Sazuna/UAT_keywords huggingface dataset:
On the Spectral Evolution of Hot White Dwarf Stars. III. The PG 1159-DO-DB-DQ Evolutionary Channel Revisited. We continue our comprehensive theoretical investigation of the spectral evolution of white dwarfs based on sophisticated simulations of element transport. In this paper, we focus on the transformation of PG 1159 stars into DO/DB white dwarfs due to the gravitational settling of heavy elements and then into DQ white dwarfs through the convective dredge-up of carbon. We study the impact of several physical parameters on the evolution of the surface carbon abundance over a wide range of effective temperatures. In the hot PG 1159 and DO phases, our calculations confirm that the temperature of the PG 1159-to-DO transition depends sensitively on the stellar mass and the wind mass-loss rate. We show that measured carbon abundances of DOZ white dwarfs are mostly accounted for by our models, with the notable exception of the coolest DOZ stars. In the cooler DB and DQ phases, the predicted atmospheric composition is strongly influenced by the stellar mass, the thickness of the envelope, the initial carbon content, the efficiency of convective overshoot, and the presence of residual hydrogen. We demonstrate that, under reasonable assumptions, our simulations reproduce very well the observed carbon abundance pattern of DQ stars, which thus allows us to constrain the extent of the overshoot region in cool helium-rich white dwarfs. We also argue that our calculations naturally explain a number of recent empirical results, such as the relative excess of low-mass DQ stars and the presence of trace hydrogen and/or carbon at the surface of most DC and DZ stars.

Keywords: DB stars, Stellar convection envelopes, White dwarf stars, DO stars, Stellar evolutionary models, Stellar diffusion, Stellar winds, Atmospheric composition, DQ stars
The Donor of the Black Hole X-Ray Binary MAXI J1820+070. We estimate the parameters of the donor of the accreting black hole binary MAXI J1820+070. The measured values of the binary period, rotational and radial velocities, and constraints on the orbital inclination imply the donor is a subgiant with the mass of ${M}_{2}\approx {0.49}_{-0.10}^{+0.10}{M}_{\odot }$ and the radius of ${R}_{2}\approx {1.19}_{-0.08}^{+0.08}{R}_{\odot }$ . We reanalyze the previously obtained optical spectrum from the Gran Telescopio Canarias and found it yields a strict lower limit on the effective temperature of T &gt; 4200 K. We compile optical and infrared fluxes observed during the quiescence of this system. From the minima r- and i-band fluxes found in Pan-STARSS1 Data Release 2 prediscovery imaging and for a distance of D ≈ 3 kpc, reddening of E(B - V) = 0.23, and R 2 ≈ 1.11R ⊙, we find T ≲ 4230 K, very close to the above lower limit. For a larger distance, the temperature can be higher, up to about 4500 K (corresponding to a K5 spectral type, preferred by previous studies) at D = 3.5 kpc, allowed by the Gaia parallax. We perform evolutionary calculations for the binary system and compare them to the observational constraints. Our model fitting the above temperature and radius constraints at D ≈ 3 kpc has a mass of 0.4M ⊙, T ≈ 4200 K, and solar metallicity. Two alternative models require D ≳ 3.3-3.4 kpc at 0.4M ⊙, T ≈ 4500 K and half-solar metallicity, and 0.5M ⊙, T ≈ 4300 K, and solar metallicity. These models yield mass-transfer rates of ~10-10 M ⊙ yr-1, compatible with those based on the estimated accreted mass of ≈2 × 1025 g and the time between the 2018 discovery and the 1934 historical outburst.

Keywords: Low-mass x-ray binary stars, X-ray binary stars, Stellar mass black holes, Astrophysical black holes
Detection of Travel Time Anisotropy from Subsurface Horizontal Magnetic Fields. A time-distance measurement technique is derived to isolate phase travel time anisotropy caused by subsurface horizontal magnetic fields; a method that uses the measured anisotropy to estimate the field's orientation is also derived. A simulation of acoustic waves propagating in a uniform, inclined magnetic field with solar background structure is used to verify the derived technique. Then the procedure is applied to a numerical simulation of a sunspot for which the subsurface state is known to provide context for the results obtained from the study of several sunspots observed by the Helioseismic and Magnetic Imager. Significant anisotropies are detected, on the order of 1 minute, and the subsurface field's azimuth is estimated and compared with the azimuth of the surface magnetic field. In all cases, the subsurface azimuth is found to be well aligned with that of the surface, and the results from the numerical simulation are used to interpret features in the detected travel time anisotropy.

