Table of contents

In a previously presented proof-of-principle study, we established a parameterized spherically symmetric explosion method (PUSH) that can reproduce many features of core-collapse supernovae. The present paper goes beyond a specific application that is able to reproduce observational properties of SN 1987A and performs a systematic study of an extensive set of nonrotating, solar metallicity stellar progenitor models in the mass range from 10.8 to 120 M_⊙. This includes the transition from neutron stars to black holes as the final result of the collapse of massive stars, and the relation of the latter to supernovae, possibly faint supernovae, and failed supernovae. We discuss the explosion properties of all models and predict remnant mass distributions within this approach. The present paper provides the basis for extended nucleosynthesis predictions in a forthcoming paper to be employed in galactic evolution models.

In a previously presented proof-of-principle study, we established a parameterized spherically symmetric explosion method (PUSH) that can reproduce many features of core-collapse supernovae (CCSNe) for a wide range of pre-explosion models. The method is based on the neutrino-driven mechanism and follows collapse, bounce, and explosion. There are two crucial aspects of our model for nucleosynthesis predictions. First, the mass cut and explosion energy emerge simultaneously from the simulation (determining, for each stellar model, the amount of Fe-group ejecta). Second, the interactions between neutrinos and matter are included consistently (setting the electron fraction of the innermost ejecta). In the present paper, we use the successful explosion models from Ebinger et al. that include two sets of pre-explosion models at solar metallicity, with combined masses between 10.8 and 120 M_⊙. We perform systematic nucleosynthesis studies and predict detailed isotopic yields. The resulting ⁵⁶Ni ejecta are in overall agreement with observationally derived values from normal CCSNe. The Fe-group yields are also in agreement with derived abundances for metal-poor star HD 84937. We also present a comparison of our results with observational trends in alpha element to iron ratios.

Cosmochemical evaluations of the initial meteoritical abundance of the short-lived radioisotope (SLRI) ²⁶Al have remained fairly constant since 1976, while estimates for the initial abundance of the SLRI ⁶⁰Fe have varied widely recently. At the high end of this range, ⁶⁰Fe initial abundances have seemed to require ⁶⁰Fe nucleosynthesis in a core-collapse supernova, followed by incorporation into primitive meteoritical components within ∼1 Myr. This paper continues the detailed exploration of this classical scenario, using models of the self-gravitational collapse of molecular cloud cores that have been struck by suitable shock fronts, leading to the injection of shock front gas into the collapsing cloud through Rayleigh–Taylor fingers formed at the shock–cloud interface. As before, these models are calculated using the FLASH three-dimensional, adaptive mesh refinement, gravitational hydrodynamical code. While the previous models used FLASH 2.5, the new models employ FLASH 4.3, which allows sink particles to be introduced to represent the newly formed protostellar object. Sink particles permit the models to be pushed forward farther in time to the phase where a ∼1 M_⊙ protostar has formed, orbited by a rotating protoplanetary disk. These models are thus able to define what type of target cloud core is necessary for the supernova triggering scenario to produce a plausible scheme for the injection of SLRIs into the presolar cloud core: a ∼3 M_⊙ cloud core rotating at a rate of ∼3 × 10⁻¹⁴ rad s⁻¹ or higher.

In this work, we present a ∼90 ks continuous monitoring of the Galactic microquasar GRS 1915 + 105 with AstroSat when the source undergoes a major transition from a nonvariable, χ class (similar to radio-quiet χ class) to a structured, large-amplitude, periodic heartbeat state (similar to ρ class). We show that such a transition takes place via an intermediate state when the large-amplitude, irregular variability of the order of hundreds of seconds in the soft X-ray band turned into 100–150 s regular, structured, nearly periodic flares. The properties of strong low-frequency (LF) quasi-periodic oscillation (QPO) in the frequency range 3–5 Hz also evolve marginally during these variability transitions. We also study time-lag and rms spectra at the QPO and harmonic component and the dynamic power spectra. We note a few important differences between the heartbeat state and the ρ class. Interestingly, the time-averaged LF QPO properties in the hard X-ray band are relatively stable in three states when compared to the significant evolution observed in the slow variability properties at millihertz frequencies. Such relative stability of LF QPOs implies that the inner disk-corona coupled accretion flow, which determines the LF QPO properties, may be uninterrupted by the launch of long, large-amplitude flares.

Using molecular line data from the Millimetre Astronomy Legacy Team 90 GHz Survey, we have searched the optically thick HCO⁺ (1–0) line for the "blue asymmetry" spectroscopic signature of infall motion in a large sample of high-mass, dense molecular clumps observed to be at different evolutionary stages of star cluster formation according to their mid-infrared appearance. To quantify the degree of the line asymmetry, we measure the asymmetry parameter $A=\displaystyle \frac{{I}_{\mathrm{blue}}-{I}_{\mathrm{red}}}{{I}_{\mathrm{blue}}+{I}_{\mathrm{red}}}$, the fraction of the integrated intensity that lies to the blueshifted side of the systemic velocity determined from the optically thin tracer N₂H⁺ (1–0). For a sample of 1093 sources, both the mean and median of A are positive ($A=0.083\pm 0.010$ and 0.065 ± 0.009, respectively) with high statistical significance, and a majority of sources (a fraction of 0.607 ± 0.015 of the sample) show positive values of A, indicating a preponderance of blue asymmetric profiles over red asymmetric profiles. Two other measures, the local slope of the line at the systemic velocity and the δv parameter of Mardones et al. (1997), also show an overall blue asymmetry for the sample, but with smaller statistical significance. This blue asymmetry indicates that these high-mass clumps are predominantly undergoing gravitational collapse. The blue asymmetry is larger ($A\sim 0.12$) for the earliest evolutionary stages (quiescent, protostellar, and compact H ii region) than for the later H ii region ($A\sim 0.06$) and photodissociation region ($A\sim 0$) classifications.

The Event Horizon Telescope, a global 230 GHz very-long-baseline interferometry array, achieves angular resolution of $\approx 20\,\mu \mathrm{as}$, sufficient to resolve the supermassive black hole Sagittarius A* (Sgr A*). This resolution may soon enable measurements of the black hole "shadow" size and asymmetry, predicted to be ≈50 and ≲3 μas, respectively. Measurements that depart from these values could indicate a violation of the "no-hair theorem." However, refractive scattering by the turbulent ionized interstellar medium distorts the image of Sgr A*, affecting its apparent size and asymmetry. In this paper, we present a general analytic approach to quantify the expected image wander, distortion, and asymmetry from refractive scattering. If the turbulence in the scattering material of Sgr A* is close to Kolmogorov, we estimate the mean refractive image wander, distortion, and asymmetry to be 0.53, 0.72, and 0.52 μas at 230 GHz. However, alternative scattering models with flatter power spectra can yield larger values, up to 2.1, 6.3, and 5.0 μas, respectively. We demonstrate that these effects can be reduced by averaging images over multiple observations. For a small number of observations, the effects of scattering can be comparable to or greater than those from black hole spin, and they determine a fundamental limit for testing general relativity via images of Sgr A*.

