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We use data from Gaia's second data release (DR2) to constrain the initial–final mass relation (IFMR) for field stars with initial masses 0.9 ≲ m_in/M_⊙ ≲ 8. Precise parallaxes have revealed unprecedented substructure in the white dwarf (WD) cooling sequence on the color–magnitude diagram (CMD). Some of this substructure stems from the diversity of WD atmospheric compositions, but the CMD remains bimodal even when only spectroscopically confirmed DA WDs are considered. We develop a generative model to predict the CMD for DA WDs as a function of the initial mass function, stellar age distribution, and a flexibly parameterized IFMR. We then fit the CMD of 1100 bright DA WDs within 100 pc, for which atmospheric composition and completeness are well understood. The resulting best-fit IFMR flattens at 3.5 ≲ m_in/M_⊙ ≲ 5.5, producing a secondary peak in the WD mass distribution at m_WD ∼ 0.8 M_⊙. Our IFMR is broadly consistent with weaker constraints obtained from binaries and star clusters in previous work but represents the clearest observational evidence obtained to date of theoretically predicted nonlinearity in the IFMR. A visibly bimodal CMD is only predicted for mixed-age stellar populations: in single-age clusters, more massive WDs reach the bottom of the cooling sequence before the first lower-mass WDs appear. This may explain why bimodal cooling sequences have thus far evaded detection in cluster CMDs.

The process by which massive galaxies transition from blue, star-forming disks into red, quiescent galaxies remains one of the most poorly understood aspects of galaxy evolution. In this investigation, we attempt to gain a better understanding of how star formation is quenched by focusing on a massive post-starburst galaxy at z = 0.747. The target has a high stellar mass and a molecular gas fraction of 30%—unusually high for its low star formation rate (SFR). We look for indicators of star formation suppression mechanisms in the stellar kinematics and age distribution of the galaxy obtained from spatially resolved Gemini Integral-field spectra and in the gas kinematics obtained from the Atacama Large Millimeter/submillimeter Array (ALMA). We find evidence of significant rotation in the stars, but we do not detect a stellar age gradient within 5 kpc. The molecular gas is aligned with the stellar component, and we see no evidence of strong gas outflows. Our target may represent the product of a merger-induced starburst or of morphological quenching; however, our results are not completely consistent with any of the prominent quenching models.

The bright, erratic black hole X-ray binary GRS 1915+105 has long been a target for studies of disk instabilities, radio/infrared jets, and accretion disk winds, with implications that often apply to sources that do not exhibit its exotic X-ray variability. With the launch of the Neutron star Interior Composition Explorer (NICER), we have a new opportunity to study the disk wind in GRS 1915+105 and its variability on short and long timescales. Here we present our analysis of 39 NICER observations of GRS 1915+105 collected during five months of the mission data validation and verification phase, focusing on Fe xxv and Fe xxvi absorption. We report the detection of strong Fe xxvi in 32 (>80%) of these observations, with another four marginal detections; Fe xxv is less common, but both likely arise in the well-known disk wind. We explore how the properties of this wind depend on broad characteristics of the X-ray lightcurve: mean count rate, hardness ratio, and fractional rms variability. The trends with count rate and rms are consistent with an average wind column density that is fairly steady between observations but varies rapidly with the source on timescales of seconds. The line dependence on spectral hardness echoes the known behavior of disk winds in outbursts of Galactic black holes; these results clearly indicate that NICER is a powerful tool for studying black hole winds.

The nature of magnetic reconnection in planetary magnetospheres may differ between various planets. We report the first observations of a rapidly evolving magnetic reconnection process in Mercury's magnetotail by the MESSENGER spacecraft. The reconnection process was initialized in the plasma sheet and then evolved into the lobe region during a ∼35 s period. The tailward reconnection fronts of primary and secondary flux ropes with clear Hall signatures and energetic electron bursts were observed. The reconnection timescale of a few seconds is substantially shorter than that of terrestrial magnetospheric plasmas. The normalized reconnection rate during a brief quasi-steady period is estimated to be ∼0.2 on average. The observations show the rapid and impulsive nature of the exceedingly driven reconnection in Mercury's magnetospheric plasma that may be responsible for the much more dynamic magnetosphere of Mercury.

