Table of contents

We present the results from Atacama Large Millimeter/submillimeter Array observations of [N ii] 205 μm, [C ii] 158 μm, and [O iii] 88 μm lines in an unlensed submillimeter galaxy at z = 4.3, COSMOS-AzTEC-1, hosting a compact starburst core with an effective radius of ∼1 kpc. The [C ii] and [N ii] emission are spatially resolved in 03-resolution (1 kpc in radius). The kinematic properties of the [N ii] emission are consistent with those of the CO(4–3) and [C ii] emission, suggesting that the ionized gas feels the same gravitational potential as the associated molecular gas and photodissociation regions (PDRs). On the other hand, the spatial extent is different among the lines and dust continuum: the [C ii] emitting gas is the most extended and the dust is the most compact, leading to a difference of the physical conditions in the interstellar medium. We derive the incident far-ultraviolet flux and the hydrogen gas density through PDR modeling by properly subtracting the contribution of ionized gas to the total [C ii] emission. The observed [C ii] emission is likely produced by dense PDRs with ${n}_{{\rm{H}}}^{\mathrm{PDR}}={10}^{5.5\mbox{--}5.75}$ cm⁻³ and G₀ = 10^3.5–3.75 in the central 1 kpc region and ${n}_{{\rm{H}}}^{\mathrm{PDR}}={10}^{5.0\mbox{--}5.25}$ cm⁻³ and G₀ = 10^3.25–3.5 in the central 3 kpc region. We have also successfully measured the line ratio of [O iii]/[N ii] in the central 3 kpc region of COSMOS-AzTEC-1 at z = 4.3, which is the highest redshift where both nitrogen and oxygen lines are detected. Under the most likely physical conditions, the measured luminosity ratio of L_{[O iii]}/L_{[N ii]} = 6.4 ± 2.2 indicates a near solar metallicity with Z_gas = 0.7–1.0 Z_⊙, suggesting a chemically evolved system at z = 4.3.

We present results from fully kinetic particle-in-cell simulations of the transport of astrophysical relativistic jets in magnetized intergalactic medium. As opposed to magnetohydrodynamic simulations, the results show that a strong charge-separation electric field, induced by the different responses between jet electrons and ions to the magnetic fields, significantly enhances the energy exchange between different species of charged particles and electromagnetic fields, thus playing a key role in determining the collimation and shape of the jet spectral energy distribution (SED). It is found that when the magnetic field strength increases, the jet collimation also increases while the power-law slope of the jet SED decreases; this provides potential enlightenment on related astrophysical observations.

Nonparametric star formation histories (SFHs) have long promised to be the "gold standard" for galaxy spectral energy distribution (SED) modeling as they are flexible enough to describe the full diversity of SFH shapes, whereas parametric models rule out a significant fraction of these shapes a priori. However, this flexibility is not fully constrained even with high-quality observations, making it critical to choose a well-motivated prior. Here, we use the SED-fitting code Prospector to explore the effect of different nonparametric priors by fitting SFHs to mock UV–IR photometry generated from a diverse set of input SFHs. First, we confirm that nonparametric SFHs recover input SFHs with less bias and return more accurate errors than do parametric SFHs. We further find that, while nonparametric SFHs robustly recover the overall shape of the input SFH, the primary determinant of the size and shape of the posterior star formation rate as a function of time (SFR(t)) is the choice of prior, rather than the photometric noise. As a practical demonstration, we fit the UV–IR photometry of ∼6000 galaxies from the Galaxy and Mass Assembly survey and measure scatters between priors to be 0.1 dex in mass, 0.8 dex in SFR_{100 Myr}, and 0.2 dex in mass-weighted ages, with the bluest star-forming galaxies showing the most sensitivity. An important distinguishing characteristic for nonparametric models is the characteristic timescale for changes in SFR(t). This difference controls whether galaxies are assembled in bursts or in steady-state star formation, corresponding respectively to (feedback-dominated/accretion-dominated) models of galaxy formation and to (larger/smaller) confidence intervals derived from SED fitting. High-quality spectroscopy has the potential to further distinguish between these proposed models of SFR(t).

We present the first three-dimensional (3D), hydrodynamic simulations of the core convection zone (CZ) and extended radiative zone spanning from 1% to 90% of the stellar radius of an intermediate-mass ($3\,{M}_{\odot }$) star. This allows us to self-consistently follow the generation of internal gravity waves (IGWs) at the convective boundary and their propagation to the surface. We find that convection in the core is dominated by plumes. The frequency spectrum in the CZ and that of IGW generation is a double power law, as seen in previous two-dimensional (2D) simulations. The spectrum is significantly flatter than theoretical predictions using excitation through Reynolds stresses induced by convective eddies alone. It is compatible with excitation through plume penetration. An empirically determined distribution of plume frequencies generally matches the one necessary to explain a large part of the observed spectrum. We observe waves propagating in the radiation zone and excited standing modes, which can be identified as gravity and fundamental modes. They show similar frequencies and node patterns to those predicted by the stellar oscillation code GYRE. The continuous part of the spectrum fulfills the IGW dispersion relation. A spectrum of tangential velocity and temperature fluctuations close to the surface is extracted, which is directly related to observable brightness variations in stars. Unlike 2D simulations, we do not see the high frequencies associated with wave breaking, likely because the 3D simulations presented in this paper are more heavily damped.

We present optical, UV, and X-ray monitoring of the short orbital period black hole X-ray binary candidate Swift J1753.5–0127, focusing on the final stages of its 12 yr long outburst that started in 2005. From 2016 September onward, the source started to fade, and within 3 months, the optical flux almost reached the quiescent level. Soon after that, using a new proposed rebrightening classification method, we recorded a mini-outburst and a reflare in the optical light curves, peaking in 2017 February (V ∼ 17.0) and May (V ∼ 17.9), respectively. Remarkably, the mini-outburst has a peak flux consistent with the extrapolation of the slow decay before the fading phase preceding it. The following reflare was fainter and shorter. We found from optical colors that the temperature of the outer disk was ∼11,000 K when the source started to fade rapidly. According to the disk instability model, this is close to the critical temperature when a cooling wave is expected to form in the disk, shutting down the outburst. The optical color could be a useful tool to predict decay rates in some X-ray transients. We notice that all X-ray binaries that show mini-outbursts following a main outburst are short orbital period systems (<7 hr). In analogy with another class of short-period binaries showing similar mini-outbursts, the cataclysmic variables of the RZ LMi type, we suggest that mini-outbursts could occur if there is a hot inner disk at the end of the outburst decay.

We explore the spiral arm structural properties in a large variety of simulated galaxy systems. We study spiral arms arising from isolated barred and unbarred galaxies, as well as from interactions with small satellites. In all these first models, galactic systems are all embedded in a spherical dark matter halo. We also study spiral arms arising from a galactic system embedded in a triaxial dark matter halo. Simulations used in this work have been obtained by using different N-body codes and initial conditions techniques. Our strategy is to study the 3D arm structure through the analysis of pitch angle, along/transverse/vertical density laws and their corresponding scale lengths, and spiral lifetime. Our main results are as follows. First, the radial density profile of all spiral arms analyzed in this work is exponential. This profile resembles the one of the disk but with a scale length that is systematically larger (5%–40%). This result suggests that spiral arm gravitational influence is important beyond the scale radius of the disk. Second, the vertical and transversal density laws of the spiral arms follow a sech². The vertical scale length is compatible with the one of the disk; this is observed in all spiral arms analyzed here, independently of their origin, i.e., bar, high-order disk perturbation, tidal interaction with satellites, or halo triaxiality. Third, in the triaxial and satellite simulations, spiral arms follow a logarithmic locus all through their lifetime; the remaining models develop transient, recurrent, and short-lived spirals with a nondefined locus. In all cases, spiral arms wind up in their lifetime with a small pitch angle reduction. It is common that newborn spirals inherit the pitch angle of the previous ones; this result challenges the dynamical relevance of arm evolution. Finally, from the analysis of public photometric observations of NGC 2543, we state that the properties of observed spiral arm structure can be consistent with our conclusions. Further and systematic comparisons with observations are needed in order to confirm our results.

We study dust concentration in axisymmetric gas rings in protoplanetary disks. Given the gas surface density, we derived an analytical total dust surface density by taking into account the differential concentration of all grain sizes. This model allows us to predict the local dust-to-gas mass ratio and the slope of the particle size distribution, as a function of radius. We test this analytical model by comparing it with a 3D magnetohydrodynamical simulation of dust evolution in an accretion disk. The model is also applied to the disk around HD 169142. By fitting the disk continuum observations simultaneously at λ = 0.87, 1.3, and 3.0 mm, we obtain a global dust-to-gas mass ratio ${\epsilon }_{\mathrm{global}}=1.05\times {10}^{-2}$ and a viscosity coefficient α = 1.35 × 10⁻². This model can be easily implemented in numerical simulations of accretion disks.

PSR J1306–40 is a millisecond pulsar (MSP) binary with a non-degenerate companion in an unusually long ∼1.097 day orbit. We present new optical photometry and spectroscopy of this system, and model these data to constrain fundamental properties of the binary such as the component masses and distance. The optical data imply a minimum neutron star mass of 1.75 ± 0.09 M_⊙ (1σ) and a high, nearly edge-on inclination. The light curves suggest a large hot spot on the companion, suggestive of a portion of the pulsar wind being channeled to the stellar surface by the magnetic field of the secondary, mediated via an intrabinary shock. The Hα line profiles switch rapidly from emission to absorption near the companion inferior conjunction, consistent with an eclipse of the compact emission region at these phases. At our optically inferred distance of 4.7 ± 0.5 kpc, the X-ray luminosity is ∼10³³ erg s⁻¹, brighter than nearly all known redbacks in the pulsar state. The long-period, subgiant-like secondary, and luminous X-ray emission suggest this system may be part of the expanding class of MSP binaries that are progenitors to typical field pulsar–white dwarf binaries.

Witt (1996) has shown that, for an elliptical potential, the four images of a quadruply lensed quasar lie on a rectangular hyperbola that passes through the unlensed quasar position and the center of the potential as well. Wynne & Schechter (2018) have shown that, for the singular isothermal elliptical potential (SIEP), the four images also lie on an "amplitude" ellipse centered on the quasar position with axes parallel to the hyperbola's asymptotes. Witt's hyperbola arises from equating the directions of both sides of the lens equation. The amplitude ellipse derives from equating the magnitudes. One can model any four points as an SIEP in three steps. (1) Find the rectangular hyperbola that passes through the points. (2) Find the aligned ellipse that also passes through them. (3) Find the hyperbola with asymptotes parallel to those of the first that passes through the center of the ellipse and the pair of images closest to each other. The second hyperbola and the ellipse give an SIEP that predicts the positions of the two remaining images where the curves intersect. Pinning the model to the closest pair guarantees a four-image model. Such models permit rapid discrimination between gravitationally lensed quasars and random quartets of stars.

Convective overshooting in super asymptotic giant branch stars has been suggested to lead to the formation of hybrid white dwarfs with carbon–oxygen cores and oxygen–neon mantles. As the white dwarf cools, this core–mantle configuration becomes convectively unstable and should mix. This mixing has been previously studied using stellar evolution calculations, but these made the approximation that convection did not affect the temperature profile of the mixed region. In this work, we perform direct numerical simulations of an idealized problem representing the core–mantle interface of the hybrid white dwarf. We demonstrate that, while the resulting structure within the convection zone is somewhat different than what is assumed in the stellar evolution calculations, the two approaches yield similar results for the size and growth of the mixed region. These hybrid white dwarfs have been invoked as progenitors of various peculiar thermonuclear supernovae. This lends further support to the idea that if these hybrid white dwarfs form, then they should be fully mixed by the time of explosion. These effects should be included in the progenitor evolution, in order to more accurately characterize the signatures of these events.

Burst oscillations are brightness asymmetries that develop in the burning ocean during thermonuclear bursts on accreting neutron stars. They have been observed during H/He-triggered (Type I) bursts and carbon-triggered superbursts. The mechanism responsible is not unknown, but the dominant burst oscillation frequency is typically within a few hertz of the spin frequency, where this is independently known. One of the best-studied burst oscillation sources, 4U 1636-536, has oscillations at 581 Hz in both its regular Type I bursts and in one superburst. Recently, however, Strohmayer & Mahmoodifar reported the discovery of an additional signal at a higher frequency, 835 Hz, during the superburst. This higher frequency is consistent with the predictions for several types of global ocean modes, one of the possible burst oscillation mechanisms. If this is the case then the same physical mechanism may operate in the normal Type I bursts of this source. In this paper we report a stacked search for periodic signals in the regular Type I bursts: we found no significant signal at the higher frequency, with upper limits for the single trial root-mean-square fractional amplitude of 0.57(6)%. Our analysis did, however, reveal that the dominant 581 Hz burst oscillation signal is present at a weak level even in the sample of bursts where it cannot be detected in individual bursts. This indicates that any cutoff in the burst oscillation mechanism occurs below the detection threshold of existing X-ray telescopes.