We present spectroscopic confirmation of five galaxy clusters at 1.25 < z < 1.5, discovered in the 2500 deg² South Pole Telescope Sunyaev–Zel'dovich (SZ) survey. These clusters, taken from a mass-limited sample with a nearly redshift-independent selection function, have multiwavelength follow-up imaging data from the X-ray to near-IR and currently form the most homogeneous massive high-redshift cluster sample known. We identify 44 member galaxies, along with 25 field galaxies, among the five clusters, and describe the full set of observations and data products from Magellan/LDSS3 multiobject spectroscopy of these cluster fields. We briefly describe the analysis pipeline and present ensemble analyses of cluster member galaxies that demonstrate the reliability of the measured redshifts. We report z = 1.259, 1.288, 1.316, 1.401, and 1.474 for the five clusters from a combination of absorption-line (Ca ii H&K doublet—λλ3968, 3934) and emission-line ([O ii] λλ3727, 3729) spectral features. Moreover, the calculated velocity dispersions yield dynamical cluster masses in good agreement with the SZ masses for these clusters. We discuss the velocity and spatial distributions of passive and [O ii]-emitting galaxies in these clusters, showing that they are consistent with velocity segregation and biases observed in lower redshift South Pole Telescope clusters. We identify modest [O ii] emission and pronounced CN and Hδ absorption in a stacked spectrum of 28 passive galaxies with Ca ii H&K-derived redshifts. This work increases the number of spectroscopically confirmed SZ-selected galaxy clusters at z > 1.25 from three to eight, further demonstrating the efficacy of SZ selection for the highest redshift massive clusters and enabling detailed study of these systems.

We report on the results of multi-frequency follow-up observations of three pulsars (PSRs J0026+6320, J2208+5500, and J2217+5733) discovered with the Giant Metrewave Radio Telescope (GMRT). These observations were carried out with the GMRT and the Ooty Radio Telescope (ORT). We report improved timing solutions for all three pulsars. For PSR J2208+5500, we estimate the nulling fraction to be 53(3)%. The steep spectrum of this pulsar, its single component profile, and narrow pulse width suggest its single component to be a core component. If so, this significant cessation of emission in a core component is inconsistent with a geometric origin of nulls, such as those due to "empty" sightline traverses, and more likely due to intrinsic changes in the pulsar magnetosphere. We have measured scatter-broadening timescales at 325 and 610 MHz for PSRs J0026+6320 and J2217+5733. The implied scatter-broadening frequency scaling index of −2.9 for both pulsars is different from that expected assuming Kolmogorov turbulence in the interstellar medium. We also report spectral indices, obtained from imaging observations, for all three pulsars for the first time. The spectra for two of these pulsars indicate a possible spectral turnover between 100 and 300 MHz. Multi-frequency timing analyses carried out for these pulsars have enabled us to determine dispersion measures with accuracies of 0.01 pc cm⁻³. This demonstrates the usefulness of quasi-simultaneous multi-frequency multi-epoch timing observations with the GMRT and the ORT for studying variations in DM for millisecond pulsars.

Observational tests of stellar and Galactic chemical evolution call for the joint knowledge of a star's physical parameters, detailed element abundances, and precise age. For cool main-sequence (MS) stars the abundances of many elements can be measured from spectroscopy, but ages are very hard to determine. The situation is different if the MS star has a white dwarf (WD) companion and a known distance, as the age of such a binary system can then be determined precisely from the photometric properties of the cooling WD. As a pilot study for obtaining precise age determinations of field MS stars, we identify nearly 100 candidates for such wide binary systems: a faint WD whose GPS1 proper motion matches that of a brighter MS star in Gaia/TGAS with a good parallax (σ_ϖ/ϖ ≤ 0.05). We model the WD's multi-band photometry with the BASE-9 code using this precise distance (assumed to be common for the pair) and infer ages for each binary system. The resulting age estimates are precise to ≤10% (≤20%) for 42 (67) MS–WD systems. Our analysis more than doubles the number of MS–WD systems with precise distances known to date, and it boosts the number of such systems with precise age determination by an order of magnitude. With the advent of the Gaia DR2 data, this approach will be applicable to a far larger sample, providing ages for many MS stars (that can yield detailed abundances for over 20 elements), especially in the age range of 2–8 $\mathrm{Gyr}$, where there are only few known star clusters.

We present an analysis of flares in mid-to-late M dwarfs in the MEarth photometric survey. We search 3,985,155 observations across 2226 stars, and detect 54 large (Δm ≥ 0.018) flares in total, distributed across 34 stars. We combine our flare measurements with recent activity and rotation period results from MEarth to show that there is an increase in flares per observation from low Rossby number (R_o < 0.04, rapid rotators) to intermediate Rossby number (0.04 < R_o < 0.44, intermediate rotators) at the 99.85% confidence level. We additionally find an increased flare rate from the high Rossby number population (R_o > 0.44, slow rotators) to the intermediate population at the 99.97% level. We posit that the rise in flare rate for intermediate R_o could be due to changing magnetic field geometry on the surface of the star.

We present an analysis of the microlensing event OGLE-2015-BLG-0232. This event is challenging to characterize for two reasons. First, the light curve is not well sampled during the caustic crossing due to the proximity of the full Moon impacting the photometry quality. Moreover, the source brightness is difficult to estimate because this event is blended with a nearby K dwarf star. We found that the light-curve deviations are likely due to a close brown dwarf companion (i.e., s = 0.55 and q = 0.06), but the exact nature of the lens is still unknown. We finally discuss the potential of follow-up observations to estimate the lens mass and distance in the future.

Supernova (SN) 2018oh (ASASSN-18bt) is the first spectroscopically confirmed Type Ia supernova (SN Ia) observed in the Kepler field. The Kepler data revealed an excess emission in its early light curve, allowing us to place interesting constraints on its progenitor system. Here we present extensive optical, ultraviolet, and near-infrared photometry, as well as dense sampling of optical spectra, for this object. SN 2018oh is relatively normal in its photometric evolution, with a rise time of 18.3 ± 0.3 days and Δm₁₅(B) = 0.96 ± 0.03 mag, but it seems to have bluer B − V colors. We construct the "UVOIR" bolometric light curve having a peak luminosity of 1.49 × 10⁴³ erg s⁻¹, from which we derive a nickel mass as 0.55 ± 0.04 M_⊙ by fitting radiation diffusion models powered by centrally located ⁵⁶Ni. Note that the moment when nickel-powered luminosity starts to emerge is +3.85 days after the first light in the Kepler data, suggesting other origins of the early-time emission, e.g., mixing of ⁵⁶Ni to outer layers of the ejecta or interaction between the ejecta and nearby circumstellar material or a nondegenerate companion star. The spectral evolution of SN 2018oh is similar to that of a normal SN Ia but is characterized by prominent and persistent carbon absorption features. The C ii features can be detected from the early phases to about 3 weeks after the maximum light, representing the latest detection of carbon ever recorded in an SN Ia. This indicates that a considerable amount of unburned carbon exists in the ejecta of SN 2018oh and may mix into deeper layers.

On 2018 February 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift of z = 0.01098 and a peak apparent magnitude of B_max = 14.31, ASASSN-18bt is the nearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here we present the discovery of ASASSN-18bt, the K2 light curve, and prediscovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System. The K2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an SN Ia light curve, and it unambiguously shows a ∼4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18bt joins a growing list of SNe Ia whose early light curves are not well described by a single power law. We show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. However, we find that current models of the interaction with a nondegenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing published models with shallow ⁵⁶Ni are able to span the observed behavior and, with tuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, more theoretical work is needed to satisfactorily model this and other early-time SNe Ia light curves. Finally, we use Swift X-ray nondetections to constrain the presence of circumstellar material (CSM) at much larger distances and lower densities than possible with the optical light curve. For a constant-density CSM, these nondetections constrain ρ < 4.5 × 10⁵ cm⁻³ at a radius of 4 × 10¹⁵ cm from the progenitor star. Assuming a wind-like environment, we place mass loss limits of $\dot{M}\lt 8\times \ {10}^{-6}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$ for v_w = 100 km s⁻¹, ruling out some symbiotic progenitor systems. This work highlights the power of well-sampled early-time data and the need for immediate multiband, high-cadence follow-up for progress in understanding SNe Ia.

