR over the PHAT survey area is 0.28 ± 0.03 ${{M}_{\odot }}$${\rm y}{{{\rm r}}^{-1}}$ with an average deprojected intensity of $7.3\times {{10}^{-4}}$${{M}_{\odot }}$${\rm y}{{{\rm r}}^{-1}}\;{\rm kp}{{{\rm c}}^{-2}}$, which yields a total SFR of ∼0.7 ${{M}_{\odot }}$${\rm y}{{{\rm r}}^{-1}}$ when extrapolated to the entire area of M31's disk. This SFR is consistent with measurements from broadband estimates.

GRAMORs are GRAvitationally highly magnified yet MORphologically regular images. An example of this phenomenon was discovered in the cluster MACS J1149.5+2223 in 2009. We investigate the lens statistics of GRAMORs in detail. Assuming a NFW profile for a sample of clusters, we calculate the expected number and redshift distribution of GRAMORs using parameters from COSMOS data for the number density of the background galaxy. A model with a cluster placed at z = 0.544 based on WMAP5 cosmology predicts the redshift of a GRAMOR at $z\simeq 1.49$ which is close to the observed z = 1.4906. These results show that the expected number of GRAMORs is about two per cluster in the most likely case, and thus a large number of GRAMORs would be observed in a systematic survey. The probability distribution function of source redshift for GRAMORs depends strongly on dark energy and may be useful for constraining the nature of dark energy.

We present deep NH₃ observations of the L1495-B218 filaments in the Taurus molecular cloud covering over a 3° angular range using the K-band focal plane array on the 100 m Green Bank Telescope. The L1495-B218 filaments form an interconnected, nearby, large complex extending over 8 pc. We observed NH₃ (1, 1) and (2, 2) with a spectral resolution of 0.038 km s⁻¹ and a spatial resolution of 31''. Most of the ammonia peaks coincide with intensity peaks in dust continuum maps at 350 and 500 μm. We deduced physical properties by fitting a model to the observed spectra. We find gas kinetic temperatures of 8–15 K, velocity dispersions of 0.05–0.25 km s⁻¹, and NH₃ column densities of 5 × 10¹² to 1 × 10¹⁴ cm⁻². The CSAR algorithm, which is a hybrid of seeded-watershed and binary dendrogram algorithms, identifies a total of 55 NH₃ structures, including 39 leaves and 16 branches. The masses of the NH₃ sources range from 0.05 to 9.5 ${{M}_{\odot }}$. The masses of NH₃ leaves are mostly smaller than their corresponding virial mass estimated from their internal and gravitational energies, which suggests that these leaves are gravitationally unbound structures. Nine out of 39 NH₃ leaves are gravitationally bound, and seven out of nine gravitationally bound NH₃ leaves are associated with star formation. We also found that 12 out of 30 gravitationally unbound leaves are pressure confined. Our data suggest that a dense core may form as a pressure-confined structure, evolve to a gravitationally bound core, and undergo collapse to form a protostar.

HH 212 is a nearby (400 pc) highly collimated protostellar jet powered by a Class 0 source in Orion. We have mapped the inner 80'' (∼0.16 pc) of the jet in SiO ($J=8-7$) and CO ($J=3-2$) simultaneously at ∼ 0 5 resolution with the Atacama Millimeter/Submillimeter Array (SMA) at unprecedented sensitivity. The jet consists of a chain of knots and bow shocks with sinuous structures in between. Compared to what we saw in our previous observations with the SMA, the jet appears to be more continuous, especially in the northern part. Some of the knots are now observed to be associated with small bow shocks, with their bow wings curving back to the jet axis, as seen in pulsed jet simulations. Two of the knots are reasonably resolved, showing kinematics consistent with sideways ejection, possibly tracing the internal working surfaces formed by a temporal variation in the jet velocity. In addition, nested shells are seen in CO around the jet axis connecting to the knots and bow shocks, driven by them. The proper motion of the jet is estimated to be ∼115 ± 50 km s⁻¹, comparing with our previous observations. The jet has a small semi-periodical wiggle with a period of ∼93 yr. The amplitude of the wiggle first increases with the distance from the central source and then stays roughly constant. One possible origin of the wiggle could be the kink instability in a magnetized jet.

Gamma-ray bursts (GRBs) are characterized by ultra-relativistic outflows, while supernovae are generally characterized by non-relativistic ejecta. GRB afterglows decelerate rapidly, usually within days, because their low-mass ejecta rapidly sweep up a comparatively larger mass of circumstellar material. However, supernovae with heavy ejecta can be in nearly free expansion for centuries. Supernovae were thought to have non-relativistic outflows except for a few relativistic ones accompanied by GRBs. This clear division was blurred by SN 2009bb, the first supernova with a relativistic outflow without an observed GRB. However, the ejecta from SN 2009bb was baryon loaded and in nearly free expansion for a year, unlike GRBs. We report the first supernova discovered without a GRB but with rapidly decelerating mildly relativistic ejecta, SN 2012ap. We discovered a bright and rapidly evolving radio counterpart driven by the circumstellar interaction of the relativistic ejecta. However, we did not find any coincident GRB with an isotropic fluence of more than one-sixth of the fluence from GRB 980425. This shows for the first time that central engines in SNe Ic, even without an observed GRB, can produce both relativistic and rapidly decelerating outflows like GRBs.

We study the task of estimating the true masses of known radial-velocity (RV) exoplanets by means of direct astrometry on coronagraphic images to measure the apparent separation between exoplanet and host star. Initially, we assume perfect knowledge of the RV orbital parameters and that all errors are due to photon statistics. We construct design reference missions for four missions currently under study at NASA: EXO-S and WFIRST-S, with external star shades for starlight suppression, EXO-C and WFIRST-C, with internal coronagraphs. These DRMs reveal extreme scheduling constraints due to the combination of solar and anti-solar pointing restrictions, photometric and obscurational completeness, image blurring due to orbital motion, and the "nodal effect," which is the independence of apparent separation and inclination when the planet crosses the plane of the sky through the host star. Next, we address the issue of nonzero uncertainties in RV orbital parameters by investigating their impact on the observations of 21 single-planet systems. Except for two—GJ 676 A b and 16 Cyg B b, which are observable only by the star-shade missions—we find that current uncertainties in orbital parameters generally prevent accurate, unbiased estimation of true planetary mass. For the coronagraphs, WFIRST-C and EXO-C, the most likely number of good estimators of true mass is currently zero. For the star shades, EXO-S and WFIRST-S, the most likely numbers of good estimators are three and four, respectively, including GJ 676 A b and 16 Cyg B b. We expect that uncertain orbital elements currently undermine all potential programs of direct imaging and spectroscopy of RV exoplanets.

Wrapping around the Milky Way, the Sagittarius stream is the dominant substructure in the halo. Our statistical selection method has allowed us to identify 106 highly likely members of the Sagittarius stream. Spectroscopic analysis of metallicity and kinematics of all members provides us with a new mapping of the Sagittarius stream. We find correspondence between the velocity distribution of stream stars and those computed for a triaxial model of the Milky Way dark matter halo. The Sagittarius trailing arm exhibits a metallicity gradient, ranging from −0.59 to −0.97 dex over 142°. This is consistent with the scenario of tidal disruption from a progenitor dwarf galaxy that possessed an internal metallicity gradient. We note high metallicity dispersion in the leading arm, causing a lack of detectable gradient and possibly indicating orbital phase mixing. We additionally report on a potential detection of the Sextans dwarf spheroidal in our data.