In this work, we investigate the reliability of the BPT diagram for excluding galaxies that host an active galactic nucleus (AGN). We determine the prevalence of X-ray AGNs in the star-forming region of the BPT diagram and discuss the reasons behind this apparent misclassification, focusing primarily on relatively massive ($\mathrm{log}({M}_{* })\gtrsim 10$) galaxies. X-ray AGNs are selected from deep XMM observations using a new method that results in greater samples with a wider range of X-ray luminosities, complete to $\mathrm{log}({L}_{{\rm{X}}})\gt 41$ for z < 0.3. Taking X-ray detectability into account, we find that the average fraction of X-ray AGNs in the BPT star-forming branch is 2%, suggesting the BPT diagram can provide a reasonably clean sample of star-forming galaxies. However, the X-ray selection is itself rather incomplete. At the tip of the AGN branch of the BPT diagram, the X-ray AGN fraction is only 14%, which may have implications for studies that exclude AGNs based only on X-ray observations. Interestingly, the X-ray AGN fractions are similar for Seyfert and LINER populations, consistent with LINERs being true AGNs. We find that neither the star formation dilution nor the hidden broad-line components can satisfactorily explain the apparent misclassification of X-ray AGNs. On the other hand, ∼40% of all X-ray AGNs have weak emission lines such that they cannot be placed on the BPT diagram at all and often have low specific SFRs. Therefore, the most likely explanation for "misclassified" X-ray AGNs is that they have intrinsically weak AGN lines, and are only placeable on the BPT diagram when they tend to have high specific SFRs.

Interstellar dust is an essential component of the interstellar medium (ISM) and plays critical roles in astrophysics. Achieving an accurate model of interstellar dust is therefore of great importance. Interstellar dust models are usually built based on observational constraints such as starlight extinction and polarization, but dynamical constraints such as grain rotation are not considered. In this paper, we show that a newly discovered effect by Hoang et al., so-called RAdiative Torque Disruption (RATD), can act as an important dynamical constraint for dust models. Using this dynamical constraint, we derive the maximum size of grains that survive in the ISM for different dust models, including contact binary, composite, silicate core and amorphous carbon mantle, and compact grain model for the different radiation fields. We find that the different dust models have different maximum sizes due to their different tensile strengths, and the largest maximum size corresponds to the compact grains with the highest tensile strength. We show that the composite grain model cannot be ruled out if constituent particles are very small with radius a_p ≤ 25 nm, but large composite grains would be destroyed if the particles are large with a_p ≥ 50 nm. We suggest that grain internal structures can be constrained with observations using the dynamical RATD constraint for strong radiation fields such as supernova, nova, or star-forming regions. Finally, our obtained results suggest that micron-sized grains perhaps have compact/core–mantle structures or have composite structures but are located in regions with slightly higher gas density and weaker radiation intensity than the average ISM.

The possibility of directly detecting ultrahigh-energy (UHE) weakly interacting massive particles (WIMPs) are considered by the WIMPs' interaction with the nuclei in the air. Because neutrinos dominate the events from the spherical crown near the Extreme Universe Space Observatory on board the Japanese Experiment Module (JEM-EUSO), all the events from this region are ignored in my work. Then the numbers of UHE WIMPs and neutrinos detected by JEM-EUSO are evaluated at different energies (1 PeV < E < 100 EeV) in 10 years, respectively. If the energy thresholds are taken to be 20 EeV, neutrino events can be almost rejected in the detection of UHE WIMPs. According to my evaluation, O(10–100) UHE WIMP events can be detected by JEM-EUSO at energies above 70 EeV in 10 years.

We present a new Bayesian hierarchical model (BHM) named Steve for performing Type Ia supernova (SN Ia) cosmology fits. This advances previous works by including an improved treatment of Malmquist bias, accounting for additional sources of systematic uncertainty, and increasing numerical efficiency. Given light-curve fit parameters, redshifts, and host-galaxy masses, we fit Steve simultaneously for parameters describing cosmology, SN Ia populations, and systematic uncertainties. Selection effects are characterized using Monte Carlo simulations. We demonstrate its implementation by fitting realizations of SN Ia data sets where the SN Ia model closely follows that used in Steve. Next, we validate on more realistic SNANA simulations of SN Ia samples from the Dark Energy Survey and low-redshift surveys (DES Collaboration et al. 2018). These simulated data sets contain more than 60,000 SNe Ia, which we use to evaluate biases in the recovery of cosmological parameters, specifically the equation of state of dark energy, w. This is the most rigorous test of a BHM method applied to SN Ia cosmology fitting and reveals small w biases that depend on the simulated SN Ia properties, in particular the intrinsic SN Ia scatter model. This w bias is less than 0.03 on average, less than half the statistical uncertainty on w. These simulation test results are a concern for BHM cosmology fitting applications on large upcoming surveys; therefore, future development will focus on minimizing the sensitivity of Steve to the SN Ia intrinsic scatter model.

The circumstellar habitable zone and its various refinements serves as a useful entry point for discussing the potential for a planet to generate and sustain life. But little attention is paid to the quality of available energy in the form of stellar photons for phototrophic (e.g., photosynthetic) life. This short paper discusses the application of the concept of exergy to exoplanetary environments and the evaluation of the maximum efficiency of energy use, or maximum work obtainable from electromagnetic radiation. Hotter stars provide temperate planets with higher maximum obtainable work with higher efficiency than cool stars, and cool planets provide higher efficiency of radiation conversion from the same stellar photons than hot planets. These statements are independent of the details of any photochemical and biochemical mechanisms and could produce systematic differences in planetary habitability, especially at the extremes of maximal or minimal biospheres, or at critical ecological tipping points. Photoautotrophic biospheres on habitable planets around M-dwarf stars may be doubly disadvantaged by lower fluxes of photosynthetically active photons, and lower exergy with lower energy conversion efficiency.

We consider the impact of electromagnetic induction and ohmic heating on a conducting planetary object that orbits a magnetic star. Power dissipated as heat saps orbital energy. If this heat is trapped by an insulating crust or mantle, interior temperatures increase substantially. We provide a quantitative description of this behavior and discuss the astrophysical scenarios in which it might occur. Magnetic fields around some main-sequence stars and white dwarfs are strong enough to cause the decay of close-in orbits of asteroids and dwarf planets, drawing them through the Roche limit on megayear timescales. We confirm that ohmic heating around neutron stars is driven by the rotation of the stellar magnetic dipole, not orbital dynamics. In any case, heating can raise interior temperatures of asteroids or dwarf planets on close-in orbits to well above liquidus. Hot material escaping to the surface may lead to volcanic ejections that can obscure the host star (as in the light curve of KIC 8462852) and pollute its atmosphere (as observed with metal-rich white dwarfs). We speculate that mixing of a volatile-rich mantle or crust with material from an induction-heated core may lead to an explosion that could destroy the asteroid prior to tidal breakup.

The conventional wisdom, dating back to 2012, is that the mass distribution of Galactic double neutron stars (DNSs) is well-fit by a Gaussian distribution with a mean of 1.33 M_⊙ and a width of 0.09 M_⊙. With the recent discovery of new Galactic DNSs and GW170817, the first neutron star merger event to be observed with gravitational waves, it is timely to revisit this model. In order to constrain the mass distribution of DNSs, we perform Bayesian inference using a sample of 17 Galactic DNSs, effectively doubling the sample used in previous studies. We expand the space of models so that the recycled neutron star need not be drawn from the same distribution as the nonrecycled companion. Moreover, we consider different functional forms including uniform, single-Gaussian, and two-Gaussian distributions. While there is insufficient data to draw firm conclusions, we find positive support (a Bayes factor (BF) of 9) for the hypothesis that recycled and nonrecycled neutron stars have distinct mass distributions. The most probable model—preferred with a BF of 29 over the conventional model—is one in which the recycled neutron star mass is distributed according to a two-Gaussian distribution, and the nonrecycled neutron star mass is distributed uniformly. We show that precise component mass measurements of ≈20 DNSs are required in order to determine with high confidence (a BF of 150) whether recycled and nonrecycled neutron stars come from a common distribution. Approximately 60 DNSs are needed in order to establish the detailed shape of the distributions.

The nearby SN 2017eaw is a Type II-P ("plateau") supernova (SN) showing early-time, moderate CSM interaction. We present a comprehensive study of this SN, including the analysis of high-quality optical photometry and spectroscopy covering the very early epochs up to the nebular phase, as well as near-ultraviolet and near-infrared spectra and early-time X-ray and radio data. The combined data of SNe 2017eaw and 2004et allow us to get an improved distance to the host galaxy, NGC 6946, of D ∼ 6.85 ± 0.63 Mpc; this fits into recent independent results on the distance of the host and disfavors the previously derived (30% shorter) distances based on SN 2004et. From modeling the nebular spectra and the quasi-bolometric light curve, we estimate the progenitor mass and some basic physical parameters for the explosion and ejecta. Our results agree well with previous reports on a red supergiant progenitor star with a mass of ∼15–16 M_⊙. Our estimation of the pre-explosion mass-loss rate ($\dot{M}\sim 3\times {10}^{-7}\mbox{--}1\times {10}^{-6}{M}_{\odot }$ yr⁻¹) agrees well with previous results based on the opacity of the dust shell enshrouding the progenitor, but it is orders of magnitude lower than previous estimates based on general light-curve modeling of Type II-P SNe. Combining late-time optical and mid-infrared data, a clear excess at 4.5 μm can be seen, supporting the previous statements on the (moderate) dust formation in the vicinity of SN 2017eaw.

Recent analyses of the excess of gamma-ray radiation emanating from the Galactic center (GC) region suggest an origin in a population of thousands of undetected millisecond pulsars (MSPs). We have conducted a search for pulsar candidates using new high-sensitivity, wide-field radio observations of the GC covering 5 deg². We conducted the search at a low frequency of ∼320 MHz in order to take advantage of the very steep spectra typical of pulsars. Additional observations at 6 GHz of the most steep-spectrum, compact sources resulted in a list of seven candidate pulsars. No pulsations were detected for any of the candidates in a search conducted with the GBT at 1.5, 2, and 6 GHz, presumably due to severe temporal scattering in the GC region or along the line of sight. We discuss the implications of the nondetections on pulse period and distance estimates using two different models of the Galactic distribution of ionized gas. For our best candidate, C1748−2827, located 43' from Sgr A*, we estimate that pulsations from a normal pulsar would have been detected up to a distance of ∼8 kpc and from an MSP up to ∼4.5 kpc.

We study the gas inflow rate (ζ_inflow) and outflow rate (ζ_outflow) evolution of local Milky Way–mass star-forming galaxies (SFGs) since z = 1.3. The stellar mass growth history of Milky Way–mass progenitor SFGs is inferred from the evolution of the star formation rate (SFR)−stellar mass (M_*) relation, and the gas mass (M_gas) is derived using the recently established gas-scaling relations. With the ${M}_{* }+{M}_{\mathrm{gas}}$ growth curve, the net inflow rate κ is quantified at each cosmic epoch. At z ∼ 1.3, κ is comparable with the SFR, whereas it rapidly decreases to ∼0.15 × SFR at z = 0. We then constrain the average outflow rate ζ_outflow of progenitor galaxies by modeling the evolution of their gas-phase metallicity. The best-fit ζ_outflow is found to be (0.5–0.8) × SFR. Combining κ and ζ_outflow, we finally investigate the evolution of ζ_inflow since z = 1.3. We find that ζ_inflow rapidly decreases by ∼80% from z = 1.3 to z = 0.5. At z < 0.5, ζ_inflow continuously decreases but with a much lower decreasing rate. Implications of these findings on galaxy evolution are discussed.

Short-period sub-Neptunes with substantial volatile envelopes are among the most common type of known exoplanets. However, recent studies of the Kepler population have suggested a dearth of sub-Neptunes on highly irradiated orbits, where they are vulnerable to atmospheric photoevaporation. Physically, we expect this "photoevaporation desert" to depend on the total lifetime X-ray and extreme ultraviolet flux, the main drivers of atmospheric escape. In this work, we study the demographics of sub-Neptunes as a function of lifetime exposure to high-energy radiation and host-star mass. We find that for a given present-day insolation, planets orbiting a 0.3 M_⊙ star experience ∼100× more X-ray flux over their lifetimes versus a 1.2 M_⊙ star. Defining the photoevaporation desert as a region consistent with zero occurrence at 2σ, the onset of the desert happens for integrated X-ray fluxes greater than 1.43 × 10²² to 8.23 × 10²⁰ as a function of planetary radii for 1.8–4 R_⊕. We also compare the location of the photoevaporation desert for different stellar types. We find much greater variability in the desert onset in the bolometric flux space compared to the integrated X-ray flux space, suggestive of photoevaporation driven by steady-state stellar X-ray emissions as the dominant control on desert location. Finally, we report tentative evidence for the sub-Neptune valley, first seen around Sun-like stars, for M&K dwarfs. The discovery of additional planets around low-mass stars from surveys such as the Transiting Exoplanet Survey Satellite mission will enable detailed exploration of these trends.