Recent works have studied the late-time light curves of Type Ia supernovae (SNe Ia) when these were older than 500 days past B-band maximum light. Of these, SN 2014J, which exploded in the nearby galaxy M82, was studied with the Advanced Camera for Surveys onboard the Hubble Space Telescope (HST) by Yang et al. Here, I report complementary photometry of SN 2014J taken with the HST Wide Field Camera 3 when it was ∼360–1300 days old. My F555W measurements are consistent with the F606W measurements of Yang et al., but the F438W measurements are ∼1 mag fainter than their F475W measurements. I corroborate their finding that, even though SN 2014J has spatially resolved light echoes, its photometry is not contaminated by an unresolved echo. Finally, I compare the F438W and F555W light curves of SN 2014J to those of the other late-time SNe Ia observed to date and show that more intrinsically luminous SNe have slower light curve decline rates. This is consistent with the correlation claimed by Graur et al., which was based on a comparison of pseudo-bolometric light curves. By conducting a direct comparison of the late-time light curves in the same filters, I remove any systematic uncertainties introduced by the assumptions that go into constructing the pseudo-bolometric light curves, thus strengthening the Graur et al. claim.

Large-scale propagating fronts are frequently observed during solar eruptions, yet whether or not they are waves is an open question, partly because the propagation is modulated by coronal structures, whose magnetic fields we still cannot measure. However, when a front impacts coronal structures, an opportunity arises for us to look into the magnetic properties of both interacting parties in the low-β corona. Here we studied large-scale EUV fronts accompanying three coronal mass ejections (CMEs), each originating from a kinking rope-like structure in the NOAA active region (AR) 12371. These eruptions were homologous and the surrounding coronal structures remained stationary. Hence we treated the events as one observed from three different viewing angles, and found that the primary front directly associated with the CME consistently transmits through (1) a polar coronal hole, (2) the ends of a crescent-shaped equatorial coronal hole, leaving a stationary front outlining its AR-facing boundary, and (3) two quiescent filaments, producing slow and diffuse secondary fronts. The primary front also propagates along an arcade of coronal loops and slows down due to foreshortening at the far side, where local plasma heating is indicated by an enhancement in 211 Å (Fe xiv) but a dimming in 193 Å (Fe xii) and 171 Å (Fe ix). The strength of coronal magnetic field is therefore estimated to be ∼2 G in the polar coronal hole and ∼4 G in the coronal arcade neighboring the AR. These observations substantiate the wave nature of the primary front and shed new light on slow fronts.

We present an improved semi-analytic model for calculation of the broad optical emission-line signatures from sub-parsec supermassive black hole binaries (SBHBs) in circumbinary disks. The second-generation model improves on the treatment of radiative transfer by taking into account the effect of the radiation-driven accretion disk wind on the properties of the emission-line profiles. Analysis of 42.5 million modeled emission-line profiles shows that correlations between the profile properties and SBHB parameters identified in the first-generation model are preserved, indicating that their diagnostic power is not diminished. The profile shapes are a more sensitive measure of the binary orbital separation and the degree of alignment of the black hole mini-disks and are less sensitive to the SBHB mass ratio and orbital eccentricity. We also find that modeled profile shapes are more compatible with the observed sample of SBHB candidates than with our control sample of regular active galactic nuclei. Furthermore, if the observed sample of SBHBs is made up of genuine binaries, it must include compact systems with comparable masses and misaligned mini-disks. We note that the model described in this paper can be used to interpret the observed emission-line profiles once a sample of confirmed SBHBs is available but cannot be used to prove that the observed SBHB candidates are true binaries.

A relativistic electron–positron pair beam can be produced in the interaction between TeV photons from a blazar and extragalactic background light. The relativistic e^± pairs lose energy through inverse-Compton scattering (ICS) photons of the cosmic microwave background or plasma instabilities. The dominant energy-loss process is under debate. Based on the assumption that the dominant energy-loss process is ICS, the resulting cascade GeV radiation is usually used to constrain the intergalactic magnetic field (IGMF). Here, we include the energy-loss due to plasma oblique instability in the calculation of cascade gamma-ray flux, and investigate the impact of the plasma instability on the constraint of IGMF. Up-to-date GeV data and archival TeV data of the blazar 1ES 0229+200 are used. The results indicate that even if the oblique instability cooling is dominating ICS cooling, the cascade flux could still be used to constrain the IGMF. It is found that with the ratio between the cooling rates of the oblique instability and the ICS varying from 0.1,1 to 10, the lower limit of the IGMF of the cascade flux and the gamma-ray data changes from 8 × 10⁻¹⁸ G, 5 × 10⁻¹⁸ G to 10⁻¹⁸ G. If the ratio between the two cooling rates is 30, the estimate of IGMF based on the cascade flux is invalid.

In the Simulations and Constructions of the Reionization of Cosmic Hydrogen project, we present new radiation-hydrodynamic simulations with updated high-redshift galaxy populations and varying radiation escape fractions. The simulations are designed to have fixed Thomson optical depth τ ≈ 0.06, consistent with recent Planck observations, and similar midpoints of reionization 7.5 ≲ z ≲ 8.0, but with different ionization histories. The galaxy luminosity functions and ionizing photon production rates in our model are in good agreement with recent HST observations. Adopting a power-law form for the radiation escape fraction ${f}_{\mathrm{esc}}(z)={f}_{8}{[(1+z)/9]}^{{a}_{8}}$, we simulate the cases for a₈ = 0, 1, and 2 and find a₈ ≲ 2 in order to end reionization in the range of 5.5 ≲ z ≲ 6.5 to be consistent with Lyα forest observations. At fixed τ and as the power-law slope a₈ increases, the reionization process starts earlier but ends later with a longer duration Δz and the decreased redshift asymmetry Az. We find a range of durations 3.9 ≲ Δz ≲ 4.6 that is currently in tension with the upper limit Δz < 2.8 inferred from a recent joint analysis of Planck and South Pole Telescope observations.

We study how star formation (SF) is quenched in low-redshift disk galaxies with integral-field spectroscopy. We select 131 face-on spiral galaxies with stellar mass greater than 3 × 10¹⁰M_⊙, and with spatially resolved spectrum from MaNGA DR13. We subdivide the sample into four groups based on the offset of their global specific star formation rate (SFR) from the star-forming main sequence and stack the radial profiles of stellar mass and SFR. By comparing the stacked profiles of quiescent and star-forming disk galaxies, we find that the decrease of the global SFR is caused by the suppression of SF at all radii, but with a more significant drop from the center to the outer regions following an inside-out pattern. As the global specific SFR decreases, the central stellar mass, the fraction of disk galaxies hosting stellar bars, and active galactic nuclei (AGNs; including both LINERs and Seyferts) all increase, indicating dynamical processes and AGN feedback are possible contributors to the inside-out quenching of SF in the local universe. However, if we include only Seyferts, or AGNs with EW(Hα) > 3 Å, the increasing trend of AGN fraction with decreasing global sSFR disappears. Therefore, if AGN feedback is contributing to quenching, we suspect that it operates in the low-luminosity AGN mode, as indicated by the increasing large bulge mass of the more passive disk galaxies.

