adsabs/SciX_UAT_keywords huggingface dataset:
Isochronic Evolution and the Radioactive Decay of r-process Nuclei

We report on the creation and application of a novel decay network that uses the latest data from experiment and evaluation. We use the network to simulate the late-time phase of the rapid neutron capture (r) process. In this epoch, the bulk of nuclear reactions, such as radiative capture, have ceased, and nuclear decays are the dominant transmutation channels. We find that the decay from short-lived to long-lived species naturally leads to an isochronic evolution in which nuclei with similar half-lives are populated at the same time. We consider random perturbations along each isobaric chain to initial solar-like r-process compositions to demonstrate the isochronic nature of the late-time phase of the r-process. Our analysis shows that detailed knowledge of the final isotopic composition allows for the prediction of late-time evolution with a high degree of confidence despite uncertainties that exist in astrophysical conditions and the nuclear physics properties of the most neutron-rich nuclei. We provide the time-dependent nuclear composition in the Appendix as supplemental material.

Keywords: r-process; nucleosynthesis; nuclear astrophysics
Reconstructing the Assembly of Massive Galaxies. II. Galaxies Develop Massive and Dense Stellar Cores as They Evolve and Head toward Quiescence at Cosmic Noon

We use the spectral energy distribution fitting code PROSPECTOR to reconstruct the nonparametric star formation history (SFH) of massive ( $\mathrm{log}{M}_{* }\gt 10.3$ ) star-forming galaxies (SFGs) and quiescent galaxies (QGs) at redshift z obs ~ 2 to investigate the joint evolution of star formation activity and structural properties. Compared to extended SFGs, compact SFGs are more likely to have experienced multiple star formation episodes, with the fractional mass formed during the older (≥1 Gyr) episode being larger, suggesting that high-redshift SFGs assembled their central regions earlier and then kept growing in central mass as they become more compact. The SFH of compact QGs does not significantly differ from the average for this category, and shows an early burst followed by a gradual decline of the star formation rate. The SFH of extended QGs, however, is similar to that of post-starburst galaxies, and their morphology is also frequently disturbed. Knowledge of the SFH also enables us to empirically reconstruct the structural evolution of individual galaxies. While the progenitor effect is clearly observed and accounted for in our analysis, it alone is insufficient to explain the observed structural evolution. We show that, as they evolve from star-forming phase to quiescence, galaxies grow massive dense stellar cores. Quenching begins at the center and then propagates outward to the rest of the structure. We discuss possible physical scenarios for the observed evolution and find that our empirical constraints are in good quantitative agreement with the model predictions from dissipative accretion of gas to the center followed by massive starbursts before final quiescence (wet compaction).

Keywords: galaxy formation; galaxy evolution; galaxy quenching; galaxy structure; high-redshift galaxies
Milky Way Satellite Census. II. Galaxy-Halo Connection Constraints Including the Impact of the Large Magellanic Cloud

The population of Milky Way (MW) satellites contains the faintest known galaxies and thus provides essential insight into galaxy formation and dark matter microphysics. Here we combine a model of the galaxy-halo connection with newly derived observational selection functions based on searches for satellites in photometric surveys over nearly the entire high Galactic latitude sky. In particular, we use cosmological zoom-in simulations of MW-like halos that include realistic Large Magellanic Cloud (LMC) analogs to fit the position-dependent MW satellite luminosity function. We report decisive evidence for the statistical impact of the LMC on the MW satellite population due to an estimated 6 ± 2 observed LMC-associated satellites, consistent with the number of LMC satellites inferred from Gaia proper-motion measurements, confirming the predictions of cold dark matter models for the existence of satellites within satellite halos. Moreover, we infer that the LMC fell into the MW within the last 2 Gyr at high confidence. Based on our detailed full-sky modeling, we find that the faintest observed satellites inhabit halos with peak virial masses below $3.2\times {10}^{8}\ {M}_{\odot }$ at 95% confidence, and we place the first robust constraints on the fraction of halos that host galaxies in this regime. We predict that the faintest detectable satellites occupy halos with peak virial masses above ${10}^{6}\ {M}_{\odot }$ , highlighting the potential for powerful galaxy formation and dark matter constraints from future dwarf galaxy searches.