There are two planetary formation scenarios: core accretion and gravitational disk instability. Based on the fact that gaseous objects are preferentially observed around metal-rich host stars, most extrasolar gaseous objects discovered to date are thought to have been formed by core accretion. Here, we present 569 samples of gaseous planets and brown dwarfs found in 485 planetary systems that span three mass regimes with boundary values at 4 and 25 Jupiter-mass masses through performing cluster analyses of these samples regarding the host-star metallicity, after minimizing the impact of the selection effect of radial-velocity measurement on the cluster analysis. The larger mass is thought to be the upper mass limit of the objects that were formed during the planetary formation processes. In contrast, the lower mass limit appears to reflect the difference between planetary formation processes around early-type and G-type stars; disk instability plays a greater role in the planetary formation process around early-type stars than that around G-type stars. Populations with masses between 4 and 25 Jupiter masses that orbit early-type stars comprise planets formed not only via the core-accretion process but also via gravitational disk instability because the population preferentially orbits metal-poor stars or is independent of the host-star metallicity. Therefore, it is essential to have a hybrid scenario for the planetary formation of the diverse systems.

Over the past decade, γ-ray observations of supernova remnants (SNRs) and accurate cosmic-ray (CR) spectral measurements have significantly advanced our understanding of particle acceleration in SNRs. In combination with multiwavelength observations of a large sample of SNRs, it has been proposed that the highest energy particles are mostly accelerated in young remnants, and the maximum energy that middle-age and old SNRs can accelerate particles to decreases rapidly with the decrease in shock speed. If SNRs dominate the CR flux observed at Earth, a large number of particles need to be accelerated in old SNRs for the soft CR spectrum even though they cannot produce very high-energy CRs. With radio, X-ray, and γ-ray observations of seven middle-age shell-type SNRs, we derive the distribution of high-energy electrons trapped in these remnants via a simple one-zone leptonic emission model and find that their spectral evolution is consistent with such a scenario. In particular, we find that particle acceleration by shocks in middle-age SNRs with age t can be described by a unified model with the maximum energy decreasing as t^−3.1 and the number of GeV electrons increasing as t^2.5 in the absence of escape from SNRs.

Molecular lines observed toward protoplanetary disks carry information about physical and chemical processes associated with planet formation. We present ALMA Band 6 observations of C₂H, HCN, and C¹⁸O in a sample of 14 disks spanning a range of ages, stellar luminosities, and stellar masses. Using C₂H and HCN hyperfine structure fitting and HCN/H¹³CN isotopologue analysis, we extract optical depth, excitation temperature, and column density radial profiles for a subset of disks. C₂H is marginally optically thick (τ ∼ 1–5) and HCN is quite optically thick (τ ∼ 5–10) in the inner 200 au. The extracted temperatures of both molecules are low (10–30 K), indicative of either subthermal emission from the warm disk atmosphere or substantial beam dilution due to chemical substructure. We explore the origins of C₂H morphological diversity in our sample using a series of toy disk models and find that disk-dependent overlap between regions with high UV fluxes and high atomic carbon abundances can explain a wide range of C₂H emission features (e.g., compact versus extended and ringed versus ringless emission). We explore the chemical relationship between C₂H, HCN, and C¹⁸O and find a positive correlation between C₂H and HCN fluxes but no relationship between C₂H or HCN with C¹⁸O fluxes. We also see no evidence that C₂H and HCN are enhanced with disk age. C₂H and HCN seem to share a common driver; however, more work remains to elucidate the chemical relationship between these molecules and the underlying evolution of C, N, and O chemistries in disks.

The Parker Solar Probe (PSP) will eventually reach and cross the Alfvén point or surface as it provides us with direct in situ measurements of the solar atmosphere. The Alfvén surface is the location at which the large-scale bulk solar wind speed ${\boldsymbol{U}}$ and the Alfvén speed ${\boldsymbol{V}}$_A are equal, and thus it separates sub-Aflvénic coronal flow $| {\boldsymbol{U}}| \ll | {{\boldsymbol{V}}}_{{\rm{A}}}|$ from super-Alfvénic solar wind flow $| {\boldsymbol{U}}| \gg | {{\boldsymbol{V}}}_{{\rm{A}}}|$. The nature of turbulence at the Alfvén surface is not fully understood, and the PSP measurements at the Alfvén surface will be revealing. We investigate turbulence at the Alfvén surface from a theoretical perspective by using the 2012 and 2017 Zank et al. turbulence transport model equations. The 2012 Zank et al. description is applicable to a large plasma beta β_p ≫ 1 regime, whereas the 2017 Zank et al. model applies to a plasma beta regime of order of β_p ∼ 1 or ≪1. The distinction in the β_p ≫ 1 and β_p ≪ 1 or ∼1 turbulence description is in a sense geometric, in that the β_p ≫ 1 description yields a fully 3D description of magnetohydrodynamic turbulence whereas β_p ≪ 1 or ∼1 describes predominantly quasi-2D (with respect to the large-scale or mean magnetic field) turbulence and a minority slab turbulence component. Our analyses suggest that turbulence at the Alfvén surface (i) turns off if the higher order plasma beta turbulence transport model equations are used and (ii) does not turn off if the lower order plasma beta, nearly incompressible turbulence transport model equations are used.

Supernova remnants (SNRs) are thought to be one of the major acceleration sites of galactic cosmic rays and an important class of objects for high-energy astrophysics. SNRs produce multiwavelength, nonthermal emission via accelerated particles at collisionless shocks generated by the interactions between the SN ejecta and the circumstellar medium (CSM). Although it is expected that the rich diversities observed in supernovae (SNe) and their CSM can result in distinct very high energy (VHE) electromagnetic signals in the SNR phase, there are only a handful of SNRs observed in both GeV and TeV γ-rays so far. A systematic understanding of particle acceleration at SNRs in different ambient environments is therefore limited. Here we explore nonthermal emission from SNRs in various circumstellar environments up to 5000 yr from explosion using hydrodynamical simulations coupled with efficient particle acceleration. We find that time evolution of emission characteristics in the VHE regime is mainly dictated by two factors: the number density of the target particles and the amplified magnetic field in the shocked medium. We also predict that the Cherenkov Telescope Array (CTA) will have sufficient sensitivity to detect VHE γ-rays from most young SNRs at distances ≲5.0 kpc. Future SNR observations with CTA will thus be promising for probing the CSM environment of SNe and hence their progenitor properties, including the mass-loss history of massive stars.

We model the history of Galactic r-process enrichment using high-redshift, high-resolution zoom cosmological simulations of a Milky Way–type halo. We assume that all r-process sources are neutron star mergers (NSMs) with a power-law delay time distribution. We model the time to mix pollutants at subgrid scales, which allows us to better compute the properties of metal-poor (MP) and carbon-enhanced metal-poor (CEMP) stars, along with statistics of their r-process-enhanced subclasses. Our simulations underpredict the cumulative ratios of r-process-enhanced MP and CEMP stars (MP-r, CEMP-r) over MP and CEMP stars by about one order of magnitude, even when the minimum coalescence time of the double neutron stars (DNSs), t_min, is set to 1 Myr. No r-process-enhanced stars form if t_min = 100 Myr. Our results show that even when we adopt the r-process yield estimates observed in GW170817, NSMs by themselves can only explain the observed frequency of r-process-enhanced stars if the birth rate of DNSs per unit mass of stars is boosted to $\approx {10}^{-4}\,{M}_{\odot }^{-1}$.

We have carried out 3D hydrodynamic simulations of a precessing jet/counterjet ejection. We have included the photoionization from the central source, considering three different ionizing photon rates (${S}_{* }={10}^{45}$, 10⁴⁶, and 10⁴⁷ phots s⁻¹), in order to determine its effect on the morphology and kinematics of the protoplanetary nebula. We have considered a time-dependent ejection density that generates dense knot structures in the jet, which are then partially photoionized by the ionizing photon field from the central source. We also explore the role of the medium in which the jet is propagated, under these conditions. The photoionization results in a larger Hα emission of the knots, and in an acceleration of the knots as a result of the so-called "rocket effect." We find that for larger values of the ionizing photon rate, a clear outwards acceleration of the knots is produced. These models are appropriate for explaining protoplanetary nebulae in which such outwards accelerations are observed.

We report measurements of parallax and proper motion for five 6.7 GHz methanol maser sources in the outer regions of the Perseus arm as part of the BeSSeL Survey of the Galaxy. By combining our results with previous astrometric results, we determine an average spiral arm pitch angle of 92 ± 15 and an arm width of 0.39 kpc for this spiral arm. For sources on the interior side of the Perseus arm, we find on average a radial inward motion in the Galaxy of 13.3 ± 5.4 km s⁻¹ and counter to Galactic rotation of 6.2 ± 3.2 km s⁻¹. These characteristics are consistent with models for spiral arm formation that involve gas entering an arm to be shocked and then to form stars. However, similar data for other spiral arms do not show similar characteristics.

We investigate Lyα transmission spikes at z > 5 in synthetic quasar spectra and discuss their connection to the properties of the intergalactic medium and their ability to constrain reionization models. We use state-of-the-art radiation-hydrodynamic simulations from the Cosmic Reionization On Computers series to predict the number of transmission spikes as a function of redshift, both in the ideal case of infinite spectral resolution and in a realistic observational setting. Transmission spikes are produced in highly ionized underdense regions located in the vicinity of UV sources. We find that most of the predicted spikes are unresolved by current observations and show that our mock spectra are consistent with observations of the quasar ULAS J1120+0641 in about 15% of the realizations. The spike height correlates with both the gas density and the ionized fraction, but the former link is erased when synthetic spectra are smoothed to realistically achievable spectral resolutions. There exists a linear relationship between spike width and the extent of the associated underdense region, with a slope that is redshift dependent. In agreement with observations, the spike transmitted flux is suppressed at small distance from bright galaxies as these reside in overdense regions. We argue that this anticorrelation can be used to constrain large-scale density modes.

The galaxy catalogs generated from low-resolution emission-line surveys often contain both foreground and background interlopers due to line misidentification, which can bias the cosmological parameter estimation. In this paper, we present a method for correcting the interloper bias by using the joint analysis of auto- and cross-power spectra of the main and the interloper samples. In particular, we can measure the interloper fractions from the cross-correlation between the interlopers and survey galaxies, because the true cross-correlation must be negligibly small. The estimated interloper fractions, in turn, remove the interloper bias in the cosmological parameter estimation. For example, in the Hobby–Eberly Telescope Dark Energy Experiment low-redshift (z < 0.5) [O ii] λ3727Å emitters contaminate high-redshift (1.9 < z < 3.5) Lyα line emitters. We demonstrate that the joint-analysis method yields a high signal-to-noise ratio measurement of the interloper fractions while only marginally increasing the uncertainties in the cosmological parameters relative to the case without interlopers. We also show that the same is true for the high-latitude spectroscopic survey of the Wide-field Infrared Survey Telescope mission where contamination occurs between the Balmer-α line emitters at lower redshifts (1.1 < z < 1.9) and oxygen ([O iii] λ5007Å) line emitters at higher redshifts (1.7 < z < 2.8).

We present the results of a suite of numerical simulations designed to explore the origin of the angular momenta of protostellar cores. Using the hydrodynamic grid code Athena with a sink implementation, we follow the formation of protostellar cores and protostars (sinks) from the subvirial collapse of molecular clouds on larger scales to investigate the range and relative distribution of core properties. We find that the core angular momenta are relatively unaffected by large-scale rotation of the parent cloud; instead, we infer that angular momenta are mainly imparted by torques between neighboring mass concentrations and exhibit a log-normal distribution. Our current simulation results are limited to size scales ∼0.05 pc (∼10⁴ au), but serve as first steps toward the ultimate goal of providing initial conditions for higher-resolution studies of core collapse to form protoplanetary disks.

We report the discovery of a hot white dwarf (WD) companion to a blue straggler star (BSS) in the globular cluster (GC) NGC 5466, based on observations from the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat. The Spectral Energy Distribution (SED) of the Far-UV detected BSS NH 84 was constructed by combining the flux measurements from four filters of UVIT, with GALEX, GAIA, and other ground-based observations. The SED of NH 84 reveals the presence of a hot companion to the BSS. The temperature and radius of the BSS (T${}_{\mathrm{eff}}={8000}_{-250}^{+1000}$ K, R/R${}_{\odot }=1.44\pm 0.05$) derived from Gemini spectra and SED fitting using Kurucz atmospheric models are consistent with each other. The temperature and radius of the hotter companion of NH 84 (T${}_{\mathrm{eff}}={\rm{32,000}}\pm 2000$ K, R/R${}_{\odot }=0.021\pm 0.007$) derived by fitting Koester WD models to the SED suggest that it is likely to be a hot WD. The radial velocity derived from the spectra along with the proper motion from GAIA DR2 confirms NH 84 to be a kinematic member of the cluster. This is the second detection of a BSS-WD candidate in a GC, and the first in the outskirts of a low-density GC. The location of this BSS in NGC 5466 along with its dynamical age supports the mass-transfer pathway for BSS formation in low-density environments.

To understand the physical origin of the close connection between supermassive black holes (BHs) and their host galaxies, it is vital to investigate star formation properties in active galaxies. Using a large data set of nearby type 1 active galactic nuclei (AGNs) with detailed structural decomposition based on high-resolution optical images obtained with the Hubble Space Telescope, we study the correlation between BH mass and bulge luminosity and the (Kormendy) relation between bulge effective radius and surface brightness. In both relations, the bulges of type 1 AGNs tend to be more luminous than those of inactive galaxies with the same BH mass or the same bulge size. This suggests that the central regions of AGN host galaxies have characteristically lower mass-to-light ratios than inactive galaxies, most likely due to the presence of a younger stellar population in active systems. In addition, the degree of luminosity excess appears to be proportional to the accretion rate of the AGN, revealing a physical connection between stellar growth and BH growth. Adopting a simple toy model for the increase of stellar mass and BH mass, we show that the fraction of young stellar population flattens out toward high accretion rates, possibly reflecting the influence of AGN-driven feedback.