Keywords: Helioseismology, Solar active region magnetic fields, Sunspots
A Highly Settled Disk around Oph163131. High dust density in the midplane of protoplanetary disks is favorable for efficient grain growth and can allow fast formation of planetesimals and planets, before disks dissipate. Vertical settling and dust trapping in pressure maxima are two mechanisms allowing dust to concentrate in geometrically thin and high-density regions. In this work, we aim to study these mechanisms in the highly inclined protoplanetary disk SSTC2D J163131.2-242627 (Oph 163131, i ~ 84°). We present new high-angular-resolution continuum and 12CO ALMA observations of Oph 163131. The gas emission appears significantly more extended in the vertical and radial direction compared to the dust emission, consistent with vertical settling and possibly radial drift. In addition, the new continuum observations reveal two clear rings. The outer ring, located at ~100 au, is well-resolved in the observations, allowing us to put stringent constraints on the vertical extent of millimeter dust particles. We model the disk using radiative transfer and find that the scale height of millimeter-sized grains is 0.5 au or less at 100 au from the central star. This value is about one order of magnitude smaller than the scale height of smaller micron-sized dust grains constrained by previous modeling, which implies that efficient settling of the large grains is occurring in the disk. When adopting a parametric dust settling prescription, we find that the observations are consistent with a turbulent viscosity coefficient of about α ≲ 10-5 at 100 au. Finally, we find that the thin dust scale height measured in Oph 163131 is favorable for planetary growth by pebble accretion: a 10 M E planet may grow within less than 10 Myr, even in orbits exceeding 50 au.

Keywords: Dust continuum emission, Radiative transfer, Protoplanetary disks, Planet formation
The Curious Case of ASASSN-20hx: A Slowly Evolving, UV- and X-Ray-Luminous, Ambiguous Nuclear Transient. We present observations of ASASSN-20hx, a nearby ambiguous nuclear transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We observed ASASSN-20hx from -30 to 275 days relative to the peak UV/optical emission using high-cadence, multiwavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite data, we determine that the ANT began to brighten on 2020 June 22.8 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 30 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity of L = (3.15 ± 0.04) × 1043 erg s-1. The subsequent decline is slower than any TDE observed to date and consistent with many other ANTs. Compared to an archival X-ray detection, the X-ray luminosity of ASASSN-20hx increased by an order of magnitude to L  x  ~ 1.5 × 1042 erg s-1 and then slowly declined over time. The X-ray emission is well fit by a power law with a photon index of Γ ~ 2.3-2.6. Both the optical and near-infrared spectra of ASASSN-20hx lack emission lines, unusual for any known class of nuclear transient. While ASASSN-20hx has some characteristics seen in both tidal disruption events and active galactic nuclei, it cannot be definitively classified with current data.

Keywords: Accretion, Black hole physics, Supermassive black holes, Tidal disruption, Active galactic nuclei
Wave-optical Effects in the Microlensing of Continuous Gravitational Waves by Star Clusters. Rapidly rotating neutron stars are promising sources for existing and upcoming gravitational-wave interferometers. While relatively dim, these systems are expected to emit continuously, allowing for signal to be accumulated through persistent monitoring over year-long timescales. If, at some point during the observational window, the source comes to lie behind a dense collection of stars, transient gravitational lensing may occur. Such events, though rare, would modulate the waveform, induce phase drifts, and ultimately affect parameter inferences concerning the nuclear equation of state and/or magnetic field structure of the neutron star. Importantly, the radiation wavelength will typically exceed the Schwarzschild radius of the individual perturbers in this scenario, implying that (micro)lensing occurs in the diffractive regime, where geometric optics does not apply. In this paper, we make use of numerical tools that borrow from Picard-Lefschetz theory to efficiently evaluate the relevant Fresnel-Kirchhoff integrals for n ≳ 102 microlenses. Modulated strain profiles are constructed both in general and for particular neutron star trajectories relative to some simulated macrolenses.