Keywords: dark matter; milky way dark matter halo; galaxy abundances
Mass Function of a Young Cluster in a Low-metallicity Environment. Sh 2-209

We present deep near-infrared (NIR) imaging of Sh 2-209 (S209), a low-metallicity ([O/H] = -0.5 dex) H II region in the Galaxy. From the NIR images, combined with astrometric data from Gaia EDR3, we estimate the distance to S209 to be 2.5 kpc. This is close enough to enable us to resolve cluster members clearly (≃1000 au separation) down to a mass-detection limit of ≃ 0.1 M ⊙, and we have identified two star-forming clusters in S209, with individual cluster scales ~1 pc. We employ a set of model luminosity functions to derive the underlying initial mass functions (IMFs) and ages for both clusters. The IMFs we obtained for both clusters exhibit slightly flat high-mass slopes (Γ ≃ -1.0) compared to the Salpeter IMF (Γ = -1.35), and their break mass of ≃0.1 M ⊙ is lower than those generally seen in the solar neighborhood (~0.3 M ⊙). In particular, because the S209 main cluster is a star-forming cluster with a larger number of members (~1500) than the number (~100) in regions previously studied in such environments, it is possible for the first time to derive the IMF in a low-metallicity environment with high accuracy over the wide mass range of 0.1-20 M ⊙.

Keywords: h ii regions; star formation; pre-main sequence stars; low mass stars; infrared sources; open star clusters; galaxy abundances; luminosity function; stellar mass functions
The Impact of Extended H2O Cross Sections on Temperate Anoxic Planet Atmospheres: Implications for Spectral Characterization of Habitable Worlds

JWST has created a new era of terrestrial exoplanet atmospheric characterization, and with it, the possibility to detect potential biosignature gases like CH4. Our interpretation of exoplanet atmospheric spectra, and the veracity of these interpretations, will be limited by our understanding of atmospheric processes and the accuracy of input modeling data. Molecular cross sections are essential inputs to these models. The photochemistry of temperate planets depends on photolysis reactions whose rates are governed by the dissociation cross sections of key molecules. H2O is one such molecule; the photolysis of H2O produces OH, a highly reactive and efficient sink for atmospheric trace gases. We investigate the photochemical effects of improved H2O cross sections on anoxic terrestrial planets as a function of host star spectral type and CH4 surface flux. Our results show that updated H2O cross sections, extended to wavelengths >200 nm, substantially impact the predicted abundances of trace gases destroyed by OH. The differences for anoxic terrestrial planets orbiting Sun-like host stars are greatest, showing changes of up to 3 orders of magnitude in surface CO levels, and over an order of magnitude in surface CH4 levels. These differences lead to observable changes in simulated planetary spectra, especially important in the context of future direct-imaging missions. In contrast, the atmospheres of planets orbiting M-dwarf stars are substantially less affected. Our results demonstrate a pressing need for refined dissociation cross-section data for H2O, where uncertainties remain, and other key molecules, especially at mid-UV wavelengths >200 nm.

Keywords: planetary atmospheres; habitable planets; exoplanet atmospheres; exoplanets; water vapor
On the Origin of GW190521-like Events from Repeated Black Hole Mergers in Star Clusters

LIGO and Virgo have reported the detection of GW190521, from the merger of a binary black hole (BBH) with a total mass around 150 M⊙. While current stellar models limit the mass of any black hole (BH) remnant to about 40-50 M⊙, more massive BHs can be produced dynamically through repeated mergers in the core of a dense star cluster. The process is limited by the recoil kick (due to anisotropic emission of gravitational radiation) imparted to merger remnants, which can escape the parent cluster, thereby terminating growth. We study the role of the host cluster metallicity and escape speed in the buildup of massive BHs through repeated mergers. Almost independent of host metallicity, we find that a BBH of about 150 M⊙ could be formed dynamically in any star cluster with escape speed ≳200 km s-1, as found in galactic nuclear star clusters as well as the most massive globular clusters and super star clusters. Using an inspiral-only waveform, we compute the detection probability for different primary masses (≥60 M⊙) as a function of secondary mass and find that the detection probability increases with secondary mass and decreases for larger primary mass and redshift. Future additional detections of massive BBH mergers will be of fundamental importance for understanding the growth of massive BHs through dynamics and the formation of intermediate-mass BHs.