The ultraviolet (UV) extinction feature at 2175 Å is ubiquitously observed in the Galaxy but is rarely detected at high redshifts. Here we report the spectroscopic detection of the 2175 Å bump on the sightline to the γ-ray burst (GRB) afterglow GRB 180325A at z = 2.2486, the only unambiguous detection over the past 10 years of GRB follow-up, at four different epochs with the Nordic Optical Telescope (NOT) and the Very Large Telescope (VLT)/X-shooter. Additional photometric observations of the afterglow are obtained with the Gamma-Ray burst Optical and Near-Infrared Detector (GROND). We construct the near-infrared to X-ray spectral energy distributions (SEDs) at four spectroscopic epochs. The SEDs are well described by a single power law and an extinction law with R_V ≈ 4.4, A_V ≈ 1.5, and the 2175 Å extinction feature. The bump strength and extinction curve are shallower than the average Galactic extinction curve. We determine a metallicity of [Zn/H] > −0.98 from the VLT/X-shooter spectrum. We detect strong neutral carbon associated with the GRB with equivalent width of W_r(λ 1656) = 0.85 ± 0.05. We also detect optical emission lines from the host galaxy. Based on the Hα emission-line flux, the derived dust-corrected star formation rate is ∼46 ± 4 M_⊙ yr⁻¹ and the predicted stellar mass is log M_*/M_⊙ ∼ 9.3 ± 0.4, suggesting that the host galaxy is among the main-sequence star-forming galaxies.

We present the first detection of OH absorption in diffuse gas at z > 0, along with another eight stringent limits on OH column densities for cold atomic gas in galaxies at 0 < z < 0.4. The absorbing gas detected toward Q0248+430 (z_q = 1.313) originates from a tidal tail emanating from a highly star-forming galaxy G0248+430 (z_g = 0.0519) at an impact parameter of 15 kpc. The measured column density is N(OH) = (6.3 ± 0.8) × 10¹³$\left(\tfrac{{T}_{\mathrm{ex}}}{3.5}\right)\left(\tfrac{1.0}{{f}_{c}^{\mathrm{OH}}}\right)$ cm⁻², where ${f}_{c}^{\mathrm{OH}}$ and T_ex are the covering factor and the excitation temperature of the absorbing gas, respectively. In our Galaxy, the column densities of OH in diffuse clouds are of the order of N(OH) ∼ 10^13–14 cm⁻². From the incidence (number per unit redshift; n₂₁) of H i 21 cm absorbers at 0.5 < z < 1 and assuming no redshift evolution, we estimate the incidence of OH absorbers (with log N(OH) > 13.6) to be n_OH = ${0.008}_{-0.008}^{+0.018}$ at z ∼ 0.1. Based on this we expect to detect ${10}_{-10}^{+20}$ such OH absorbers from the MeerKAT Absorption Line Survey (MALS). Using H i 21 cm and OH 1667 MHz absorption lines detected toward Q0248+430, we estimate (ΔF/F) = (5.2 ± 4.5) × 10⁻⁶, where $F\equiv {g}_{p}{({\alpha }^{2}/\mu )}^{1.57}$, α is the fine structure constant, μ is the electron–proton mass ratio, and g_p is the proton gyromagnetic ratio. This corresponds to Δα/α(z = 0.0519) = (1.7 ± 1.4) × 10⁻⁶, which is among the stringent constraints on the fractional variation of α.