Keywords: Neutron stars, Gravitational wave sources, Gravitational waves, Gravitational microlensing, Magnetars, Magnetic fields
Quantifying Feedback from Narrow Line Region Outflows in Nearby Active Galaxies. IV. The Effects of Different Density Estimates on the Ionized Gas Masses and Outflow Rates. Active galactic nuclei (AGN) can launch outflows of ionized gas that may influence galaxy evolution, and quantifying their full impact requires spatially resolved measurements of the gas masses, velocities, and radial extents. We previously reported these quantities for the ionized narrow-line region outflows in six low-redshift AGN, where the gas velocities and extents were determined from Hubble Space Telescope long-slit spectroscopy. However, calculating the gas masses required multicomponent photoionization models to account for radial variations in the gas densities, which span ~6 orders of magnitude. To simplify this method for larger samples with less spectral coverage, we compare these gas masses with those calculated from techniques in the literature. First, we use a recombination equation with three different estimates for the radial density profiles. These include constant densities, those derived from [S II], and power-law profiles based on constant values of the ionization parameter (U). Second, we use single-component photoionization models with power-law density profiles based on constant U, and allow U to vary with radius based on the [O III]/Hβ ratios. We find that assuming a constant density of n H = 102 cm-3 overestimates the gas masses for all six outflows, particularly at small radii where the outflow rates peak. The use of [S II] marginally matches the total gas masses, but also overestimates at small radii. Overall, single-component photoionization models where U varies with radius are able to best match the gas mass and outflow rate profiles when there are insufficient emission lines to construct detailed models.

Keywords: AGN host galaxies, Supermassive black holes, Markarian galaxies, Active galaxies, Emission line galaxies, Seyfert galaxies, Galaxy winds, Galaxy kinematics, Active galactic nuclei, Interstellar medium wind
SpeX Near-infrared Spectroscopic Extinction Curves in the Milky Way. Interstellar dust extinction curves provide valuable information about dust properties, including the composition and size of the dust grains, and are essential to correct observations for the effects of interstellar dust. In this work, we measure a representative sample of near-infrared (NIR; 0.8-5.5 μm) spectroscopic extinction curves for the first time, enabling us to investigate the extinction at wavelengths where it is usually only measured in broad photometric bands. We use IRTF/SpeX spectra of a sample of reddened and comparison stars to measure 15 extinction curves with the pair method. Our sample spans A(V) values from 0.78 to 5.65 and R(V) values from 2.43 to 5.33. We confirm that the NIR extinction curves are well fit by a power law, with indices and amplitudes differing from sight line to sight line. Our average diffuse NIR extinction curve can be represented by a single power law with index α = 1.7, but because of the sight line-to-sight line variations, the shape of any average curve will depend on the parental sample. We find that most of the variation in our sample can be linked to the ratio of total-to-selective extinction R(V), a rough measurement of the average dust grain size. Two sight lines in our sample clearly show the ice extinction feature at 3 μm, which can be fitted by a modified Drude profile. We find tentative ice detections with slightly over 3σ significance in two other sight lines. In our average diffuse extinction curve, we measure a 3σ upper limit of A(ice)/A(V) = 0.0021 for this ice feature.