Keywords: gravitational waves; gravitational wave sources; gravitational wave detectors; gravitational wave astronomy; astrophysical black holes; black holes; intermediate-mass black holes; black hole physics; globular star clusters; open star clusters; star clusters; superclusters
Winter Weakening of Titan's Stratospheric Polar Vortices

Polar vortices are a prominent feature in Titan's stratosphere. The Cassini mission has provided a detailed view of the breakdown of the northern polar vortex and formation of the southern vortex, but the mission did not observe the full annual cycle of the evolution of the vortices. Here we use a TitanWRF general circulation model simulation of an entire Titan year to examine the full annual cycle of the polar vortices. The simulation reveals a winter weakening of the vortices, with a clear minimum in polar potential vorticity and midlatitude zonal winds between winter solstice and spring equinox. The simulation also produces the observed postfall equinox cooling followed by rapid warming in the upper stratosphere. This warming is due to strong descent and adiabatic heating, which also leads to the formation of an annular potential vorticity structure. The seasonal evolution of the polar vortices is very similar in the two hemispheres, with only small quantitative differences that are much smaller than the seasonal variations, which can be related to Titan's orbital eccentricity. This suggests that any differences between observations of the northern hemisphere vortex in late northern winter and the southern hemisphere vortex in early winter are likely due to the different observation times with respect to solstice, rather than fundamental differences in the polar vortices.

Keywords: atmospheric science; planetary atmospheres; atmospheric circulation; titan; saturnian satellites; stratosphere
Comparison of Kinematics of Solar Eruptive Prominences and Spatial Distribution of the Magnetic Decay Index

Theoretical studies of electric current instability explaining solar prominence eruptions show that the loss of equilibrium may develop in a case when the surrounding magnetic field decreases sufficiently rapidly with height. The magnetic decay index, a parameter indicating whether the external magnetic field has a configuration that may lead to a certain type of electric current instability, is a useful instrument for predicting the behavior of prominences. In our study, we consider three eruptive prominences. We perform potential-field extrapolation to obtain the spatial distribution of the magnetic decay index in the coronal space identified with the prominences. Analysis of time-dependent height profiles of the prominences revealed that eruptions started at heights close to those, where the computed magnetic decay index exceeded a value equal to 1.5. This indicates that the torus instability is a possible mechanism of the considered eruptive events.

Keywords: solar prominences; solar magnetic fields; eruptive phenomena
Determination of the Abundance of Mercury from the Hg II Line at 5677.10 Å in Late B-type Stars. II. HD 143807 and HD 16727

The weak line of Hg II located at 5677.10 Å is used to measure the mercury abundance in two HgMn stars, HD 143807 (ι CrB) and HD 16727 (11 Per). This line is detected at 3σ above the noise in the adjacent continuum in HD 143807 but not detected in HD 16727. The synthesis of this line, using the current atomic parameters available in NIST, confirms the large overabundance of mercury in HD 143807, about +4.60 dex, in very good agreement with the excess derived in earlier studies from the 3983.93 Å line. At the overabundance expected for HD 16727 (3.90 dex), the line is predicted to contribute a 1% absorption feature which is not seen in the observed spectrum.

Keywords: chemically peculiar stars; mercury-manganese stars
The Ups and Downs of Accreting X-Ray Pulsars: Decade-long Observations with the Fermi Gamma-Ray Burst Monitor

We review more than 10 yr of continuous monitoring of accreting X-ray pulsars with the all-sky Gamma-ray Burst Monitor (GBM) aboard the Fermi Gamma-ray Space Telescope. Our work includes data from the start of GBM operations in 2008 August, through to 2019 November. Pulsations from 39 accreting pulsars are observed over an energy range of 10-50 keV by GBM. The GBM Accreting Pulsars Program performs data reduction and analysis for each accreting pulsar and makes histories of the pulse frequency and pulsed flux publicly available. We examine in detail the spin histories, outbursts, and torque behaviors of the persistent and transient X-ray pulsars observed by GBM. The spin period evolution of each source is analyzed in the context of disk-accretion and quasi-spherical settling accretion-driven torque models. Long-term pulse frequency histories are also analyzed over the GBM mission lifetime and compared to those available from the previous Burst and Transient Source Experiment all-sky monitoring mission, revealing previously unnoticed episodes in some of the analyzed sources (such as a torque reversal in 2S 1845-024). We obtain new, or update known, orbital solutions for three sources. Our results demonstrate the capabilities of GBM as an excellent instrument for monitoring accreting X-ray pulsars and its important scientific contribution to this field.