Using Gaia data release 2 (DR2), we analyzed the distribution of stars in the close vicinity of the Sun in the full 3D position–velocity space. We have found no evidence of incomplete phase mixing in the vertical direction of the disk, which could have originated from external events. We show that the vertical phase-space spiral Z–V_z is produced by the well-known moving groups (MGs), mainly by Coma Berenices, Pleiades–Hyades, and Sirius, when the statistical characteristics (mean, median, or mode) of the azimuthal velocity V_φ are used to analyze the distribution in the vertical position–velocity plane. This result does not invoke external perturbations and is independent of the internal dynamical mechanisms from which the MGs originate. Our conclusions counterbalance current arguments in favor of short-lived (between 300 and 900 Myr) structures in the solar neighborhood. Contrarily, they support the hypothesis of a longer formation timescale (around a few gigayears) for the MGs.

We present a detailed morphological study of TeV gamma-rays, synchrotron radiation, and interstellar gas in the young Type Ia supernova remnant (SNR) RCW 86. We find that the interstellar atomic gas shows good spatial correlation with the gamma-rays, indicating that the TeV gamma-rays from RCW 86 are likely predominantly of hadronic origin. In contrast, the spatial correlation between the interstellar molecular cloud and the TeV gamma-rays is poor in the southeastern shell of the SNR. We argue that this poor correlation can be attributed to the low-energy cosmic rays (∼1 TeV) not penetrating into the dense molecular cloud due to an enhancement of the turbulent magnetic field around the dense cloud of ∼10–100 μG. We also find that the southwestern shell, which is bright in both synchrotron X-ray and radio continuum radiation, shows a significant gamma-ray excess compared with the interstellar proton column density, suggesting that leptonic gamma-rays via inverse Compton scattering possibly contribute alongside the hadronic gamma-rays. The total cosmic-ray energies of the young TeV gamma-ray SNRs—RX J1713.7−3946, Vela Jr, HESS J1731−347, and RCW 86—are roughly similar, which indicates that cosmic rays can be accelerated in both the core-collapse and Type Ia supernovae. The total energy of cosmic rays derived using the gas density, ∼10⁴⁸–10⁴⁹ erg, gives a safe lower limit due mainly to the low filling factor of interstellar gas within the shell.

Radiative association cross sections and rates are computed, using a quantum approach, for the formation of C₂ molecules (dicarbon) during the collision of two ground-state C(³P) atoms. We find that transitions originating in the C ¹Π_g, d ³Π_g, and 1 ⁵Π_u states are the main contributors to the process. The results are compared and contrasted with previous results obtained from a semiclassical approximation. New ab initio potential curves and transition dipole moment functions have been obtained for the present work using the multi-reference configuration interaction approach with the Davidson correction (MRCI+Q) and aug-cc-pCV5Z basis sets, substantially increasing the available molecular data on dicarbon. Applications of the current computations to various astrophysical environments and laboratory studies are briefly discussed, focusing on these rates.

We present multiobject spectroscopic observations of 23 globular cluster candidates (GCCs) in the prototypical megamaser galaxy NGC 4258, carried out with the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy instrument at the 10.4 m Gran Telescopio Canarias. The candidates have been selected based on the (u* − i') versus (i' − K_s) diagram, in the first application of the u*i'K_s method to a spiral galaxy. In the spectroscopy presented here, 70% of the candidates are confirmed as globular clusters (GCs). Our results validate the efficiency of the u*i'K_s method in the sparser GC systems of spirals, and given the downward correction to the total number of GCs, the agreement of the galaxy with the correlations between black hole mass and the total number and mass of GCs is actually improved. We find that the GCs, mostly metal-poor, corotate with the H i disk, even at large galactocentric distances.

We study the star-forming (SF) population of galaxies within a sample of 209 IR-selected galaxy clusters at 0.3 ≤ z ≤ 1.1 in the ELAIS-N1 and XMM-LSS fields, exploiting the first HSC-SSP data release. The large area and depth of these data allow us to analyze the dependence of the SF fraction, f_SF, on stellar mass and environment separately. Using R/R₂₀₀ to trace environment, we observe a decrease in f_SF from the field toward the cluster core, which strongly depends on stellar mass and redshift. The data show an accelerated growth of the quiescent population within the cluster environment: the f_SF versus stellar mass relation of the cluster core (R/R₂₀₀ ≤ 0.4) is always below that of the field (4 ≤ R/R₂₀₀ < 6). Finally, we find that environmental and mass quenching efficiencies depend on galaxy stellar mass and distance to the center of the cluster, demonstrating that the two effects are not separable in the cluster environment. We suggest that the increase of the mass quenching efficiency in the cluster core may emerge from an initial population of galaxies formed "in situ." The dependence of the environmental quenching efficiency on stellar mass favors models in which galaxies exhaust their reservoir of gas through star formation and outflows, after new gas supply is truncated when galaxies enter the cluster.

We report extensive differential V-band photometry and high-resolution spectroscopic observations of the early F-type, 1.06-day detached eclipsing binary V506 Oph. The observations, along with times of minimum light from the literature, are used to derive a very precise ephemeris and the physical properties for the components, with the absolute masses and radii being determined to 0.7% or better. The masses are 1.4153 ± 0.0100 M_☉ and 1.4023 ± 0.0094 M_☉ for the primary and secondary, the radii are 1.725 ± 0.010 R_☉ and 1.692 ± 0.012 R_☉, and the effective temperatures are 6840 ± 150 K and 6780 ± 110 K, respectively. The orbit is circular and the stars are rotating synchronously. The accuracy of the radii and temperatures is supported by the resulting distance estimate of 564 ± 30 pc, which is in excellent agreement with the value implied by the trigonometric parallax listed in the Gaia/Data Release 2 catalog. Current stellar evolution models from the Modules for Experiments in Stellar Astrophysics (MESA) Isochrones and Stellar Tracks series for a composition of [Fe/H] = −0.04 match the properties of both stars in V506 Oph very well at an age of 1.83 Gyr and indicate they are halfway through their core hydrogen-burning phase.

We present the 850 μm polarization observations toward the IC 5146 filamentary cloud taken using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) and its associated polarimeter (POL-2), mounted on the James Clerk Maxwell Telescope, as part of the B-fields In STar forming Regions Observations. This work is aimed at revealing the magnetic field morphology within a core-scale (≲1.0 pc) hub-filament structure (HFS) located at the end of a parsec-scale filament. To investigate whether the observed polarization traces the magnetic field in the HFS, we analyze the dependence between the observed polarization fraction and total intensity using a Bayesian approach with the polarization fraction described by the Rice likelihood function, which can correctly describe the probability density function of the observed polarization fraction for low signal-to-noise ratio data. We find a power-law dependence between the polarization fraction and total intensity with an index of 0.56 in A_V ∼ 20–300 mag regions, suggesting that the dust grains in these dense regions can still be aligned with magnetic fields in the IC 5146 regions. Our polarization maps reveal a curved magnetic field, possibly dragged by the contraction along the parsec-scale filament. We further obtain a magnetic field strength of 0.5 ± 0.2 mG toward the central hub using the Davis–Chandrasekhar–Fermi method, corresponding to a mass-to-flux criticality of ∼1.3 ± 0.4 and an Alfvénic Mach number of <0.6. These results suggest that gravity and magnetic field are currently of comparable importance in the HFS and that turbulence is less important.

We present chemical abundances for the elements carbon, sodium, and fluorine in 15 red giants of the globular cluster M4, as well as six red giants of the globular cluster ω Centauri. The chemical abundances were calculated in LTE via spectral synthesis. The spectra analyzed are high-resolution spectra obtained in the near-infrared region around 2.3 μm with the Phoenix spectrograph on the 8.1 m Gemini South Telescope, the IGRINS spectrograph on the McDonald Observatory 2.7 m Telescope, and the CRIRES spectrograph on the ESO 8.2 m Very Large Telescope. The results indicate a significant reduction in the fluorine abundances when compared to previous values from the literature for M4 and ω Centauri, due to a downward revision in the excitation potentials of the HF (1−0) R9 line used in the analysis. The fluorine abundances obtained for the M4 red giants are found to be anticorrelated with those of Na, following the typical pattern of abundance variations seen in globular clusters between distinct stellar populations. In M4, as the Na abundance increases by ∼+0.4 dex, the F abundance decreases by ∼−0.2 dex. A comparison with abundance predictions from two sets of stellar evolution models finds that the models predict somewhat less F depletion (∼−0.1 dex) for the same increase of +0.4 dex in Na.

The magnetic fields of low-mass stars are observed to be variable on decadal timescales, ranging in behavior from cyclic to stochastic. The changing strength and geometry of the magnetic field should modify the efficiency of angular momentum loss by stellar winds, but this has not been well quantified. In Finley et al. (2018), we investigated the variability of the Sun and calculated the time-varying angular momentum-loss rate in the solar wind. In this work, we focus on four low-mass stars that have all had their surface magnetic fields mapped for multiple epochs. Using mass-loss rates determined from astrospheric Lyα absorption, in conjunction with scaling relations from the MHD simulations of Finley & Matt (2018), we calculate the torque applied to each star by their magnetized stellar winds. The variability of the braking torque can be significant. For example, the largest torque for Eri is twice its decadal averaged value. This variation is comparable to that observed in the solar wind, when sparsely sampled. On average, the torques in our sample range from 0.5 to 1.5 times their average value. We compare these results to the torques of Matt et al. (2015), who use observed stellar rotation rates to infer the long-time-averaged torque on stars. We find that our stellar wind torques are systematically lower than the long-time-averaged values, by a factor of ∼3–30. Stellar wind variability appears unable to resolve this discrepancy, implying that there remain some problems with observed wind parameters, stellar wind models, or the long-term evolution models, which have yet to be understood.

We report the first high-resolution (subarcminute) large-scale mapping ¹²CO and ¹³CO observations of the molecular clouds associated with the giant outer Galaxy H ii region CTB 102 (KR 1). These observations were made using a newly commissioned receiver system on the 13.7 m radio telescope at the Taeduk Radio Astronomy Observatory. Our observations show that the molecular clouds have a spatial extent of 60 × 35 pc and a total mass of 10^4.8–10^5.0M_⊙. Infrared data from the Wide-field Infrared Survey Explorer and Two Micron All Sky Survey were used to identify and classify the young stellar object (YSO) population associated with ongoing star formation activity within the molecular clouds. We directly detect 18 class I/class II YSOs and six transition disk objects. Moving away from the H ii region, there is an age/class gradient consistent with sequential star formation. The infrared and molecular-line data were combined to estimate the star formation efficiency (SFE) of the entire cloud as well as the SFE for various subregions of the cloud. We find that the overall SFE is between ∼5% and 10%, consistent with previous observations of giant molecular clouds. One of the subregions, region 1a, is a clear outlier, with a SFE of 17%–35% on a 5 pc spatial scale. This high SFE is more typical for much smaller (subparsec scale) star-forming cores, and we think region 1a is likely an embedded massive protocluster.

Suprathermal ions form from interstellar gas that is first ionized into pickup ions and then accelerated to tens and hundreds of keV in energy. The resulting suprathermal ion spectra with hundreds of keV have been previously observed throughout the heliosphere; however, measurements at lower energies, around the pickup ion cutoff energy where they are accelerated from, were limited to <10 au. Here we present a statistical study of suprathermal ions in the keV to hundred keV energy range. We use the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on the New Horizons spacecraft, which recorded observations at a wide range of heliocentric distances, and compare these measurements to charge energy mass spectrometer (CHEMS) observations on Cassini, which cruised to and remained at Saturn. We find that the power-law exponents of suprathermal ion intensity over energy are between −1 and −2, change abruptly close to discontinuities that are likely corotating merged interaction regions, correlate with the solar wind bulk speed, and show a long-term evolution on the timescale of the solar cycle. The independent measurements from New Horizons and Cassini are consistent, confirming the first fully calibrated measurements from the New Horizons/PEPSSI instrument.

We present data of 10 penumbral microjets (PMJs) observed in a Hα, Ca ii 8542 Å, and Fe i 6302 Å line pair with the Swedish 1 m Solar Telescope (SST) with CRISP and Ca ii K with SST/CHROMIS in active region NOAA 12599 on 2016 October 12 at μ = 0.68. All four Stokes parameters of the Ca ii 8542 Å and Fe i 6302 Å lines were observed and a series of test pixels were inverted using the Stockholm inversion code. Our analysis revealed for the first time that PMJs are visible in Hα, where they appear as dark features with average line-of-sight (LOS) upflows of 1.1 ± 0.6 km s⁻¹, matching the LOS velocities from the inversions. Based on the Hα observations we extend the previous average length and lifetime of PMJs to 2815 ± 530 km and 163 ± 25 s, respectively. The plane-of-sky (POS) velocities of our PMJs of up to 17 km s⁻¹ tend to give increased velocities with distance traveled. Furthermore, two of our PMJs with significant Stokes V signal indicate that the PMJs possess an increased LOS magnetic field of up to 100 G compared to the local pre-/post- PMJ magnetic field, which propagates as quickly as the PMJs' POS velocities. Finally, we present evidence that PMJs display an on average 1 minute gradual precursory brightening that only manifests itself in the cores of the Ca ii lines. We conclude that PMJs are not ordinary jets but likely are manifestations of heat fronts that propagate at the local Alfvén velocity.