Keywords: Interstellar dust, Interstellar dust extinction, Reddening law, Extinction, Interstellar reddening, Interstellar medium, Interstellar extinction, Reddened stars, Milky Way Galaxy, Near infrared astronomy
Massive Young Stellar Objects in the Galactic Center. II. Seeing Through the Ice-rich Envelopes. To study the demographics of interstellar ices in the Central Molecular Zone (CMZ) of the Milky Way, we obtain near-infrared spectra of 109 red point sources using NASA IRTF/SpeX at Maunakea. We select the sample from near- and mid-infrared photometry, including 12 objects in the previous paper of this series, to ensure that these sources trace a large amount of absorption through clouds in each line of sight. We find that most of the sample (100 objects) show CO band-head absorption at 2.3 μm, tagging them as red (super-) giants. Despite the photospheric signature, however, a fraction of the sample with L-band spectra (9/82 = 0.11) exhibit large H2O ice column densities (N &gt; 2 × 1018 cm-2), and six of them also reveal CH3OH ice absorption. As one of such objects is identified as a young stellar object (YSO) in our previous work, these ice-rich sight lines are likely associated with background stars in projection to an extended envelope of a YSO or a dense cloud core. The low frequency of such objects in the early stage of stellar evolution implies a low star-formation rate (≲0.02 M ⊙ yr-1), reinforcing the previous claim on the suppressed star-formation activity in the CMZ. Our data also indicate that the strong "shoulder" CO2 ice absorption at 15.4 μm observed in YSO candidates in the previous paper arises from CH3OH-rich ice grains having a large CO2 concentration [N(CO2)/N(CH3OH) ≈ 1/3].

Keywords: Galactic center, Ice formation, Interstellar abundances, Astrochemistry, Young stellar objects, Protostars
Relativistic Description of Dense Matter Equation of State and Compatibility with Neutron Star Observables: A Bayesian Approach. The general behavior of the nuclear equation of state (EOS), relevant for the description of neutron stars (NSs), is studied within a Bayesian approach applied to a set of models based on a density-dependent relativistic mean-field description of nuclear matter. The EOS is subjected to a minimal number of constraints based on nuclear saturation properties and the low-density pure neutron matter EOS obtained from a precise next-to-next-to-next-to-leading order (N3LO) calculation in chiral effective field theory (χEFT). The posterior distributions of the model parameters obtained under these minimal constraints are employed to construct the distributions of various nuclear matter properties and NS properties such as radii, tidal deformabilities, central energy densities, speeds of sound, etc. We found that a 90% confidence interval for the allowed NS mass-radius relationship and tidal deformabilities is compatible with GW170817 and recent Neutron star Interior Composition ExploreR observations, without invoking the exotic degrees of freedom. A central speed of sound of the order of $\sqrt{2/3}$ c is obtained. The maximum NS mass allowed by the model is 2.5 M ⊙.

Keywords: Neutron stars, Nuclear astrophysics, Neutron star cores, Nuclear physics
The NICER "Reverberation Machine": A Systematic Study of Time Lags in Black Hole X-Ray Binaries. We perform the first systematic search of all NICER archival observations of black hole (and candidate) low-mass X-ray binaries for signatures of reverberation. Reverberation lags result from the light travel time difference between the direct coronal emission and the reflected disk component, and therefore their properties are a useful probe of the disk-corona geometry. We detect new signatures of reverberation lags in eight sources, increasing the total sample from three to 11, and study the evolution of reverberation lag properties as the sources evolve in outbursts. We find that in all of the nine sources with more than one reverberation lag detection, the reverberation lags become longer and dominate at lower Fourier frequencies during the hard-to-soft state transition. This result shows that the evolution in reverberation lags is a global property of the state transitions of black hole low-mass X-ray binaries, which is valuable in constraining models of such state transitions. The reverberation lag evolution suggests that the corona is the base of a jet that vertically expands and/or gets ejected during state transition. We also discover that in the hard state, the reverberation lags get shorter, just as the quasiperiodic oscillations (QPOs) move to higher frequencies, but then in the state transition, while the QPOs continue to higher frequencies, the lags get longer. We discuss the implications of the coronal geometry and physical models of QPOs in light of this new finding.