Keywords: neutron stars; accretion; stellar accretion disks; x-ray transient sources
The Chemical Structure of Young High-mass Star-forming Clumps. II. Parsec-scale CO Depletion and Deuterium Fraction of HCO+

The physical and chemical properties of cold and dense molecular clouds are key to understanding how stars form. Using the IRAM 30 m and NRO 45 m telescopes, we carried out a Multiwavelength line-Imaging survey of the 70 μm-dArk and bright clOuds (MIAO). At a linear resolution of 0.1-0.5 pc, this work presents a detailed study of parsec-scale CO depletion and HCO+ deuterium (D-) fractionation toward four sources (G11.38+0.81, G15.22-0.43, G14.49-0.13, and G34.74-0.12) included in our full sample. In each source with T < 20 K and nH ∼ 104-105 cm-3, we compared pairs of neighboring 70 μm bright and dark clumps and found that (1) the H2 column density and dust temperature of each source show strong spatial anticorrelation; (2) the spatial distribution of CO isotopologue lines and dense gas tracers, such as 1-0 lines of H13CO+ and DCO+, are anticorrelated; (3) the abundance ratio between C18O and DCO+ shows a strong correlation with the source temperature; (4) both the C18O depletion factor and D-fraction of HCO+ show a robust decrease from younger clumps to more evolved clumps by a factor of more than 3; and (5) preliminary chemical modeling indicates that chemical ages of our sources are ∼8 × 104 yr, which is comparable to their free-fall timescales and smaller than their contraction timescales, indicating that our sources are likely dynamically and chemically young.

Keywords: interstellar medium; interstellar molecules; star formation; massive stars; astrochemistry
Chandra, NuSTAR, and Optical Observations of the Cataclysmic Variables IGR J17528-2022 and IGR J20063+3641

We report on Chandra, NuSTAR, and MDM observations of two International Gamma-ray Astrophysics Laboratory (INTEGRAL) sources, namely IGR J17528-2022 and IGR J20063+3641. IGR J17528-2022 is an unidentified INTEGRAL source, while IGR J20063+3641 was recently identified as a magnetic cataclysmic variable (mCV) by Halpern et al. The Chandra observation of IGR J17528-2022 has allowed us to locate the optical counterpart to the source and to obtain its optical spectrum, which shows a strong Hα emission line. The optical spectrum and flickering observed in the optical time-series photometry in combination with the X-ray spectrum, which is well fit by an absorbed partially covered thermal bremsstrahlung model, suggest that this source is a strong mCV candidate. The X-ray observations of IGR J20063+3641 reveal a clear modulation with a period of 172.46 ± 0.01 s, which we attribute to the white dwarf spin period. Additional MDM spectroscopy of the source has also allowed for a clear determination of the orbital period at 0.731 ± 0.015 days. The X-ray spectrum of this source is also well fit by an absorbed partially covered thermal bremsstrahlung model. The X-ray spectrum, spin periodicity, and orbital periodicity allow this source to be further classified as an intermediate polar.

Keywords: cataclysmic variable stars; x-ray binary stars; x-ray sources
Direct Imaging Explorations for Companions around Mid-Late M Stars from the Subaru/IRD Strategic Program