In nuclear star clusters, the potential is governed by the central massive black hole (MBH), so that stars move on nearly Keplerian orbits and the total potential is almost stationary in time. Yet, the deviations of the potential from the Keplerian one, due to the enclosed stellar mass and general relativity, will cause the stellar orbits to precess. Moreover, as a result of the finite number of stars, small deviations of the potential from spherical symmetry induce residual torques that can change the stars' angular momentum faster than the standard two-body relaxation. The combination of these two effects drives a stochastic evolution of orbital angular momentum, a process named "resonant relaxation" (RR). Owing to recent developments in the description of the relaxation of self-gravitating systems, we can now fully describe scalar resonant relaxation (relaxation of the magnitude of the angular momentum) as a diffusion process. In this framework, the potential fluctuations due to the complex orbital motion of the stars are described by a random correlated noise with statistical properties that are fully characterized by the stars' mean field motion. On long timescales, the cluster can be regarded as a diffusive system with diffusion coefficients that depend explicitly on the mean field stellar distribution through the properties of the noise. We show here, for the first time, how the diffusion coefficients of scalar RR, for a spherically symmetric system, can be fully calculated from first principles, without any free parameters. We also provide an open source code that evaluates these diffusion coefficients numerically.

We carry out high-resolution calculations for the stellar convection zone. The main purpose of this Letter is to investigate the effect of a small-scale dynamo on the differential rotation. The solar differential rotation deviates from the Taylor–Proudman state in which the angular velocity does not change along the rotational axis. To break the Taylor–Proudman state deep in the convection zone, it is thought that a latitudinal entropy gradient is required. In this Letter, we find that the small-scale dynamo has three roles in the deviation of the stellar differential rotation from the Taylor–Proudman state. 1) The shear of the angular velocity is suppressed. This leads to a situation where the latitudinal entropy gradient efficiently breaks the Taylor–Proudman state. 2) The perturbation of the entropy increases with the suppression of the turbulent velocity between upflows and downflows. 3) The convection velocity is reduced. This increases the effect of the rotation on the convection. The second and third factors increase the latitudinal entropy gradient and break the Taylor–Proudman state. We find that an efficient small-scale dynamo has a significant impact on the stellar differential rotation.

Using the observations from the Optical and Near-infrared Solar Eruption Tracer (ONSET) and the Solar Dynamics Observatory (SDO), we study an M5.7 flare in AR 11476 on 2012 May 10 and a micro-flare in the quiet Sun on 2017 March 23. Before the onset of each flare, there is a reverse S-shaped filament above the polarity inversion line, then the filaments become unstable and begin to rise. The rising filaments gain the upper hand over the tension force of the dome-like overlying loops and thus successfully erupt outward. The footpoints of the reconnecting overlying loops successively brighten and are observed as two flare ribbons, while the newly formed low-lying loops appear as post-flare loops. These eruptions are similar to the classical model of successful filament eruptions associated with coronal mass ejections (CMEs). However, the erupting filaments in this study move along large-scale lines and eventually reach the remote solar surface; i.e., no filament material is ejected into the interplanetary space. Thus, both the flares are confined. These results reveal that some successful filament eruptions can trigger confined flares. Our observations also imply that this kind of filament eruption may be ubiquitous on the Sun, from active regions (ARs) with large flares to the quiet Sun with micro-flares.

The Sun occasionally goes through Maunder-like extended grand minima when its magnetic activity drops considerably from the normal activity level for several decades. Many possible theories have been proposed to explain the origin of these minima. However, how the Sun managed to recover from such inactive phases every time is even more enigmatic. The Babcock–Leighton type dynamos, which are successful in explaining many features of the solar cycle remarkably well, are not expected to operate during grand minima due to the lack of a sufficient number of sunspots. In this Letter, we explore the question of how the Sun could recover from grand minima through the Babcock–Leighton dynamo. In our three-dimensional dynamo model, grand minima are produced spontaneously as a result of random variations in the tilt angle of emerging active regions. We find that the Babcock–Leighton process can still operate during grand minima with only a minimal number of sunspots, and that the model can emerge from such phases without the need for an additional generation mechanism for the poloidal field. The essential ingredient in our model is a downward magnetic pumping, which inhibits the diffusion of the magnetic flux across the solar surface.

This Letter reports the discovery of five new globular clusters (GCs) in the Galactic bulge (Camargo 1102, 1103, 1104, 1105, and 1106) using Wide-field Infrared Survey Explorer (WISE) images. Their natures are established by using 2MASS and Gaia second data release (DR2) photometry. The new findings are old and metal-poor GCs located less than 4 kpc from the Galactic center. Camargo 1102 seems to be located over the Galactic bar on the far side of the Milky Way and at a vertical distance lower than 1 kpc. The other four clusters lie even closer to the Milky Way mid-plane. The old ages and low metallicities suggest that the newly discovered GCs may have the potential of providing important clues on the early inner Galaxy formation and its subsequent evolution, as well as the current bulge structure and kinematics.