A spectrum of the four-decade solar irradiance record has a prominent cluster of power for periodicities near 1 yr. Correlating irradiance with a bandpass filter showed that periodicity values were not constant, but varied sinusoidally with each cycle lasting 14 ± 1 yr. The large modulation amplitude makes solar frequencies ≥1 yr⁻¹ hard to detect at the solar surface. After removing the modulation, a Lomb–Scargle spectrum exposed two true periodicities: 1.006 and 0.920 yr. They are interpreted as the synodic rotation periods of r modes of lowest angular degree (ℓ = 1). The first propagates in the stable interior and the second in the convective envelope perturbed by its several flow fields. The rotational beat period of the two modes is about 10.9 yr. This is close to the average length of a solar cycle and possibly controls this average. The 1.006 yr periodicity dominates most of the filtered irradiance record but an abrupt change to about 0.8 yr occurs in mid-2010. Also found was evidence for higher-degree r modes (ℓ = 2 to 8) and a curious sawtooth modulation with a recurrence period of 2.6 yr.

Uncertainties in atomic models will introduce noticeable additional systematics in calculating the flux of weak dielectronic recombination (DR) satellite lines, affecting the detection and flux measurements of other weak spectral lines. One important example is the Ar xvii Heβ DR, which is expected to be present in emission from the hot intracluster medium of galaxy clusters and could impact measurements of the flux of the 3.5 keV line that has been suggested as a secondary emission from a dark matter interaction. We perform a set of experiments using the Lawrence Livermore National Laboratory's electron beam ion trap (EBIT-I) and the X-ray Spectrometer quantum calorimeter (XRS/EBIT) to test the Ar xvii Heβ DR origin of the 3.5 keV line. We measured the X-ray emission following resonant DR onto helium-like and lithium-like Argon using EBIT-I's Maxwellian simulator mode at a simulated electron temperature of T_e = 1.74 keV. The measured flux of the Ar xvii Heβ DR lined is too weak to account for the flux in the 3.5 keV line, assuming reasonable plasma parameters. We, therefore, rule out Ar xvii Heβ DR as a significant contributor to the 3.5 keV line. A comprehensive comparison between the atomic theory and the EBIT experiment results is also provided.

It has been shown that the rate of angular momentum loss (AML) in cataclysmic variables (CVs) below the period gap is about 2.47 times that caused by gravitational radiation (GR), suggesting an extra AML mechanism aside from GR. Several potential mechanisms have been proposed but none of them has been verified. In this work, we examine whether AML caused by friction between the expanding nova envelope and the donor star can account for the required AML rate. By adopting various expanding velocities of the envelope, we have calculated the evolution of CVs with typical initial parameters. Our results show that this friction interaction unlikely solves the extra AML problem unless the expanding velocities are extremely low. Thus, there should be a more efficient AML mechanism that plays a role in CV evolution.

The rapid neutron-capture ("r-") process is responsible for synthesizing many of the heavy elements observed in both the solar system and Galactic metal-poor halo stars. Simulations of r-process nucleosynthesis can reproduce abundances derived from observations with varying success, but so far they fail to account for the observed overenhancement of actinides, present in about 30% of r-process-enhanced stars. In this work, we investigate actinide production in the dynamical ejecta of a neutron star merger (NSM) and explore whether varying levels of neutron-richness can reproduce the actinide boost. We also investigate the sensitivity of actinide production on nuclear physics properties: fission distribution, β-decay, and mass model. For most cases, the actinides are overproduced in our models if the initial conditions are sufficiently neutron-rich for fission cycling. We find that actinide production can be so robust in the dynamical ejecta that an additional lanthanide-rich, actinide-poor component is necessary in order to match observations of actinide-boost stars. We present a simple actinide-dilution model that folds in estimated contributions from two nucleosynthetic sites within a merger event. Our study suggests that while the dynamical ejecta of an NSM are likely production sites for the formation of actinides, a significant contribution from another site or sites (e.g., the NSM accretion disk wind) is required to explain abundances of r-process-enhanced, metal-poor stars.

A comprehensive, hyperfine-resolved rotation–vibration line list for the ammonia molecule (¹⁴NH₃) is presented. The line list, which considers hyperfine nuclear quadrupole coupling effects, has been computed using robust, first principles methodologies based on a highly accurate empirically refined potential energy surface. Transitions between levels with energies below 8000 cm⁻¹ and total angular momentum F ≤ 14 are considered. The line list shows excellent agreement with a range of experimental data and will significantly assist future high-resolution measurements of NH₃, both astronomically and in the laboratory.

Telescope slew and settle time markedly reduce the efficiency of wide-field multi-epoch surveys for sensitive interferometers with small fields of view. The overheads can be mitigated through the use of on-the-fly mosaicking (OTFM), where the the antennas are driven at a non-sidereal rate and visibilities are recorded continuously. Here we introduce the OTFM technique for the Very Large Array (VLA), and describe its implementation for the Caltech-NRAO Stripe 82 Survey (CNSS), a dedicated five-epoch survey for slow transients at the S band (2–4 GHz). We also describe the OTFSim tool for planning dynamically scheduled OTFM observations on the VLA, the latest imaging capabilities for OTFM in CASA, and present a comparison of OTFM observations with pointed observations. Using the subset of our observations from the CNSS pilot and final surveys, we demonstrate that the wide-band and wide-field OTFM observations with the VLA can be imaged accurately, and that this technique offers a more efficient alternative to standard mosaicking for multi-epoch shallow surveys such as the CNSS and the VLA Sky Survey. We envisage that the new OTFM mode will facilitate new synoptic surveys and high-frequency mapping experiments on the VLA.

We present an analysis of a sample of 69 local obscured Swift/Burst Alert Telescope active galactic nuclei (AGNs) with X-ray spectra from NuSTAR and infrared (IR) spectral energy distributions from Herschel and WISE. We combine this X-ray and IR phenomenological modeling and find a significant correlation between reflected hard X-ray emission and IR AGN emission, with suggestive indications that this correlation may be stronger than the one between intrinsic hard X-ray and IR emissions. This relation between the IR and reflected X-ray emission suggests that both are the result of the processing of intrinsic emission from the corona and accretion disk by the same structure. We explore the resulting implications on the underlying distribution of covering fraction for all AGNs, by generating mock observables for the reflection parameter and IR luminosity ratio using empirical relations found for the covering fraction with each quantity. We find that the observed distributions of the reflection parameter and IR-to-X-ray ratio are reproduced with broad distributions centered around covering fractions of at least ∼40%–50%, whereas narrower distributions match our observations only when centered around covering fractions of ∼70%–80%. Our results are consistent with both independent estimates of the covering fractions of individual objects and the typical covering fraction obtained on the basis of obscured fractions for samples of AGNs. These results suggest that the level of reprocessing in AGNs, including X-ray reflection, is related in a relatively straightforward way to the geometry of the obscuring material.

The reality of a field Argus Association has been doubted in some papers in the literature. We apply Gaia DR2 data to stars previously suggested to be Argus members and conclude that a true association exists with age 40–50 Myr and containing many stars within 100 pc of Earth; β Leo and 49 Cet are two especially interesting members. Based on youth and proximity to Earth, Argus is one of the better nearby moving groups to target in direct imaging programs for dusty debris disks and young planets.