We present far-IR photometry and the infrared spectrum of the z = 3.9114 quasar/starburst composite system APM 08279+5255, obtained using the Stratospheric Observatory for Infrared Astronomy (SOFIA)/High-resolution Airborne Wideband Camera+ (HAWC+) and the Spitzer Space Telescope Infrared Spectrograph. We decompose the IR-to-radio spectral energy distribution (SED), sampled in 51 bands, using (i) a model comprised of two-temperature modified blackbodies and radio power laws and (ii) a semi-analytic model, which also accounts for emission from a clumpy torus. The latter is more realistic but requires a well-sampled SED, which is possible here. In the former model, we find temperatures of ${T}_{d}^{\mathrm{warm}}$ = 296${}_{-15}^{+17}$ K and ${T}_{d}^{\mathrm{cold}}$ = 110${}_{-3}^{+3}$ K for the warm and cold dust components, respectively. This model suggests that the cold dust component dominates the far-infrared (FIR) energy budget (66%) but contributes only 17% to the total IR luminosity. Based on the torus models, we infer an inclination angle of i = 15${}_{-8}^{+8}$° and the presence of silicate emission, in accordance with the Type-1 active galactic nucleus nature of APM 08279+5255. Accounting for the torus' contribution to the FIR luminosity, we find a lensing-corrected star formation rate of SFR = 3075 × (4/μ_L) M_⊙ yr⁻¹. We find that the central quasar contributes 30% to the FIR luminosity but dominates the total IR luminosity (93%). The 30% correction is in contrast to the 90% reported in previous work. In addition, the IR luminosity inferred from the torus model is a factor of two higher. These differences highlight the importance of adopting physically motivated models to properly account for IR emission in high-z quasars, which is now possible with SOFIA/HAWC+.

Despite many decades of study, the kinematics of the broad-line region of 3C 273 are still poorly understood. We report a new, high signal-to-noise, reverberation mapping campaign carried out from 2008 November to 2018 March that allows the determination of time lags between emission lines and the variable continuum with high precision. The time lag of variations in Hβ relative to those of the 5100 Å continuum is ${146.8}_{-12.1}^{+8.3}$ days in the rest frame, which agrees very well with the Paschen-α region measured by the GRAVITY at The Very Large Telescope Interferometer. The time lag of the Hγ emission line is found to be nearly the same as that for Hβ. The lag of the Fe ii emission is ${322.0}_{-57.9}^{+55.5}$ days, longer by a factor of ∼2 than that of the Balmer lines. The velocity-resolved lag measurements of the Hβ line show a complex structure that can be possibly explained by a rotation-dominated disk with some inflowing radial velocity in the Hβ-emitting region. Taking the virial factor of f_BLR = 1.3, we derive a BH mass of ${M}_{\bullet }={4.1}_{-0.4}^{+0.3}\times {10}^{8}\,{M}_{\odot }$ and an accretion rate of $9.3\,{L}_{\mathrm{Edd}}\,{c}^{-2}$ from the Hβ line. The decomposition of its Hubble Space Telescope images yields a host stellar mass of ${M}_{* }={10}^{11.3\pm 0.7}\,{M}_{\odot }$, and a ratio of ${M}_{\bullet }/{M}_{* }\approx 2.0\times {10}^{-3}$ in agreement with the Magorrian relation. In the near future, it is expected to compare the geometrically thick BLR discovered by the GRAVITY in 3C 273 with its spatially resolved torus in order to understand the potential connection between the BLR and the torus.

We report the results of a Sloan Digital Sky Survey IV eBOSS program to target X-ray sources and mid-infrared-selected Wide-field Infrared Survey Explorer (WISE) active galactic nucleus (AGN) candidates in a 36.8 deg² region of Stripe 82. About half this survey (15.6 deg²) covers the largest contiguous portion of the Stripe 82 X-ray survey. This program represents the largest spectroscopic survey of AGN candidates selected solely by their WISE colors. We combine this sample with X-ray and WISE AGNs in the field identified via other sources of spectroscopy, producing a catalog of 4847 sources that is 82% complete to r ∼ 22. Based on X-ray luminosities or WISE colors, 4730 of these sources are AGNs, with a median sample redshift of z ∼ 1. About 30% of the AGNs are optically obscured (i.e., lack broad lines in their optical spectra). BPT analysis, however, indicates that 50% of the WISE AGNs at z < 0.5 have emission line ratios consistent with star-forming galaxies, so whether they are buried AGNs or star-forming galaxy contaminants is currently unclear. We find that 61% of X-ray AGNs are not selected as mid-infrared AGNs, with 22% of X-ray AGNs undetected by WISE. Most of these latter AGNs have high X-ray luminosities (L_x > 10⁴⁴ erg s⁻¹), indicating that mid-infrared selection misses a sizable fraction of the highest luminosity AGNs, as well as lower luminosity sources where AGN-heated dust is not dominating the mid-infrared emission. Conversely, ∼58% of WISE AGNs are undetected by X-rays, though we do not find that they are preferentially redder than the X-ray-detected WISE AGNs.

The emergence of active regions (ARs) leads to various dynamic activities. Using high-resolution and long-lasting Hα observations from the New Vacuum Solar Telescope, we report the dynamics of NOAA AR 12700 in its emerging phase on 2018 February 26 in detail. In this AR, constant interchange reconnections (IRs) between emerging fibrils and preexisting ones were detected. Driven by the flux emergence, small-scale fibrils observed in Hα wavelength continuously emerged at the center of the AR and reconnected with the ambient preexisting fibrils, forming new longer fibrils. We investigate three IR scenarios that occurred over two hours. Specially, the third scenario of reconnection resulted in the formation of longer fibrils that show pronounced rotation motion. To derive the evolution of the magnetic structure during the reconnections, we perform nonlinear force-free field extrapolations. The extrapolated three-dimensional magnetic fields clearly depict a set of almost potential emerging loops, two preexisting flux ropes at 03:00 UT before the second reconnection scenario, and a set of newly formed loops with less twist at 03:48 UT after the third reconnection scenario. All of these extrapolated structures are consistent with the fibrils detected at the Hα wavelength. The aforementioned observations and extrapolation results suggest that the constant IRs resulted in the magnetic twist being redistributed from preexisting flux ropes toward the newly formed system with longer magnetic structure and weaker twist.

Halos and galaxies acquire their angular momentum during the collapse of the surrounding large-scale structure. This process imprints alignments between galaxy spins and nearby filaments and sheets. Low-mass halos grow by accretion onto filaments, aligning their spins with the filaments, whereas high-mass halos grow by mergers along filaments, generating spins perpendicular to the filament. We search for this alignment signal using filaments identified with the "Cosmic Web Reconstruction" algorithm applied to the Sloan Digital Sky Survey Main Galaxy Sample and galaxy spins from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) integral-field unit survey. MaNGA produces a map of the galaxy's rotational velocity, allowing direct measurement of the galaxy's spin direction, or unit angular momentum vector projected onto the sky. We find no evidence for alignment between galaxy spins and filament directions. We do find hints of a mass-dependent alignment signal, which is in 2σ–3σ tension with the mass-dependent alignment signal in the MassiveBlack-II and Illustris hydrodynamical simulations. However, the tension vanishes when galaxy spin is measured using the Hα emission line velocity rather than stellar velocity. Finally, in simulations we find that the mass-dependent transition from aligned to anti-aligned dark matter halo spins is not necessarily present in stellar spins: we find a stellar spin transition in Illustris but not in MassiveBlack-II, highlighting the sensitivity of spin-filament alignments to feedback prescriptions and subgrid physics.

Based on archival Chandra observations with a total exposure of 1.3 Ms, we study X-ray point sources in the Fornax cluster of galaxies, with the primary aim of searching for intracluster X-ray source populations. We detect 1177 point sources out to a projected radius of ∼30 arcmin (∼180 kpc) from the cluster center and down to a limiting 0.5–8 keV luminosity of ∼3 × 10³⁷ erg s⁻¹. We construct the source surface density profile, after excluding sources associated with foreground stars, known globular clusters, ultra-compact dwarfs, and galactic nuclei. From this profile we statistically identify ∼183 excess sources that are not associated with the bulk stellar content of the individual member galaxies of Fornax, nor with the cosmic X-ray background. Taking into account Poisson error and cosmic variance, the cumulative significance of this excess is at the ≳2σ level (with a maximum of 3.6σ) outside three effective radii of the central giant elliptical, NGC 1399. The luminosity function of the excess sources is found to be significantly steeper than that of the GC-hosting sources (presumably low-mass X-ray binaries (LMXBs)), disfavoring the possibility that unidentified GCs are primarily responsible for the excess. We show that a large fraction of the excess can be related to the extended stellar halo of NGC 1399 and/or the diffuse intracluster light, thus providing strong evidence for the presence of intracluster X-ray sources in Fornax, the second unambiguous case for a galaxy cluster after Virgo. Other possible origins of the excess, including supernova-kicked LMXBs and stripped nucleated dwarf galaxies are discussed.

We discuss a systematic effect associated with measuring polarization with a continuously rotating half-wave plate (HWP). The effect was identified with the data from the E and B Experiment, which was a balloon-borne instrument designed to measure the polarization of the cosmic microwave background (CMB) as well as that from Galactic dust. The data show polarization fractions larger than 10%, while less than 3% were expected from instrumental polarization. We give evidence that the excess polarization is due to detector nonlinearity in the presence of a continuously rotating HWP. The nonlinearity couples intensity signals to polarization. We develop a map-based method to remove the excess polarization. Applying this method to the 150 (250) GHz band data, we find that 81% (92%) of the excess polarization was removed. Characterization and mitigation of this effect are important for future experiments aiming to measure the CMB B-modes with a continuously rotating HWP.

Any unambiguous detection of a stochastic gravitational wave background (GWB) by a pulsar timing array will rest on the measurement of a characteristic angular correlation between pulsars. The ability to measure this correlation will depend on the geometry of the array. However, spatially correlated sources of noise, such as errors in the planetary ephemeris or clock errors, can produce false-positive correlations. The severity of this contamination will also depend on the geometry of the array. This paper quantifies these geometric effects with a spherical harmonic analysis of the pulsar timing residuals. At least nine well-spaced pulsars are needed to simultaneously measure a GWB and separate it from ephemeris and clock errors. Uniform distributions of pulsars can eliminate the contamination for arrays with large numbers of pulsars, but pulsars following the galactic distribution of known millisecond pulsars will always be affected. We quantitatively demonstrate the need for arrays to include many pulsars and for the pulsars to be distributed as uniformly as possible. Finally, we suggest a technique to cleanly separate the effect of ephemeris and clock errors from the gravitational wave signal.

We present an analytic formalism to compute the fluctuating component of the H i signal and extend it to take into account the effects of partial Lyα coupling during the era of cosmic dawn. We use excursion set formalism to calculate the size distribution of randomly distributed self-ionized regions. These ionization bubbles are surrounded by partially heated and Lyα coupled regions, which create spin temperature T_S fluctuations. We use the ratio of number of Lyα to ionizing photons (f_L) and number of X-ray photons emitted per stellar baryon (N_heat) as modeling parameters. Using our formalism, we compute the global H i signal, its autocorrelation, and its power spectrum in the redshift range 10 ≤ z ≤ 30 for the ΛCDM model. We check the validity of this formalism for various limits and simplified cases. Our results agree reasonably well with existing results from N-body simulations, in spite of following a different approach and requiring orders of magnitude less computation power and time. We further apply our formalism to study the fluctuating component corresponding to the recent observation by the Experiment to Detect the Global Epoch of reionization Signature (EDGES) that shows an unexpectedly deep absorption trough in the global H i signal in the redshift range 15 < z < 19. We show that, generically, the EDGES observation predicts a larger signal in this redshift range but a smaller signal at higher redshifts. We also explore the possibility of negative real-space autocorrelation of spin temperature and show that it can be achieved for partial Lyα coupling in many cases corresponding to simplified models and a complete model without density perturbations.

Previous studies indicate that interplanetary small magnetic flux ropes (SMFRs) are manifestations of microflare-associated small coronal mass ejections (CMEs), and the hot material with high-charge states heated by related microflares are found in SMFRs. Ordinary CMEs are frequently associated with prominence eruptions, and cool prominence materials are found within some magnetic clouds (MCs). Therefore, the predicted small CMEs may also be frequently associated with small prominence eruptions. In this work, we aim to search for cool prominence materials within SMFRs. We examined all the O⁵⁺ and Fe⁶⁺ fraction data obtained by the Advanced Composition Explorer (ACE) spacecraft during 1998–2008 and found that 13 SMFRs might exhibit low-charge-state signatures of unusual O⁵⁺ and/or Fe⁶⁺ abundances. One of the 13 SMFRs also exhibited signatures of high ionic charge states. We also reported a SMFR with high Fe⁶⁺ fraction, but the values of Fe⁶⁺ is a little lower than the threshold defining unusual Fe⁶⁺. However, the Solar Dynamics Observatory/ Atmospheric Imaging Assembly observations confirmed that the progenitor CME of this SMFR is associated with a small eruptive prominence, and the observations also supported the prominence materials were embedded in the CME. These observations are at the edge of the capabilities of ACE/Solar Wind Ion Composition Spectrometer and it cannot be ruled out that they are solely caused by instrumental effects. If these observations are real, they provide new evidence for the conjecture that SMFRs are small-scale MCs but also imply that the connected small CMEs could be associated with flares and prominence eruptions.