Keywords: Accretion, Stellar mass black holes, Astrophysical black holes
Integral Field Spectroscopy with the Solar Gravitational Lens. The prospect of combining integral field spectroscopy with the solar gravitational lens (SGL) to spectrally and spatially resolve the surfaces and atmospheres of extrasolar planets is investigated. The properties of hyperbolic orbits visiting the focal region of the SGL are calculated analytically, demonstrating trade-offs between departure velocity and time of arrival, as well as gravity assist maneuvers and heliocentric angular velocity. Numerical integration of the solar barycentric motion demonstrates that navigational acceleration of ${dv}\lesssim 80\tfrac{{\rm{m}}}{{\rm{s}}}+6.7\tfrac{{\rm{m}}}{{\rm{s}}}\tfrac{t}{\mathrm{year}}$ is needed to obtain and maintain alignment. Obtaining target ephemerides of sufficient precision is an open problem. The optical properties of an oblate gravitational lens are reviewed, including calculations of the magnification and the point-spread function that forms inside a telescope. Image formation for extended, incoherent sources is discussed when the projected image is smaller than, approximately equal to, and larger than the critical caustic. Sources of contamination that limit observational signal-to-noise ratio (S/N) are considered in detail, including the Sun, the solar corona, the host star, and potential background objects. A noise mitigation strategy of spectrally and spatially separating the light using integral field spectroscopy is emphasized. A pseudo-inverse-based image reconstruction scheme demonstrates that direct reconstruction of an Earth-like source from single measurements of the Einstein ring is possible when the critical caustic and observed S/N are sufficiently large. In this arrangement, a mission would not require multiple telescopes or navigational symmetry breaking, enabling continuous monitoring of the atmospheric composition and dynamics on other planets.

Keywords: Gravitational lensing, Astronomical instrumentation, Exoplanets, Spectroscopy
A Misfired Outburst in the Neutron Star X-Ray Binary Centaurus X-4. We report on a long-term optical monitoring of the neutron star X-ray binary Centaurus X-4 performed during the last 13.5 yr. This source has been in quiescence since its outburst in 1979. Our monitoring reveals the overall evolution of the accretion disk; we detect short-duration flares, likely originating also in the disk, superimposed with a small-amplitude (&lt;0.1 mag) ellipsoidal modulation from the companion star due to geometrical effects. A long-term (~2300 days) downward trend, followed by a shorter (~1000 days) upward one, is observed in the disk light curve. Such a rise in the optical has been observed for other X-ray binaries preceding outbursts, as predicted by the disk instability model. For Cen X-4, the rise of the optical flux proceeded for ~3 yr, and culminated in a flux increase at all wavelengths (optical-UV-X-rays) at the end of 2020. This increase faded after ~2 weeks, without giving rise to a full outburst. We suggest that the propagation of an inside-out heating front was ignited due to a partial ionization of hydrogen in the inner disk. The propagation might have stalled soon after the ignition due to the increasing surface density in the disk that the front encountered while propagating outward. The stall was likely eased by the low-level irradiation of the outer regions of the large accretion disk, as shown by the slope of the optical/X-ray correlation, suggesting that irradiation does not play a strong role in the optical, compared to other sources of emission.

Keywords: Neutron stars, Low-mass x-ray binary stars, Stellar accretion disks
The Solar Cycle Temporal Variation of the Solar Wind Charge Exchange X-Ray Lines. Solar wind charge exchange (SWCX) is the primary contamination of soft X-ray emission lines from the Milky Way (MW) hot gas. We report a solar cycle (≈10 yr) temporal variation of observed O VII and O VIII emission line measurements in the XMM-Newton archive, which is tightly correlated with the solar cycle traced by the sunspot number (SSN). This temporal variation is expected to be associated with the heliospheric SWCX. Another observed correlation is that higher solar wind (SW) fluxes lead to higher O VII or O VIII fluxes, which is due to the magnetospheric SWCX. We construct an empirical model to reproduce the observed correlation between the line measurements and the solar activity (i.e., the SW flux and the SSN). With this model we discovered a lag of ${0.91}_{-0.22}^{+0.20}$ yr between the O VII flux and the SSN. This time lag is a combination of the SW transit time within the heliosphere, the lag of the neutral gas distribution responding to solar activity, and the intrinsic lag between the SSN and the launch of a high-energy SW (i.e., O7+ and O8+). MW O VII and O VIII fluxes have mean values of 5.4 L.U. and 1.7 L.U., which are reduced by 50% and 30%, compared to studies where the SWCX contamination is not removed. This correction also changes the determination of the density distribution and the temperature profile of the MW hot gas.

Keywords: Charge exchange ionization, Solar wind, Milky Way Galaxy, Circumgalactic medium