I show that, by assuming a standard Blandford–Königl jet, it is possible to determine the bulk Lorentz factor and angle to the line of sight of self-similar parsec-scale blazar jets by using five measured quantities: redshift, core radio flux, extended radio flux, the magnitude of the core shift between two frequencies, and apparent jet opening angle. From the bulk Lorentz factor and angle computed with this method, one can compute other jet properties such as the Doppler factor, magnetic field strength, and intrinsic jet opening angle. I use data taken from the literature and marginalize over nuisance parameters associated with the electron distribution and equipartition to compute these quantities, although the errors are large. Results are generally consistent with constraints from other methods. Primary sources of uncertainty are the errors on the core shift measurements and the uncertainty in the electron spectral index.

Birefringence in ionized, magnetized media is usually measured as Faraday rotation of linearly polarized radiation. However, pulses propagating through regions with very large Faraday rotation measures (RMs) can split into circularly polarized components with measurable differences in arrival times ∝ν⁻³ RM, where ν is the radio frequency. Differential refraction from gradients in DM (dispersion measure) and RM can contribute a splitting time $\propto | {{\boldsymbol{\nabla }}}_{\perp }\mathrm{DM}| | {{\boldsymbol{\nabla }}}_{\perp }\mathrm{RM}| \,{\nu }^{-5}$. Regardless of whether the emitted pulse is unpolarized or linearly polarized, net circular polarization will be measured when splitting is a significant fraction of the pulse width. However, the initial polarization may be inferable from the noise statistics of the bursts. Extreme multipath scattering that broadens pulses can mask splitting effects. We discuss particular cases such as the Galactic center magnetar J1745−2900, and the repeating fast radio burst source FRB 121102. Both lines of sight have $| \mathrm{RM}| \sim {10}^{5}\,\mathrm{rad}\,{{\rm{m}}}^{-2}$, which yields millisecond splittings at frequencies well below ∼1 GHz. We also consider the splitting of nanosecond shot pulses in giant pulses from the Crab pulsar and the minimal effects of birefringence on precision pulsar timing. Finally, we explore the utility of two-dimensional coherent dedispersion with DM and RM as parameters.

Source imaging of solar radio bursts can be used to track energetic electrons and associated magnetic structures. Here we present a combined analysis of data at different wavelengths for an eruption associated with a moving type IV (t-IVm) radio burst. In the inner corona, the sources are correlated with a hot and twisted eruptive EUV structure, while in the outer corona, the sources are associated with the top front of the bright core of a white-light coronal mass ejection (CME). This reveals the potential of using t-IVm imaging data to continuously track the CME by lighting up the specific component containing radio-emitting electrons. It is found that the t-IVm burst presents a clear spatial dispersion with observing frequencies. The burst manifests broken power law–like spectra in brightness temperature, which is as high as 10⁷–10⁹ K, while the polarization level is generally weak. In addition, the t-IVm burst starts during the declining phase of the flare with a duration as long as 2.5 hr. From the differential emission measure analysis of AIA data, the density of the T-IVm source is found to be at the level of 10⁸ cm⁻³ at the start of the burst, and the temperature may reach up to several MK. These observations do not favor gyrosynchrotron to be the radiation mechanism but are in line with a coherent plasma emission excited by energetic electrons trapped within the source. Further studies are demanded to elucidate the emission mechanism and explore the full diagnostic potential of t-IVm bursts.

We quantify the luminosity contribution of active galactic nuclei (AGNs) to the 12 μm, mid-infrared (MIR; 5–38 μm), and total IR (5–1000 μm) emission in the local AGNs detected in the all-sky 70 month Swift/Burst Alert Telescope (BAT) ultrahard X-ray survey. We decompose the IR spectral energy distributions (SEDs) of 587 objects into the AGN and starburst components using templates for an AGN torus and a star-forming galaxy. This enables us to recover the emission from the AGN torus including the low-luminosity end, down to $\mathrm{log}({L}_{14-150}/\mathrm{erg}\,{{\rm{s}}}^{-1})\simeq 41$, which typically has significant host galaxy contamination. The sample demonstrates that the luminosity contribution of the AGN to the 12 μm, the MIR, and the total IR bands is an increasing function of the 14–150 keV luminosity. We also find that for the most extreme cases, the IR pure-AGN emission from the torus can extend up to 90 μm. The total IR AGN luminosity obtained through the IR SED decomposition enables us to estimate the fraction of the sky obscured by dust, i.e., the dust covering factor. We demonstrate that the median dust covering factor is always smaller than the median X-ray obscuration fraction above an AGN bolometric luminosity of $\mathrm{log}({L}_{\mathrm{bol}}^{(\mathrm{AGN})}/\mathrm{erg}\,{{\rm{s}}}^{-1})\simeq 42.5$. Considering that the X-ray obscuration fraction is equivalent to the covering factor coming from both the dust and gas, this indicates that an additional neutral gas component, along with the dusty torus, is responsible for the absorption of X-ray emission.

The Gaia mission has opened a new window into the internal kinematics of young star clusters at the sub-km s⁻¹ level, with implications for our understanding of how star clusters form and evolve. We use a sample of 28 clusters and associations with ages from ∼1–5 Myr, where lists of members are available from previous X-ray, optical, and infrared studies. Proper motions from Gaia DR2 reveal that at least 75% of these systems are expanding; however, rotation is only detected in one system. Typical expansion velocities are on the order of ∼0.5 km s⁻¹, and in several systems, there is a positive radial gradient in expansion velocity. Systems that are still embedded in molecular clouds are less likely to be expanding than those that are partially or fully revealed. One-dimensional velocity dispersions, which range from ${\sigma }_{1{\rm{D}}}=1$ to 3 km s⁻¹, imply that most of the stellar systems in our sample are supervirial and that some are unbound. In star-forming regions that contain multiple clusters or subclusters, we find no evidence that these groups are coalescing, implying that hierarchical cluster assembly, if it occurs, must happen rapidly during the embedded stage.

A primary aim of the ${Nuclear}\,{Spectroscopic}\,{Telescope}\,{Array}$ (NuSTAR) mission is to find and characterize heavily obscured Active Galactic Nuclei (AGNs). Based on mid-infrared photometry from the Wide-Field Infrared Survey Explorer (WISE) and optical photometry from the Sloan Digital Sky Surveys, we have selected a large population of luminous obscured AGNs (i.e., "obscured quasars"). Here we report NuSTAR observations of four WISE-selected heavily obscured quasars for which we have optical spectroscopy from the Southern African Large Telescope and W. M. Keck Observatory. Optical diagnostics confirm that all four targets are AGNs. With NuSTAR hard X-ray observations, three of the four objects are undetected, while the fourth has a marginal detection. We confirm that these objects have observed hard X-ray (10–40 keV) luminosities at or below ∼10⁴³ erg s⁻¹. We compare X-ray and IR luminosities to obtain estimates of the hydrogen column densities (N_H) based on the suppression of the hard X-ray emission. We estimate N_H of these quasars to be at or larger than 10²⁵ cm⁻², confirming that WISE and optical selection can identify very heavily obscured quasars that may be missed in X-ray surveys, and they do not contribute significantly to the cosmic X-ray background. From the optical Balmer decrements, we found that our three extreme obscured targets lie in highly reddened host environments. This galactic extinction cannot adequately explain the more obscured AGNs, but it may imply a different scale of obscuration in the galaxy.