We report the latest view of Kepler solar-type (G-type main-sequence) superflare stars, including recent updates with Apache Point Observatory (APO) 3.5 m telescope spectroscopic observations and Gaia-DR2 data. First, we newly conducted APO 3.5 m spectroscopic observations of 18 superflare stars found from Kepler 1-minute time-cadence data. More than half (43 stars) are confirmed to be "single" stars, among 64 superflare stars in total that have been spectroscopically investigated so far in this APO 3.5 m and our previous Subaru/HDS observations. The measurements of v sin i (projected rotational velocity) and chromospheric lines (Ca ii H and K and Ca iiλ8542) support that the brightness variation of superflare stars is caused by the rotation of a star with large starspots. We then investigated the statistical properties of Kepler solar-type superflare stars by incorporating Gaia-DR2 stellar radius estimates. As a result, the maximum superflare energy continuously decreases as the rotation period P_rot increases. Superflares with energies ≲5 × 10³⁴ erg occur on old, slowly rotating Sun-like stars (P_rot ∼ 25 days) approximately once every 2000–3000 yr, while young, rapidly rotating stars with P_rot ∼ a few days have superflares up to 10³⁶ erg. The maximum starspot area does not depend on the rotation period when the star is young, but as the rotation slows down, it starts to steeply decrease at P_rot ≳ 12 days for Sun-like stars. These two decreasing trends are consistent since the magnetic energy stored around starspots explains the flare energy, but other factors like spot magnetic structure should also be considered.

Using archival Chandra observations with a total exposure of 510 ks, we present an updated catalog of point sources for globular cluster (GC) 47 Tucanae. Our study covers an area of ∼1767 ² (i.e., with R ≲ 75) with 537 X-ray sources. We show that the surface density distribution of X-ray sources in 47 Tuc is highly peaked in the cluster center, rapidly decreases at intermediate radii, and finally rises again at larger radii, with two distribution dips at R ∼ 100'' and R ∼ 170'' for the faint (L_X ≲ 5.0 × 10³⁰ erg s⁻¹) and bright (L_X ≳ 5.0 × 10³⁰ erg s⁻¹) groups of X-ray sources, respectively. These distribution features are similar to those of blue straggler stars (BSSs), where the distribution dip is located at R ∼ 200''. By fitting the radial distribution of each group of sources with a "generalized King model," we estimated an average mass of 1.51 ± 0.17 M_⊙, 1.44 ± 0.15 M_⊙ and 1.16 ± 0.06 M_⊙ for the BSSs, bright X-ray sources, and faint X-ray sources, respectively. These results are consistent with the mass segregation effect of heavy objects in GCs, where more massive objects drop to the cluster center faster and their distribution dip propagates outward further. Besides, the peculiar distribution profiles of X-ray sources and BSSs are also consistent with the mass segregation model of binaries in GCs, which suggests that, in addition to the dynamical formation channel, primordial binaries are also a significant contributor to the X-ray source population in GCs.

The gyro-resonant cosmic-ray (CR) streaming instability is believed to play a crucial role in CR transport, leading to the growth of Alfvén waves at small scales that scatter CRs, and impacts the interaction of CRs with the interstellar medium (ISM) on large scales. However, extreme scale separation (λ ≪ pc), low CR number density (n_CR/n_ISM ∼ 10⁻⁹), and weak CR anisotropy (∼v_A/c) pose strong challenges for proper numerical studies of this instability on a microphysical level. Employing the recently developed magnetohydrodynamic particle-in-cell method, which has unique advantages to alleviate these issues, we conduct 1D simulations that quantitatively demonstrate the growth and saturation of the instability in the parameter regime consistent with realistic CR streaming in the large-scale ISM. Our implementation of the δf method dramatically reduces Poisson noise and enables us to accurately capture wave growth over a broad spectrum equally shared between left- and right-handed Alfvén modes. We are also able to accurately follow the quasi-linear diffusion of CRs subsequent to wave growth, which is achieved by employing phase randomization across periodic boundaries. Full isotropization of the CRs in the wave frame requires the pitch angles of most CRs to efficiently cross 90° and can be captured in simulations with relatively high wave amplitude and/or spatial resolution. We attribute this crossing to nonlinear wave–particle interaction (rather than mirror reflection) by investigating individual CR trajectories. We anticipate that our methodology will open up opportunities for future investigations that incorporate additional physics.

We present results of three-dimensional MHD simulations of recurrent eruptions in emerging flux regions. The initial numerical setup is the same as that in the work by Syntelis et al. Here, we perform a parametric study on the magnetic field strength (B₀) of the emerging field. The kinetic energy of the produced ejective eruptions in the emerging flux region ranges from 10²⁶ to 10²⁸ erg, reaching up to the energies of small coronal mass ejections. The kinetic and magnetic energies of the eruptions scale linearly in a logarithmic plot. We find that the eruptions are triggered earlier for higher B₀ and that B₀ is not directly correlated to the frequency of occurrence of the eruptions. Using large numerical domains, we show the initial stage of the partial merging of two colliding erupting fields. The partial merging occurs partly by the reconnection between the field lines of the following and the leading eruption at the interface between them. We also find that tether-cutting reconnection of the field lines of the leading eruption underneath the following eruption magnetically links the two eruptions. Shocks develop inside the leading eruption during the collision.

We present maps of the dust properties in the Small and Large Magellanic Clouds (SMC, LMC) from fitting Spitzer and Herschel observations with the Draine & Li dust model. We derive the abundance of the small carbonaceous grain (or polycyclic aromatic hydrocarbon; PAH) component. The global PAH fraction (${q}_{{\rm{PAH}}}$, the fraction of the dust mass in the form of PAHs) is smaller in the SMC (${1.0}_{-0.3}^{+0.3}$ %) than in the LMC (${3.3}_{-1.3}^{+1.4}$ %). We measure the PAH fraction in different gas phases (H ii regions, ionized gas outside of H ii regions, molecular gas, and diffuse neutral gas). H ii regions appear as distinctive holes in the spatial distribution of the PAH fraction. In both galaxies, the PAH fraction in the diffuse neutral medium is higher than in the ionized gas, but similar to the molecular gas. Even at equal radiation field intensity, the PAH fraction is lower in the ionized gas than in the diffuse neutral gas. We investigate the PAH life-cycle as a function of metallicity between the two galaxies. The PAH fraction in the diffuse neutral medium of the LMC is similar to that of the Milky Way (∼4.6%), while it is significantly lower in the SMC. Plausible explanations for the higher PAH fraction in the diffuse neutral medium of the LMC compared to the SMC include: more effective PAH production by fragmentation of large grains at higher metallicity, and/or the growth of PAHs in molecular gas.

HH 212 is one of the well-studied protostellar systems, showing the first vertically resolved disk with a warm atmosphere around the central protostar. Here we report a detection of nine organic molecules (including newly detected ketene, formic acid, deuterated acetonitrile, methyl formate, and ethanol) in the disk atmosphere, confirming that the disk atmosphere is, for HH 212, the chemically rich component, identified before at a lower resolution as a "hot corino." More importantly, we report the first systematic survey and abundance measurement of organic molecules in the disk atmosphere within ∼40 au of the central protostar. The relative abundances of these molecules are similar to those in the hot corinos around other protostars and in Comet Lovejoy. These molecules can be either (i) originally formed on icy grains and then desorbed into gas phase or (ii) quickly formed in the gas phase using simpler species ejected from the dust mantles. The abundances and spatial distributions of the molecules provide strong constraints on models of their formation and transport in star formation. These molecules are expected to form even more complex organic molecules needed for life and deeper observations are needed to find them.

SNe Ia play a critical role in astrophysics, yet their origin remains mysterious. A crucial physical mechanism in any SN Ia model is the initiation of the detonation front that ultimately unbinds the white dwarf progenitor and leads to the SN Ia. We demonstrate, for the first time, how a carbon detonation may arise in a realistic three-dimensional turbulent electron-degenerate flow, in a new mechanism we refer to as turbulently driven detonation. Using both analytic estimates and three-dimensional numerical simulations, we show that strong turbulence in the distributed burning regime gives rise to intermittent turbulent dissipation that locally enhances the nuclear burning rate by orders of magnitude above the mean. This turbulent enhancement to the nuclear burning rate leads in turn to supersonic burning and a detonation front. As a result, turbulence plays a key role in preconditioning the carbon–oxygen fuel for a detonation. The turbulently driven detonation initiation mechanism leads to a wider range of conditions for the onset of carbon detonation than previously thought possible, with important ramifications for SNe Ia models.

Using Gaia Data Release 2 photometry, we report the detection of extended main-sequence turnoff (eMSTO) regions in the color–magnitude diagrams (CMDs) of the ∼14 Myr old double clusters h and χ Persei (NGC 869 and NGC 884). We find that stars with masses below ∼1.3 M_⊙ in both h and χ Persei populate narrow main sequences (MSs), while more massive stars define the eMSTO, closely mimicking observations of young Galactic and Magellanic Cloud clusters (with ages older than ∼30 Myr). Previous studies based on clusters older than ∼30 Myr found that rapidly rotating MS stars are redder than slow rotators of similar luminosity, suggesting that stellar rotation may be the main driver of the eMSTO. By combining photometry and projected rotational velocities from the literature of stars in h and χ Persei, we find no obvious relation between the rotational velocities and colors of non-emission-line eMSTO stars, in contrast with what is observed in older clusters. Similar to what is observed in Magellanic Cloud clusters, most of the extremely rapidly rotating stars, identified by their strong Hα emission lines, are located in the red part of the eMSTOs. This indicates that stellar rotation plays a role in the color and magnitude distribution of MSTO stars. By comparing the observations with simulated CMDs, we find that a simple population composed of coeval stars that span a wide range of rotation rates is unable to reproduce the color spread of the cluster's MSs. We suggest that variable stars, binary interactions, and stellar rotation affect the eMSTO morphology of these very young clusters.

The observation of Type Ia supernovae (SNe Ia) plays an essential role in probing the expansion history of the universe. But the possible presence of cosmic opacity can degrade the quality of SNe Ia. The gravitational-wave (GW) standard sirens, produced by the coalescence of double neutron stars and black hole–neutron star binaries, provide an independent way to measure the distances of GW sources, which are not affected by cosmic opacity. In this paper, we first propose that combining the GW observations of third-generation GW detectors with SN Ia data in similar redshift ranges offers a novel and model-independent method to constrain cosmic opacity. Through Monte Carlo simulations, we find that one can constrain the cosmic opacity parameter κ with an accuracy of σ_κ ∼ 0.046 by comparing the distances from 100 simulated GW events and 1048 current Pantheon SNe Ia. The uncertainty of κ can be further reduced to ∼0.026 if 800 GW events are considered. We also demonstrate that combining 2000 simulated SNe Ia and 1000 simulated GW events could result in much severer constraints on the transparent universe, for which κ = 0.0000 ± 0.0044. Compared to previous opacity constraints involving distances from other cosmic probes, our method using GW standard sirens and SN Ia standard candles at least achieves competitive results.

We present a critical review of the determination of fundamental parameters of white dwarfs discovered by the Gaia mission. We first reinterpret color–magnitude and color–color diagrams using photometric and spectroscopic information contained in the Montreal White Dwarf Database (MWDD), combined with synthetic magnitudes calculated from a self-consistent set of model atmospheres with various atmospheric compositions. The same models are then applied to measure the fundamental parameters of white dwarfs using the so-called photometric technique, which relies on the exquisite Gaia trigonometric parallax measurements, and photometric data from Panoramic Survey Telescope And Rapid Response System, Sloan Digital Sky Survey, and Gaia. In particular, we discuss at length the systematic effects induced by these various photometric systems. We then study in great detail the mass distribution as a function of effective temperature for the white dwarfs spectroscopically identified in the MWDD, as well as for the white dwarf candidates discovered by Gaia. We pay particular attention to the assumed atmospheric chemical composition of cool, non-DA stars. We also briefly revisit the validity of the mass–radius relation for white dwarfs and the recent discovery of the signature of crystallization in the Gaia color–magnitude diagram for DA white dwarfs. We finally present evidence that the core composition of most of these white dwarfs is, in bulk, a mixture of carbon and oxygen, an expected result from stellar evolution theory, but never empirically well established before.