We report on a Neutron star Interior Composition Explorer (NICER) observation of the Galactic X-ray binary and stellar-mass black hole candidate, MAXI J1535−571. The source was likely observed in an "intermediate" or "very high" state, with important contributions from both an accretion disk and hard X-ray corona. The 2.3–10 keV spectrum shows clear hallmarks of relativistic disk reflection. Fits with a suitable model strongly indicate a near-maximal spin parameter of $a={cJ}/{{GM}}^{2}=0.994(2)$ and a disk that extends close to the innermost stable circular orbit, $r/{r}_{\mathrm{ISCO}}=1.08(8)$ (1σ statistical errors). In addition to the relativistic spectrum from the innermost disk, a relatively narrow Fe K emission line is also required. The resolution of NICER reveals that the narrow line may be asymmetric, indicating a specific range of emission radii. Fits with a relativistic line model suggest an inner radius of $r={144}_{-60}^{+140}\,{GM}/{c}^{2}$ for the putative second reflection geometry; full reflection models suggest that radii a few times larger are possible. The origin of the narrow line is uncertain, but a warp likely provides the most physically plausible explanation. We discuss our results in terms of the potential for NICER to reveal new features of the inner and intermediate accretion disk around black holes.

We report on the detection of waves of magnetic-field variations that were associated with flare-excited sunquake waves. An X-9.3 flare that occurred on 2017 September 6 excited strong sunquakes, and the sunquake waves were observed sweeping across the flare's host active region. This rare event gives us an unprecedented opportunity to study responses of magnetic field to passing sunquake waves. A wave of magnetic-field variations was observed in each of the two sunspots that the sunquake waves swept through, and the time–distance relations for the waves observed in magnetic field and Doppler velocity are similar. The phase relations measured between, as well as the oscillatory power distributions calculated from, the Doppler velocity variations and magnetic-field variations associated with the sunquake waves are compared with those obtained from the background waves in the same areas of the sunspot umbra and penumbra separately. The phase relations seem to favor the theory that the waves of magnetic variations are owing to opacity changes associated with the passing sunquake waves. The comparisons of phases and power distributions indicate that the background magnetic variations observed in sunspots are a combination of various wave modes, and fast magnetoacoustic waves only account for a fraction of those magnetic variations.

Proxima b is a terrestrial-mass planet in the habitable zone of Proxima Centauri. Proxima Centauri's high stellar activity, however, casts doubt on the habitability of Proxima b: sufficiently bright and frequent flares and any associated proton events may destroy the planet's ozone layer, allowing lethal levels of UV flux to reach its surface. In 2016 March, the Evryscope observed the first naked-eye-brightness superflare detected from Proxima Centauri. Proxima increased in optical flux by a factor of ∼68 during the superflare and released a bolometric energy of 10^33.5 erg, ∼10× larger than any previously detected flare from Proxima. Over the last two years the Evryscope has recorded 23 other large Proxima flares ranging in bolometric energy from 10^30.6 to 10^32.4 erg; coupling those rates with the single superflare detection, we predict that at least five superflares occur each year. Simultaneous high-resolution High Accuracy Radial velocity Planet Searcher (HARPS) spectroscopy during the Evryscope superflare constrains the superflare's UV spectrum and any associated coronal mass ejections. We use these results and the Evryscope flare rates to model the photochemical effects of NO_x atmospheric species generated by particle events from this extreme stellar activity, and show that the repeated flaring may be sufficient to reduce the ozone of an Earth-like atmosphere by 90% within five years; complete depletion may occur within several hundred kyr. The UV light produced by the Evryscope superflare would therefore have reached the surface with ∼100× the intensity required to kill simple UV-hardy microorganisms, suggesting that life would have to undergo extreme adaptations to survive in the surface areas of Proxima b exposed to these flares.