The Magnetospheric Multiscale spacecraft encountered an electron diffusion region (EDR) in a symmetric reconnection in the Earth's magnetotail. The EDR contained a guide field of about 2 nT, which was 13% of the magnetic field in the inflow region, and its thickness was about 2 local electron inertial lengths. Intense energy dissipation, a super-Alfvénic electron jet, electron nongyrotropy, and crescent-shaped electron velocity distributions were observed in association with this EDR. These features are similar to those of the EDRs in asymmetric reconnection at the dayside magnetopause. Electrons gained about 50% of their energy from the immediate upstream to the EDR. Crescent electron distributions were seen at the boundary of the EDR, while highly curved magnetic field lines inside the EDR may have gyrotropized the electrons. The EDR was characterized by a parallel current that was carried by antiparallel drifting electrons that were probably accelerated by a parallel electric field along the guide field. These results reveal the essential electron physics of the EDR and provide a significant example of an EDR in symmetric reconnection with a weak guide field.

We report on the investigation of a very high-energy, Galactic γ-ray source recently discovered at >50 GeV using the Large Area Telescope on board Fermi. This object, 2FHL J0826.1−4500, displays one of the hardest >50 GeV spectra (photon index Γ_γ ∼ 1.6) in the 2FHL catalog, and a follow-up observation with XMM-Newton has uncovered diffuse, soft thermal emission at the position of the γ-ray source. A detailed analysis of the available multi-wavelength data shows that this source is located on the western edge of the Vela supernova remnant (SNR): the observations and the modeling of the spectral energy distribution support a scenario where this γ-ray source is the byproduct of the interaction between the SNR shock and a neutral hydrogen cloud. If confirmed, this shock–cloud interaction would make 2FHL J0826.1−4500 a promising candidate for efficient particle acceleration.

The bRing robotic observatory network was built to search for circumplanetary material within the transiting Hill sphere of the exoplanet β Pic b across its bright host star β Pic. During the bRing survey of β Pic, it simultaneously monitored the brightnesses of thousands of bright stars in the southern sky (V ≃ 4–8, δ ≲ −30°). In this work, we announce the discovery of δ Scuti pulsations in the A-type star HD 156623 using bRing data. HD 156623 is notable as it is a well-studied young star with a dusty and gas-rich debris disk, previously detected using ALMA. We present the observational results on the pulsation periods and amplitudes for HD 156623, discuss its evolutionary status, and provide further constraints on its nature and age. We find strong evidence of frequency regularity and grouping. We do not find evidence of frequency, amplitude, or phase modulation for any of the frequencies over the course of the observations. We show that HD 156623 is consistent with other hot and high-frequency pre-main sequence and early zero-age main sequence (ZAMS) δ Scutis as predicted by theoretical models and corresponding evolutionary tracks, although we observe that HD 156623 lies hotter than the theoretical blue edge of the classical instability strip. This, coupled with our characterization and Sco–Cen membership analyses, suggests that the star is most likely an outlying ZAMS member of the ∼16 Myr Upper Centaurus-Lupus subgroup of the Sco–Cen association.

We explore the gas morphology and excitation mechanisms of the ionization cones of the type II Seyfert galaxy NGC 5728. Kinematics derived from near-infrared and optical data from the SINFONI and MUSE Integral Field Units on the VLT reveal active galactic nucleus (AGN)-driven outflows powered by a supermassive black hole (SMBH) of mass 3.4 × 10⁷M_⊙, bolometric luminosity of 1.46 × 10⁴⁴ erg s⁻¹, Eddington ratio of 3.3 × 10⁻², and an accretion rate of 2.7 × 10⁻²M_⊙ yr⁻¹. The symmetric bicone outflows show rapid acceleration to ±250 km s⁻¹ at ∼250 pc, decelerating to ∼130 km s⁻¹ at 500 pc from the AGN, with an estimated mass outflow rate of 38 M_⊙ yr⁻¹; the mass ratio of outflows to accretion is 1415. The kinetic power is ∼1.5 × 10⁴² erg s⁻¹, 1% of the bolometric luminosity. Over the AGN active lifetime of ∼10⁷ yr, 1.6 × 10⁸ M_⊙ of gas can become gravitationally unbound from the galaxy, a large proportion of the gas mass available for star formation in the nuclear region. The bicone internal opening angle (502) and the inclination to the line of sight (476) were determined from [Fe ii] line profiles; the outflow axis is nearly parallel to the plane of the galaxy. This geometry supports the unified model of AGNs, as these angles preclude seeing the accretion disk, which is obscured by the dusty torus.

We perform multiwavelength light-curve modeling of the recently discovered low-luminosity gamma-ray burst (GRB) 171205A. The emission model is based on the relativistic ejecta–circumstellar medium (CSM) interaction scenario. The collision of freely expanding spherical ejecta traveling at mildly relativistic velocities with the CSM produces the reverse and forward shocks, which dissipate a part of the kinetic energy of the mildly relativistic ejecta. We show that the early gamma-ray emission followed by an X-ray tail can be well explained by the radiation diffusing out from the shocked gas. Mildly relativistic ejecta with a kinetic energy of 5 × 10⁵⁰ erg and a wind-like CSM with a mass-loss rate of a few 10⁻⁴M_⊙ yr⁻¹ for a wind velocity of 10³ km s⁻¹, which extends up to ∼3 × 10¹³ cm, are required to account for the gamma-ray luminosity and duration of GRB 171205A. We also calculate the photospheric and nonthermal emission after the optically thick stage, which can fit the late-time X-ray, optical, and radio light curves. Our results suggest that the relativistic ejecta–CSM interaction can be a potential power source for low-luminosity GRBs and other X-ray-bright transients.

We present the results of CO interferometric observations of the southern elliptical galaxy NGC 3557 with ALMA. We have detected both the CO(1–0) emission line and a relatively strong continuum at 3 mm. The continuum shows a flat-spectrum central unresolved source (at our angular resolution of 07) and two jets, associated with the larger-scale emission observed at lower frequencies. The molecular gas in NGC 3557 appears to be concentrated within 250 pc of the center, and shows evidence of organized rotation along the same axis as the stellar component and the symmetry axis of the nuclear dust absorption reported in the literature. We obtained ${M}_{{{\rm{H}}}_{2}}=(9.0\pm 2.0)\times {10}^{7}\,{M}_{\odot }$ of molecular gas, which has an average CO(2–1) to CO(1–0) line ratio of 0.7, which is relatively high when compared with the values reported in the literature for bona fide ellipticals observed with single-dish telescopes. NGC 3557 shows further a high excitation peak (i.e., CO(2–1)/CO(1–0) ≈ 1.1 ± 0.3) offset 07 from the center, which appears to be associated with a region of higher velocity dispersion that does not share the overall rotation pattern of the molecular gas, but aligned with the radio jet. The molecular gas disk in this object appears to be stable to local gravitational instabilities.

We study the polarization properties of the velocity fluctuations in solar wind turbulence using high-resolution data from the Spektr-R spacecraft. The ratio of perpendicular to parallel velocity fluctuations in the inertial range is smaller than the equivalent ratio for magnetic fluctuations, but gradually increases throughout this range. In the kinetic range, there is a large decrease in the ratio, similar to the magnetic fluctuations. We compare the measurements to numerical solutions for a combination of kinetic Alfvén waves and slow waves, finding that both the slow increase and sharp decrease in the ratio are consistent with a majority population of Alfvén waves and minority population of slow waves in critical balance. Furthermore, the beta-dependence of this scale-dependent ratio can be successfully captured in the model when incorporating a beta-dependent Alfvén to slow wave ratio similar to that observed in the solar wind.