The advent of large-scale spectroscopic surveys underscores the need to develop robust techniques for determining stellar properties ("labels," i.e., physical parameters and elemental abundances). However, traditional spectroscopic methods that utilize stellar models struggle to reproduce cool (<4700 K) stellar atmospheres due to an abundance of unconstrained molecular transitions, making modeling via synthetic spectral libraries difficult. Because small, cool stars such as K and M dwarfs are both common and good targets for finding small, cool planets, establishing precise spectral modeling techniques for these stars is of high priority. To address this, we apply The Cannon, a data-driven method of determining stellar labels, to Keck High Resolution Echelle Spectrometer spectra of 141 cool (<5200 K) stars from the California Planet Search. Our implementation is capable of predicting labels for small (<1 R_⊙) stars of spectral types K and later with accuracies of 68 K in effective temperature (T_eff), 5% in stellar radius (R_*), and 0.08 dex in bulk metallicity ([Fe/H]), and maintains this performance at low spectral resolutions (R < 5000). As M dwarfs are the focus of many future planet-detection surveys, this work can aid efforts to better characterize the cool star population and uncover correlations between cool star abundances and planet occurrence for constraining planet formation theories.

Ultra-hot Jupiters are the most highly irradiated gas giant planets, with equilibrium temperatures from 2000 to over 4000 K. Ultra-hot Jupiters are amenable to characterization due to their high temperatures, inflated radii, and short periods, but their atmospheres are atypical for planets in that the photosphere possesses large concentrations of atoms and ions relative to molecules. Here we evaluate how the atmospheres of these planets respond to irradiation by stars of different spectral type. We find that ultra-hot Jupiters exhibit temperature inversions that are sensitive to the spectral type of the host star. The slope and temperature range across the inversion both increase as the host star effective temperature increases due to enhanced absorption at short wavelengths and low pressures. The steep temperature inversions in ultra-hot Jupiters around hot stars result in increased thermal dissociation and ionization compared to similar planets around cooler stars. The resulting increase in H⁻ opacity leads to a transit spectrum that has muted absorption features. The emission spectrum, however, exhibits a large contrast in brightness temperature, a signature that will be detectable with both secondary eclipse observations and high-dispersion spectroscopy. We also find that the departures from local thermodynamic equilibrium in the stellar atmosphere can affect the degree of heating caused by atomic metals in the planet's upper atmosphere. Additionally, we further quantify the significance of heating by different opacity sources in ultra-hot Jupiter atmospheres.

G79.3+0.3 is an infrared dark cloud in the Cygnus-X complex that is home to massive deeply embedded young stellar objects (YSOs). We have produced a Submillimeter Array (SMA) 1.3 mm continuum image and ¹²CO line maps of the eastern section of G79.3+0.3 in which we detect five separate YSOs. We have estimated physical parameters for these five YSOs and others in the vicinity of G79.3+0.3 by fitting existing photometry from Spitzer, Herschel, and ground-based telescopes to spectral energy distribution models. Through these model fits we find that the most massive YSOs seen in the SMA 1.3 mm continuum emission have masses in the 5–6 M_☉ range. One of the SMA sources was observed to power a massive collimated ¹²CO outflow extending at least 0.94 pc in both directions from the protostar, with a total mass of 0.83 M_☉ and a dynamical timescale of 23 kyr.

We present a new method for determining the thermal state of the intergalactic medium based on Voigt profile decomposition of the Lyα forest. The distribution of Doppler parameter and column density (b–N_{H i} distribution) is sensitive to the temperature–density relation T = T₀(ρ/ρ₀)^γ−1, and previous work has inferred T₀ and γ by fitting its low-b cutoff. This approach discards the majority of available data and is susceptible to systematics related to cutoff determination. We present a method that exploits all information encoded in the b–N_{H i} distribution by modeling its entire shape. We apply kernel density estimation to discrete absorption lines to generate model probability density functions, and then we use principal component decomposition to create an emulator that can be evaluated anywhere in thermal parameter space. We introduce a Bayesian likelihood based on these models enabling parameter inference via Markov Chain Monte Carlo. The method's robustness is tested by applying it to a large grid of thermal history simulations. By conducting 160 mock measurements, we establish that our approach delivers unbiased estimates and valid uncertainties for a 2D (T₀, γ) measurement. Furthermore, we conduct a pilot study applying this methodology to real observational data at z = 2. Using 200 absorbers, equivalent in path length to a single Lya forest spectrum, we measure $\mathrm{log}{T}_{0}={4.092}_{-0.055}^{+0.050}$ and $\gamma ={1.49}_{-0.074}^{+0.073}$ in excellent agreement with cutoff fitting determinations using the same data. Our method is far more sensitive than cutoff fitting, enabling measurements of log T₀ and γ with precision on $\mathrm{log}{T}_{0}$ (γ) nearly two (three) times higher for current data set sizes.

The nature and abundance of sulfur chemistry in protoplanetary disks (PPDs) may impact the sulfur inventory on young planets and therefore their habitability. PPDs also offer an interesting test bed for sulfur chemistry models, since each disk shows a diverse set of environments. In this context, we present new sulfur molecule observations in PPDs and new S-disk chemistry models. With the Atacama Large Millimeter/submillimeter Array we observed the CS 5–4 rotational transition toward five PPDs (DM Tau, DO Tau, CI Tau, LkCa 15, MWC 480) and the CS 6–5 transition toward three PPDs (LkCa 15, MWC 480, and V4046 Sgr). Across this sample, CS displays a range of radial distributions, from centrally peaked to gaps and rings. We also present the first detection in PPDs of ¹³CS 6–5 (LkCa 15 and MWC 480), C³⁴S 6–5 (LkCa 15), and H₂CS 8₁₇–7₁₆, 9₁₉–8₁₈, and 9₁₈–8₁₇ (MWC 480) transitions. Using LTE models to constrain column densities and excitation temperatures, we find that either ¹³C and ³⁴S are enhanced in CS or CS is optically thick despite its relatively low brightness temperature. Additional lines and higher spatial resolution observations are needed to distinguish between these scenarios. Assuming that CS is optically thin, CS column density model predictions reproduce the observations within a factor of a few for both MWC 480 and LkCa 15. However, the model underpredicts H₂CS by 1–2 orders of magnitude. Finally, comparing the H₂CS/CS ratio observed toward the MWC 480 disk and toward different interstellar medium sources, we find the closest match with prestellar cores.

Deflection of coronal mass ejections (CMEs) in the interplanetary space, especially in the ecliptic plane, serves as an important factor deciding whether CMEs arrive at the Earth. Observational studies have shown evidence for deflection, whose detailed dynamic processes, however, remain obscure. Here we developed a 2.5D ideal magnetohydrodynamic simulation to study the propagation of CMEs traveling with different speeds in the heliospheric equatorial plane. The simulation confirms the existence of the CME deflection in the interplanetary space, which is related to the difference between the CME speed (v_r) and the solar wind speed (v_sw): a CME will propagate radially as v_r is close to v_sw but eastward or westward when v_r is larger or smaller than v_sw; the greater the difference is, the larger the deflection angle will be. This result supports the model for CME deflection in the interplanetary space (DIPS) proposed by Wang et al., predicting that an isolated CME can be deflected due to the pileup of solar wind plasma ahead of or behind the CME. Furthermore, the deflection angles, which are derived by inputting v_r and v_sw from the simulation into the DIPS model, are found to be consistent with those in the simulation.

Faraday rotation–conversion is the simultaneous rotation of all three Stokes polarization parameters, Q, U, and V, as an electromagnetic wave propagates through a magnetized plasma. In this regime, the Faraday plasma screen is characterized by more than just a rotation measure. We define the conversion measure that characterizes the wavelength-dependent conversion between the linear and circular polarization. In a cold plasma, the conversion occurs at the localized regions along the wave's path, where the large-scale magnetic field is perpendicular to the propagation direction. We show that the number of these regions along the line of sight through the screen, and their individual contributions to the conversion measure, can be inferred from the polarization measurements. We argue that the simultaneous measurement of wavelength-dependent linear and circular polarization might give an important insight into the magnetic-field geometry of the Faraday screen in FRB 121102 and other repeating fast radio bursts.

Changing-look (CL) quasars are a newly discovered class of luminous active galactic nuclei that undergo rapid (≲10 yr) transitions between Type 1 and Type 1.9/2, with an associated change in their continuum emission. We characterize the host galaxies of four faded CL quasars using broadband optical imaging. We use gri images obtained with the Gemini Multi-Object Spectrograph on Gemini North to characterize the surface brightness profiles of the quasar hosts and search for [O iii] λ4959, λ5007 emission from spatially extended regions, or voorwerpjes, with the goal of using them to examine past luminosity history. Although we do not detect, voorwerpjes surrounding the four quasar host galaxies, we take advantage of the dim nuclear emission to characterize the colors and morphologies of the host galaxies. Three of the four galaxies show morphological evidence of merger activity or tidal features in their residuals. The three galaxies that are not highly distorted are fit with a single Sérsic profile to characterize their overall surface brightness profiles. The single-Sérsic fits give intermediate Sérsic indices between the n = 1 of disk galaxies and the n = 4 of ellipticals. On a color–magnitude diagram, our CL quasar host galaxies reside in the blue cloud, with other active galactic nucleus (AGN) host galaxies and star-forming galaxies. On a color-Sérsic index diagram the CL quasar hosts reside with other AGN hosts in the "green valley." Our analysis suggests that the hosts of CL quasars are predominantly disrupted or merging galaxies that resemble AGN hosts, rather than inactive galaxies.

GRB 160709A is one of the few bright short gamma-ray bursts detected by both the Gamma-ray Burst Monitor and the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. The γ-ray prompt emission of GRB 160709A is adequately fitted by combinations of three distinct components: (i) a nonthermal component described by a power law (PL) with a high-energy exponential cutoff, (ii) a thermal component modeled with a Planck function, and (iii) a second nonthermal component shaped by an additional PL crossing the whole γ-ray spectrum. While the thermal component dominates during ∼0.12 s of the main emission episode of GRB 160709A with an unusually high temperature of ∼340 keV, the nonthermal components dominate in the early and late time. The thermal component is consistent with the photospheric emission resulting in the following parameters: the size of the central engine, ${R}_{0}={3.8}_{-1.8}^{+5.9}$ × 10⁸ cm, the size of the photosphere, ${R}_{\mathrm{ph}}={7.4}_{-1.2}^{+0.8}$ × 10¹⁰ cm, and a bulk Lorentz factor, ${\rm{\Gamma }}={728}_{-93}^{+75}$, assuming a redshift of 1. The slope of the additional PL spectrum stays unchanged throughout the burst duration; however, its flux decreases continuously as a function of time. A standard external shock model has been tested for the additional PL component using the relation between the temporal and spectral indices (the closure relation). Each set of spectral and temporal indices from two energy bands (200 keV–40 MeV and 100 MeV–10 GeV) satisfies a distinct closure relation. From the closure relation test we derived the index for the electron spectral distribution, p = 2.5 ± 0.1. The interaction of the jet with the interstellar environment is preferred over the interaction with the wind medium.

We compile a sample of 93 long gamma-ray bursts (GRBs) from the Fermi satellite and 131 from Konus-Wind that have measured redshifts and well-determined spectra, and estimate their pseudo-Lorentz factors (Γ₀) using the tight L_iso–E_p–Γ₀ correlation. The statistical properties and pair correlations of the temporal and spectral parameters are studied in the observer frame, rest frame, and comoving frame, respectively. We find that the distributions of the duration, peak energy, isotropic energy, and luminosity in the different frames are basically log-normal, and that their distributions in the comoving frame are narrow, clustering around ${T}_{90}^{{\prime} }$ ∼ 4000 s, ${E}_{{\rm{p}},c}^{{\prime} }$ ∼ 0.7 keV, ${E}_{\mathrm{iso},c}^{{\prime} }$ ∼ 8 × 10⁴⁹ erg, and ${L}_{\mathrm{iso},c}^{{\prime} }$ ∼ 2.5 × 10⁴⁶ erg s⁻¹, where the redshift evolution effect has been taken into account. We also find that the values of Γ₀ are broadly distributed between a few tens and several hundreds, with median values of ∼270. We further analyze the pair correlations of all the quantities, confirm the E_iso–E_p, L_iso–E_p, L_iso–Γ₀, and E_iso–Γ₀ relations, and find that the corresponding relations in the comoving frame do still exist, but with large dispersions. This suggests not only that the well-known spectrum–energy relations are intrinsic correlations, but also that the observed correlations are governed by the Doppler effect. In addition, the peak energies of long GRBs are independent of duration both in the rest frame and in the comoving frame, and there is a weak anticorrelation between the peak energy and Lorentz factor.

We present a comprehensive study of the applications of the pixel color–magnitude diagram (pCMD) technique for measuring star formation histories (SFHs) and other stellar population parameters of galaxies, and we demonstrate that the technique can also constrain distances. SFHs have previously been measured through either the modeling of resolved-star CMDs or of integrated-light spectral energy distributions, yet neither approach can easily be applied to galaxies in the "semi-resolved regime." The pCMD technique has previously been shown to have the potential to measure stellar populations and SFHs in semi-resolved galaxies. Here we present Pixel Color–Magnitude Diagrams with Python (PCMDPy), a graphics processing unit (GPU)-accelerated package that makes significant computational improvements to the original code and includes more realistic physical models. These advances include the simultaneous fitting of distance, modeling a Gaussian metallicity distribution function, and an observationally motivated dust model. GPU acceleration allows these more realistic models to be fit roughly 7× faster than the simpler models in the original code. We present results from a suite of mock tests, showing that with proper model assumptions, the code can simultaneously recover SFH, $[\mathrm{Fe}/{\rm{H}}]$, distance, and dust extinction. Our results suggest the code, applied to observations with Hubble Space Telescope-like resolution, should constrain these properties with high precision within 10 Mpc and can be applied to systems out to as far as 100 Mpc. pCMDs open a new window to studying the stellar populations of many galaxies that cannot be readily studied through other means.