The Subaru telescope is currently performing a strategic program (SSP) using the high-precision near-infrared (NIR) spectrometer IRD to search for exoplanets around nearby mid/late M dwarfs via radial velocity (RV) monitoring. As part of the observing strategy for the exoplanet survey, signatures of massive companions such as RV trends are used to reduce the priority of those stars. However, this RV information remains useful for studying the stellar multiplicity of nearby M dwarfs. To search for companions around such "deprioritized" M dwarfs, we observed 14 IRD-SSP targets using Keck/NIRC2 with pyramid wave-front sensing at NIR wavelengths, leading to high sensitivity to substellar-mass companions within a few arcseconds. We detected two new companions (LSPM J1002+1459 B and LSPM J2204+1505 B) and two new candidates that are likely companions (LSPM J0825+6902 B and LSPM J1645+0444 B), as well as one known companion. Including two known companions resolved by the IRD fiber injection module camera, we detected seven (four new) companions at projected separations between ~2 and 20 au in total. A comparison of the colors with the spectral library suggests that LSPM J2204+1505 B and LSPM J0825+6902 B are located at the boundary between late M and early L spectral types. Our deep high-contrast imaging for targets where no bright companions were resolved did not reveal any additional companion candidates. The NIRC2 detection limits could constrain potential substellar-mass companions (~10-75 M Jup) at 10 au or further. The failure with Keck/NIRC2 around the IRD-SSP stars having significant RV trends makes these objects promising targets for further RV monitoring or deeper imaging with the James Webb Space Telescope to search for smaller-mass companions below the NIRC2 detection limits.

Keywords: exoplanets; brown dwarfs; m dwarf stars; direct imaging
Footprints of Population III Stars in the Gravitational-wave Background

We investigate detection prospects of the gravitational-wave background (GWB) that originates from the merging of compact objects formed by the collapse of Population III stars. Younger Population I/II stars lead to a GWB in the LIGO/Virgo frequency band at the inspiral phase, while Population III stars would likely show up at the later merger and ringdown phases. We show that, using a network of third-generation detectors, we may be able to separate a Population I/II signal from a Population III one, provided we can subtract individual coalescence events. A detection of a Population III GWB could reveal important information, such as the average redshifted total mass.

Keywords: population iii stars; gravitational wave astronomy
Diffusive Shock Acceleration: Breakdown of Spatial Diffusion and Isotropy

We point out that particles accelerated in a nonrelativistic shock of compression ratio r do not necessarily attain the standard, p = (r + 2)/(r - 1) spectral index. Previous derivations of the spectrum, based on the approximations of spatial diffusion or negligible anisotropy, are shown to rely on unjustified implicit assumptions. We prove analytically that the standard result is nevertheless valid in the limit of an isotropic medium. For an anisotropic medium, the problem generally requires a numerical treatment; the standard result remains valid as long as the anisotropy is not too strong, but p can substantially deviate from the standard result for sufficiently anisotropic scattering, even in the small-angle scattering limit. Additional spectral modifications, for example, by motions of scattering modes at intermediate optical depths from the shock, are discussed.

Keywords: shocks; cosmic rays; magnetic fields
Speckle Interferometry at SOAR in 2019

The results of speckle-interferometric observations at the 4.1 m Southern Astrophysical Research Telescope in 2019 are given, totaling 2555 measurements of 1972 resolved pairs with separations from 15 mas (median 0"21) and magnitude difference up to 6 mag, and non-resolutions of 684 targets. We resolved for the first time 90 new pairs or subsystems in known binaries. This work continues our long-term speckle program. Its main goal is to monitor orbital motion of close binaries, including members of high-order hierarchies and Hipparcos pairs in the solar neighborhood. We give a list of 127 orbits computed using our latest measurements. Their quality varies from excellent (25 orbits of grades 1 and 2) to provisional (47 orbits of grades 4 and 5).

Keywords: multiple stars; binary stars; visual binary stars
The Behavior of Bp Si Stars in the Far UV-Paper V: HD 92664

The analysis of spectra of the B8 Si-rich HD 92664 recorded by the SWP camera on board the International Ultraviolet Explorer reveals variations of the far-ultraviolet spectral energy distribution shortwards of 1700 Å over a time interval of about 2 months. In contrast, the mid-UV flux does not vary which suggests that changes of the silicon continuous and line opacity are the cause of the variations. The iron-peak elements, chromium, manganese and iron play a lesser role than silicon in these variations. Phased on previous ephemeris, the FUV minimum occurs at the rotational phase, ϕ = 0.0, which corresponds to maximum light in the V-band and minimum longitudinal magnetic field.

Keywords: chemically peculiar stars; bp stars