Asteroseismic large frequency separations possess great diagnostic value. However, their expressions as scaling relations are predicated on homology arguments that may not hold in general, resulting in mass- and temperature-dependent deviations. The first-order asymptotic expressions, which should in principle account for this structural evolution, also deviate more from fitted frequency-separation estimates than the simple scaling relations and exhibit qualitatively different behavior. We present a modified asymptotic estimator and show that these discrepancies can be accounted for by the evolution of the acoustic turning points of the asteroseismic mode cavity, which is typically neglected in first-order asymptotic analysis. This permits us to use a single expression to accurately estimate the large frequency separations of main-sequence, ascending red giant branch, and red clump stellar models, except at transition points between two asymptotic regimes during the subgiant phase of evolution, where the WKB approach fails. The existence of such transition points provides theoretical justification for separately calibrated scaling relations for stars in different evolutionary stages.

We present spatial- and velocity-resolved spectroscopy of NGC 7009 acquired with the UVES spectrograph at the VLT UT2/Kueyen. We use these data to determine the structure of the electron temperature and electron density based upon O ii permitted (recombination) lines. We find a strong gradient in the O ii-based electron temperature. It agrees with the electron temperature determined by forbidden (collisionally excited) lines in part of the nebular volume, but also differs by more than 6000 K in other parts of the nebular volume. This result supports the hypothesis that NGC 7009 contains two plasma components, one of which emits both forbidden and permitted lines and the other that emits only permitted lines. For the component that emits only permitted lines, we find a lower limit to the electron density of 10⁴ cm⁻³ from the O ii permitted lines, which is higher than derived from forbidden lines. We are unable to determine whether the two plasma components are in pressure equilibrium from our data, but there exist temperature and density combinations that allow this equilibrium for temperatures between 600 and 6000 K. For most of the temperature and density conditions allowed for the component that emits only permitted lines, its mass of O²⁺ is less than that of the plasma component that emits forbidden lines.

The solution of the Poisson equation is a ubiquitous problem in computational astrophysics. Most notably, the treatment of self-gravitating flows involves the Poisson equation for the gravitational field. In hydrodynamics codes using spherical polar grids, one often resorts to a truncated spherical harmonics expansion for an approximate solution. Here we present a non-iterative method that is similar in spirit, but uses the full set of eigenfunctions of the discretized Laplacian to obtain an exact solution of the discretized Poisson equation. This allows the solver to handle density distributions for which the truncated multipole expansion fails, such as off-center point masses. In 3D, the operation count of the new method is competitive with a naive implementation of the truncated spherical harmonics expansion with N_ℓ ≈ 15 multipoles. We also discuss the parallel implementation of the algorithm. The serial code and a template for the parallel solver are made publicly available.

As a young massive cluster in the central molecular zone, the Arches cluster is a valuable probe of the stellar initial mass function (IMF) in the extreme Galactic center environment. We use multi-epoch Hubble Space Telescope observations to obtain high-precision proper-motion and photometric measurements of the cluster, calculating cluster membership probabilities for stars down to ∼1.8 M_⊙ between cluster radii of 0.25 and 3.0 pc. We achieve a cluster sample with just ∼6% field contamination, a significant improvement over photometrically selected samples that are severely compromised by the differential extinction across the field. Combining this sample with K-band spectroscopy of five cluster members, we forward model the Arches cluster to simultaneously constrain its IMF and other properties (such as age and total mass) while accounting for observational uncertainties, completeness, mass segregation, and stellar multiplicity. We find that the Arches IMF is best described by a one-segment power law that is significantly top-heavy: α = 1.80 ± 0.05 (stat) ± 0.06 (sys), where dN/dm ∝ m^−α, though we cannot discount a two-segment power-law model with a high-mass slope only slightly shallower than local star-forming regions $(\alpha ={2.04}_{-0.19}^{+0.14}\pm 0.04)$ but with a break at ${5.8}_{-1.2}^{+3.2}\pm 0.02\,{M}_{\odot }$. In either case, the Arches IMF is significantly different than the standard IMF. Comparing the Arches to other young massive clusters in the Milky Way, we find tentative evidence for a systematically top-heavy IMF at the Galactic center.

We have carried out deep and wide field imaging observations with narrow bands, targeting 11 quasar fields to systematically study the possible photoevaporation effect of quasar radiation on surrounding low mass galaxies at z ∼ 2–3. We focused on Lyα emitters (LAEs) at the same redshifts as quasars that lie within the quasar proximity zones, where the UV radiation from the quasars is higher than the average background at that epoch. We found that LAEs with high rest-frame equivalent width of Lyα emission (EW₀) of ≳150 Å with low stellar mass (≲10⁸M_⊙) are predominantly scarce in the quasar proximity zones, suggesting that quasar photoevaporation effects may be taking place. The halo mass of LAEs with EW₀ > 150 Å is estimated to be ${3.6}_{-2.3}^{+12.7}\times {10}^{9}{M}_{\odot }$ either from spectral energy distribution fitting or the main sequence. Based on a hydrodynamical simulation, the predicted delay in star formation under a local UV background intensity with $J({\nu }_{L})\gtrsim {10}^{-21}$ erg s⁻¹ cm⁻² Hz⁻¹ sr⁻¹ for galaxies having less than this halo mass is about >20 Myr, which is longer than the expected age of LAEs with EW₀ > 150 Å. On the other hand, photoevaporation seems to be less effective around very luminous quasars, which is consistent with the idea that these are still in an early stage of activity.

In this paper we compare the molecular gas depletion times and midplane hydrostatic pressure in turbulent, star-forming disk galaxies to internal properties of these galaxies. For this analysis we use 17 galaxies from the DYNAMO sample of nearby (z ∼ 0.1) turbulent disks. We find a strong correlation, such that galaxies with lower molecular gas depletion time (t_dep) have higher gas velocity dispersion (σ). Within the scatter of our data, our observations are consistent with the prediction that ${t}_{\mathrm{dep}}\propto {\sigma }^{-1}$ made in theories of feedback-regulated star formation. We also show a strong, single power-law correlation between midplane pressure (P) and star formation rate surface density (Σ_SFR), which extends for 6 orders of magnitude in pressure. Disk galaxies with lower pressure are found to be roughly in agreement with theoretical predictions. However, in galaxies with high pressure we find P/Σ_SFR values that are significantly larger than theoretical predictions. Our observations could be explained with any of the following: (1) the correlation of Σ_SFR−P is significantly sublinear; (2) the momentum injected from star formation feedback (p_*/m_*) is not a single, universal value; or (3) alternate sources of pressure support are important in gas-rich disk galaxies. Finally, using published survey results, we find that our results are consistent with the cosmic evolution of t_dep(z) and σ(z). Our interpretation of these results is that the cosmic evolution of t_dep may be regulated not just by the supply of gas but also by the internal regulation of star formation via feedback.

Helioseismology has revealed the internal density and rotation profiles of the Sun. Yet, knowledge of its magnetic fields and meridional circulation is confined much closer to the surface, and latitudinal entropy gradients are below detectable limits. While numerical simulations can offer insight into the interior dynamics and help identify which ingredients are necessary to reproduce particular observations, some features of the Sun can be understood analytically from an equilibrium perspective. Examples of such features include: the 1D density profile arising from steady-state energy transport from the core to the surface, and the tilting of isorotation contours in the convection zone (CZ) due to baroclinic forcing. To help answer the question of which features can be explained by equilibrium, we propose analyzing stationary axisymmetric ideal magnetohydrodynamic flows in the solar regime. By prescribing an appropriate entropy at the surface, we recover a rotation profile that reasonably matches observations in the bulk of the CZ. Additionally, by including the effects of poloidal flow, we reproduce a feature that is reminiscent of the near surface shear layer. However, no tachocline-like feature is seen in hydrodynamic equilibrium, suggesting the importance of either dynamics or magnetic fields in its description.