Giant radio relics in the outskirts of galaxy clusters are known to be lit up by the relativistic electrons produced via diffusive shock acceleration (DSA) in shocks with low sonic Mach numbers, M_s ≲ 3. The particle acceleration at these collisionless shocks critically depends on the kinetic plasma processes that govern the injection to DSA. Here, we study the preacceleration of suprathermal electrons in weak, quasi-perpendicular (Q_⊥) shocks in the hot, high-β (β = P_gas/P_B) intracluster medium (ICM) through two-dimensional particle-in-cell simulations. Guo et al. showed that, in high-β Q_⊥-shocks, some of the incoming electrons could be reflected upstream and gain energy via shock drift acceleration (SDA). The temperature anisotropy due to the SDA-energized electrons then induces the electron firehose instability (EFI), and oblique waves are generated, leading to a Fermi-like process and multiple cycles of SDA in the preshock region. We find that such electron preacceleration is effective only in shocks above a critical Mach number ${M}_{\mathrm{ef}}^{* }\approx 2.3$. This means that, in ICM plasmas, Q_⊥-shocks with M_s ≲ 2.3 may not efficiently accelerate electrons. We also find that, even in Q_⊥-shocks with M_s ≳ 2.3, electrons may not reach high enough energies to be injected to the full Fermi-I process of DSA, because long-wavelength waves are not developed via the EFI alone. Our results indicate that additional electron preaccelerations are required for DSA in ICM shocks, and the presence of fossil relativistic electrons in the shock upstream region may be necessary to explain observed radio relics.

We develop and exploit a new catalog of coronal pressure waves modeled in 3D to study the potential role of these waves in accelerating solar energetic particles (SEPs) measured in situ. Our sample comprises modeled shocks and SEP events detected during solar cycle 24 observed over a broad range of longitudes. From the 3D reconstruction of shock waves using coronagraphic observations we derived the 3D velocity along the entire front as a function of time. Combining new reconstruction techniques with global models of the solar corona, we derive the 3D distribution of basic shock parameters such as Mach numbers, compression ratios, and shock geometry. We then model in a time-dependent manner how the shock wave connects magnetically with spacecraft making in situ measurements of SEPs. This allows us to compare modeled shock parameters deduced at the magnetically well-connected regions, with different key parameters of SEPs such as their maximum intensity. This approach accounts for projection effects associated with remote-sensing observations and constitutes the most extensive study to date of shock waves in the corona and their relation to SEPs. We find a high correlation between the maximum flux of SEPs and the strength of coronal shock waves quantified, for instance, by the Mach number. We discuss the implications of that work for understanding particle acceleration in the corona.

In this work, we present the analysis of the binary microlensing event OGLE-2018-BLG-0022 that is detected toward the Galactic bulge field. The dense and continuous coverage with the high-quality photometry data from ground-based observations combined with the space-based Spitzer observations of this long timescale event enables us to uniquely determine the masses M₁ = 0.40 ± 0.05 M_⊙ and M₂ = 0.13 ± 0.01 M_⊙ of the individual lens components. Because the lens-source relative parallax and the vector lens-source relative proper motion are unambiguously determined, we can likewise unambiguously predict the astrometric offset between the light centroid of the magnified images (as observed by the Gaia satellite) and the true position of the source. This prediction can be tested when the individual-epoch Gaia astrometric measurements are released.

We present a machine-learning (ML) approach for estimating galaxy cluster masses from Chandra mock images. We utilize a Convolutional Neural Network (CNN), a deep ML tool commonly used in image recognition tasks. The CNN is trained and tested on our sample of 7896 Chandra X-ray mock observations, which are based on 329 massive clusters from the ${\text{}}{IllustrisTNG}$ simulation. Our CNN learns from a low resolution spatial distribution of photon counts and does not use spectral information. Despite our simplifying assumption to neglect spectral information, the resulting mass values estimated by the CNN exhibit small bias in comparison to the true masses of the simulated clusters (−0.02 dex) and reproduce the cluster masses with low intrinsic scatter, 8% in our best fold and 12% averaging over all. In contrast, a more standard core-excised luminosity method achieves 15%–18% scatter. We interpret the results with an approach inspired by Google DeepDream and find that the CNN ignores the central regions of clusters, which are known to have high scatter with mass.

Rotational scaling relationships are examined for the degree of equipartition between magnetic and kinetic energies in stellar convection zones. These scaling relationships are approached from two paradigms, with first a glance at scaling relationship built on an energy-balance argument and second a look at a force-based scaling. The latter implies a transition between a nearly constant inertial scaling when in the asymptotic limit of minimal diffusion and magnetostrophy, whereas the former implies a weaker scaling with convective Rossby number. Both scaling relationships are then compared to a suite of 3D convective dynamo simulations with a wide variety of domain geometries, stratifications, and range of convective Rossby numbers.

On 2011 November 5, Venus Express observed the impact of an extremely strong interplanetary coronal mass ejection (ICME) on Venus. As a result, the Venusian induced magnetosphere dramatically fluctuated during the ICME passage: the bow shock was compressed and broadened by the sheath and the body of the ICME, respectively; an atypically strong magnetic barrier (over 250 nT) of Venus was detected; and the plasma sheet in the magnetotail flapped so rapidly that it was crossed by Venus Express 5 times within 1.5 minutes. The ionosphere was totally magnetized because of the very high magnetic pressure of the induced magnetosphere. However, the altitude of the ionopause did not decrease with respect to those in neighboring orbits, which is inconsistent with the ionopause descents reported by previous studies. We found that the ionosphere was greatly excited by the ICME as evidenced by the much higher heavy ion density. That is why the balance between the ionospheric thermal pressure and the strong magnetic pressure can be maintained at a relatively high altitude. We propose that a much stronger massloading effect resulting from the excited ionosphere is responsible for the anomalously high magnetic barrier because much more magnetic field lines were anchored. Our results also suggest that such ICMEs that can excite the ionosphere are substantially efficient in enhancing the atmospheric loss of Venus.

We present an improved determination of the Hubble constant from Hubble Space Telescope (HST) observations of 70 long-period Cepheids in the Large Magellanic Cloud (LMC). These were obtained with the same WFC3 photometric system used to measure extragalactic Cepheids in the hosts of SNe Ia. Gyroscopic control of HST was employed to reduce overheads while collecting a large sample of widely separated Cepheids. The Cepheid period–luminosity relation provides a zero-point-independent link with 0.4% precision between the new 1.2% geometric distance to the LMC from detached eclipsing binaries (DEBs) measured by Pietrzyński et al. and the luminosity of SNe Ia. Measurements and analysis of the LMC Cepheids were completed prior to knowledge of the new DEB LMC distance. Combined with a refined calibration of the count-rate linearity of WFC3-IR with 0.1% precision, these three improved elements together reduce the overall uncertainty in the geometric calibration of the Cepheid distance ladder based on the LMC from 2.5% to 1.3%. Using only the LMC DEBs to calibrate the ladder, we find H₀ = 74.22 ± 1.82 km s⁻¹ Mpc⁻¹ including systematic uncertainties, 3% higher than before for this particular anchor. Combining the LMC DEBs, masers in NGC 4258, and Milky Way parallaxes yields our best estimate: H₀ = 74.03 ± 1.42 km s⁻¹ Mpc⁻¹, including systematics, an uncertainty of 1.91%–15% lower than our best previous result. Removing any one of these anchors changes H₀ by less than 0.7%. The difference between H₀ measured locally and the value inferred from Planck CMB and ΛCDM is 6.6 ± 1.5 km s⁻¹ Mpc⁻¹ or 4.4σ (P = 99.999% for Gaussian errors) in significance, raising the discrepancy beyond a plausible level of chance. We summarize independent tests showing that this discrepancy is not attributable to an error in any one source or measurement, increasing the odds that it results from a cosmological feature beyond ΛCDM.

The purpose of this work is to develop a procedure to obtain the normal modes of a coronal loop from time-dependent numerical simulations with the aim of better understanding observed transverse loop oscillations. To achieve this goal, in this paper we present a new method and test its performance with a problem for which the normal modes can be computed analytically. In a follow-up paper, the application to the simulations of Rial et al. is tackled. The method proceeds iteratively and at each step consists of (i) a time-dependent numerical simulation followed by (ii) the Complex Empirical Orthogonal Function (CEOF) analysis of the simulation results. The CEOF analysis provides an approximation to the normal mode eigenfunctions that can be used to set up the initial conditions for the numerical simulation of the following iteration, in which an improved normal mode approximation is obtained. The iterative process is stopped once the global difference between successive approximate eigenfunctions is below a prescribed threshold. The equilibrium used in this paper contains material discontinuities that result in one eigenfunction with a jump across these discontinuities and two eigenfunctions whose normal derivatives are discontinuous there. After six iterations, the approximations to the frequency and eigenfunctions are accurate to ≲0.7% except for the eigenfunction with discontinuities, which displays a much larger error at these positions.

In this paper we report the discovery of a double blue straggler star (BSS) sequence in the core of the core-collapsed cluster M15 (NGC 7078). We performed a detailed photometric analysis of the extremely dense core of the cluster using a set of images secured with the Advanced Camera for Survey in the High Resolution Channel mode on board the Hubble Space Telescope. The proper combination of the large number of single frames in the near-UV (F220W), and blue (F435W) filters allowed us to perform a superb modeling of the point-spread function and an accurate deblending procedure. The color–magnitude diagram revealed the presence of two distinct parallel sequences of blue stragglers. In particular, the blue BSS sequence is characterized by the intriguing presence of two different branches. The first branch appears extremely narrow, it extends up to 2.5 mag brighter than the cluster main-sequence turnoff (MS-TO) point, and it is nicely reproduced by a 2 Gyr old collisional isochrone. The second branch extends up to 1.5 mag from the MS-TO and it is reproduced by a 5.5 Gyr old collisional isochrone. Our observations suggest that each of these branches is mainly constituted by a population of nearly coeval collisional BSSs of different masses generated during two episodes of high collisional activity. We discuss the possibility that the oldest episode corresponds to the core-collapse (CC) event (occurred about 5.5 Gyr ago), while the most recent one (occurred about 2 Gyr ago) is associated with a core oscillation in the post-CC evolution. The discovery of these features provides further strong evidence in support of the connection between the BSS properties and globular cluster dynamical evolution, and it opens new perspectives on the study of CC and post-CC evolution.

We have analyzed spectral properties and abundances of ∼0.02–3.0 MeV nucleon⁻¹ suprathermal (ST) H–Fe ions in 41 stream interaction regions (SIRs) near 1 au observed by Wind and ACE spacecraft from 1995 January through 2008 December. We find that, (i) the event-averaged spectral index is γ ∼ 2.44, with a standard deviation (σ) of 0.67, (ii) γ's are poorly correlated with the magnetic compression ratios, and 17% of the events group around γ ∼ 1.5, (iii) γ's for both O and Fe at ∼0.02–0.09 MeV nucleon⁻¹ and 0.09–0.3 MeV nucleon⁻¹ are correlated, but do not exhibit any systematic steepening or flattening as a function of energy, (iv) the ST heavy ion abundance ratios remain constant with increasing energy, implying that the spectral rollovers, defined by the e-folding energy E₀, are independent of the ion's mass per charge (M/Q), and (v) SIR ST abundances are similar to the corresponding solar wind values, and do not exhibit any systematic behavior when plotted versus the ion's M/Q or first ionization potential. The above results pose challenges for (1) particle acceleration models that invoke either a corotating interaction region or SIR shocks between ∼3 and 5 au, (2) particle transport models that predict M/Q-dependent spectral rollovers due to interplanetary turbulence effects, and (3) the notion that SIR ST ions originate directly from the bulk solar wind. Instead, we suggest that the SIR ST ions are accelerated out of a pool of material that includes particles accelerated in solar energetic particle events and processed or heated solar wind ions.

We present a search for gamma-ray bursts in the Fermi-GBM 10 yr catalog that show similar characteristics to GRB 170817A, the first electromagnetic counterpart to a GRB identified as a binary neutron star (BNS) merger via gravitational wave observations. Our search is focused on a nonthermal pulse, followed by a thermal component, as observed for GRB 170817A. We employ search methods based on the measured catalog parameters and Bayesian Block analysis. Our multipronged approach, which includes examination of the localization and spectral properties of the thermal component, yields a total of 13 candidates, including GRB 170817A and the previously reported similar burst, GRB 150101B. The similarity of the candidates is likely caused by the same processes that shaped the gamma-ray signal of GRB 170817A, thus providing evidence of a nearby sample of short GRBs resulting from BNS merger events. Some of the newly identified counterparts were observed by other space telescopes and ground observatories, but none of them have a measured redshift. We present an analysis of this subsample, and we discuss two models. From uncovering 13 candidates during a time period of 10 yr we predict that Fermi-GBM will trigger on-board on about one burst similar to GRB 170817A per year.