The phenomenon of subpulse drifting offers unique insights into the emission geometry of pulsars, and is commonly interpreted in terms of a rotating carousel of "spark" events near the stellar surface. We develop a detailed geometric model for the emission columns above a carousel of sparks that is entirely calculated in the observer's inertial frame, and which is consistent with the well-understood rotational effects of aberration and retardation. We explore the observational consequences of the model, including (1) the appearance of the reconstructed beam pattern via the cartographic transform, and (2) the morphology of drift bands and how they might evolve as a function of frequency. The model, which is implemented in the software package PSRGEOM, is applicable to a wide range of viewing geometries, and we illustrate its implications using PSRs B0809+74 and B2034+19 as examples. Some specific predictions are made with respect to the difference between subpulse evolution and microstructure evolution, which provides a way to further test our model.

Strong magnetic fields in the magnetospheres of neutron stars (NSs) (especially magnetars) and other astrophysical objects may release their energy in violent, intense episodes of magnetic reconnection. While reconnection has been studied extensively, the extreme field strength near NSs introduces new effects: radiation cooling and electron–positron pair production. Using massively parallel particle-in-cell simulations that self-consistently incorporate these new radiation and quantum-electrodynamic effects, we investigate relativistic magnetic reconnection in the strong-field regime. We show that reconnection in this regime can efficiently convert magnetic energy to X-ray and gamma-ray radiation and thus power bright, high-energy astrophysical flares. Rapid radiative cooling causes strong plasma and magnetic field compression in compact plasmoids. In the most extreme cases, the field can approach the quantum limit, leading to copious pair production.

There is a correlation between the bulge mass of the three main galaxies of the Local Group (LG), i.e., M31, Milky Way (MW), and M33, and the number of their dwarf spheroidal galaxies. A similar correlation has also been reported for spiral galaxies with comparable luminosities outside the LG. These correlations do not appear to be expected in standard hierarchical galaxy formation. In this paper, and for the first time, we present a quantitative investigation of the expectations of the standard model of cosmology for this possible relation using a galaxy catalog based on the Millennium-II simulation. Our main sample consists of disk galaxies at the centers of halos with a range of virial masses similar to M33, MW, and M31. For this sample, we find an average trend (though with very large scatter) similar to that observed in the LG; disk galaxies in heavier halos on average host heavier bulges and a larger number of satellites. In addition, we study sub-samples of disk galaxies with very similar stellar or halo masses (but spanning a range of 2–3 orders of magnitude in bulge mass) and find no obvious trend in the number of satellites versus bulge mass. We conclude that, while for a wide galaxy mass range a relation arises (which seems to be a manifestation of the satellite number–halo mass correlation), for a narrow range there is no relation between number of satellites and bulge mass in the standard model. Further studies are needed to better understand the expectations of the standard model for this possible relation.

Matching astronomical catalogs in crowded regions of the sky is challenging both statistically and computationally due to the many possible alternative associations. Budavári & Basu modeled the two-catalog situation as an assignment problem and used the famous Hungarian algorithm to solve it. Here we treat cross-identification of multiple catalogs by introducing a different approach based on integer linear programming. We first test this new method on problems with two catalogs and compare with the previous results. We then test the efficacy of the new approach on problems with three catalogs. The performance and scalability of this approach is discussed in the context of large surveys.

We investigate the properties of plasma turbulence by means of two-dimensional Hall-magnetohydrodynamic (HMHD) and hybrid particle-in-cell (HPIC) numerical simulations. We find that the HMHD simulations exhibit spectral properties that are in most cases in agreement with the results of the HPIC simulations and with solar wind observations. The energy spectra of magnetic fluctuations exhibit a double power law with spectral index −5/3 at MHD scales and −3 at kinetic scales, while for velocity fluctuations the spectral index is −3/2 at MHD scales. The break between the MHD and the kinetic scales occurs at the same scale in both simulations. In the MHD range the slopes of the total energy and residual energy spectra satisfy a fast Alfvén-dynamo balance. The development of a turbulent cascade is concurrently characterized by magnetic reconnection events taking place in thin current sheets that form between large eddies. A statistical analysis reveals that reconnection is qualitatively the same and fast in both the HMHD and HPIC models, characterized by inverse reconnection rates much smaller than the characteristic large-eddy nonlinear time. The agreement extends to other statistical properties, such us the kurtosis of the magnetic field. Moreover, the observation of a direct energy transfer from the large vortices to the small sub-ion scales, triggered by magnetic reconnection, further supports the existence of a reconnection-mediated turbulent regime at kinetic scales. We conclude that the HMHD fluid description captures to a large extent the transition of the turbulent cascade between the large MHD scales and the sub-ion scales.

We present HST/ACS narrowband images of a low-z sample of 19 3C radio galaxies to study the Hα and [O iii] emissions from the narrow-line region. Based on nuclear emission-line ratios, we divide the sample into high- and low-excitation galaxies (HEGs and LEGs). We observe different line morphologies, extended [O iii] emission, large [O iii]/Hα scatter across the galaxies, and a radio-line alignment. In general, HEGs show more prominent emission-line properties than LEGs: larger, more disturbed, more luminous, and more massive regions of ionized gas with slightly larger covering factors. We find evidence of correlations between line luminosities and (radio and X-ray) nuclear luminosities. All of these results point to a main common origin, the active nucleus, which ionizes the surrounding gas. However, the contribution of additional photoionization mechanisms (jet shocks and star formation) is needed to account for the different line properties of the two classes. A relationship between the accretion, photoionization, and feedback modes emerges from this study. For LEGs (hot-gas accretors), the synchrotron emission from the jet represents the main source of ionizing photons. The lack of cold gas and star formation in their hosts accounts for the moderate ionized-gas masses and sizes. For HEGs (cold-gas accretors), an ionizing continuum from a standard disk and shocks from the powerful jets are the main sources of photoionization, with a contribution from star formation. These components, combined with the large reservoir of cold/dust gas brought from a recent merger, account for the properties of their extended emission-line regions.

We report the results of long-term simultaneous X-ray and UV monitoring of the nearby (z = 0.03145) Seyfert 1.5 galaxy Mrk 817 using the Neil Gehrels Swift Observatory XRT and UVOT. Prior work has revealed that the X-ray flux from Mrk 817 has increased by a factor of  40 over the last 40 yr, whereas the UV emission has changed by a factor of  2.3. The X-ray emission of Mrk 817 now compares to some of the brightest Seyferts, but it has been poorly studied in comparison. We find that the X-ray (0.3–10.0 keV) and the UVM2 (roughly 2000–2500 Å) fluxes have fractional variability amplitudes of 0.35 and 0.18, respectively, over the entire monitoring period (2017 January 2 to 2018 April 20). A cross-correlation analysis is performed on the X-ray (0.3–10.0 keV) and UVM2 light curves over the entire monitoring period, a period of less frequent monitoring (2017 January 2–December 11), and a period of more frequent monitoring (2018 January 12–April 20). The analysis reveals no significant correlation between the two at any given lag for all monitoring periods. Especially given that reverberation studies have found significant lags between optical/UV continuum bands and broad optical lines in Mrk 817, the lack of a significant X-ray–UV correlation may point to additional complexities in the inner or intermediate disk. Mechanical (e.g., a funnel in the inner disk) and/or relativistic beaming of the X-ray emission could potentially explain the lack of a correlation. Alternatively, scattering in an equatorial wind could also diminish the ability of more isotropic X-ray emission to heat the disk itself.