adsabs/SciX_UAT_keywords huggingface dataset:
Asteroseismology of a Double-mode High-amplitude δ Scuti Star TIC 448892817

We propose that TIC 448892817 is a double-mode high-amplitude δ Scuti star. The radial modes detected in this star provide a unique opportunity to exploit asteroseismic techniques up to their limits. 30 significant frequencies are detected by frequency analysis, while two of them are independent frequencies, i.e., F0 = 13.43538(2) day-1 and F1 = 17.27007(4) day-1. The ratio of f 1/f 2 is measured to be 0.777957(2), suggesting that this target is a double-mode δ Scuti star. Nearly all the light variation is due to these two modes and their combination frequencies, but several other frequencies of very low amplitude are also present. The stellar evolutionary models were constructed with different mass M and metallicity Z using Modules for Experiments in Stellar Astrophysics (MESA). The frequency ratio f 1/f 2 obtained by the model is smaller than those obtained by observation. This might be caused by the rotation of the star pointing that rotational effects are more important than previously thought in HADS stars. This is something that deserves to be investigated in future works with models including rotational effects for moderate to intermediate rotators such as FILOU. On the other hand, the parameters obtained from MESA agree well with previous results as well as by observational spectra. The best-fitting model shows that TIC 448892817 is close to entering the first turnoff of the main sequence. In order to accurately determine the effective temperature and metallicities, thus further narrowing the parameter space of this star, we suggest high-resolution spectra is highly desired in the future.

Keywords: asteroseismology; delta scuti variable stars; stellar oscillations
The Origin of Switchbacks in the Solar Corona: Linear Theory

The origin, structure, and propagation characteristics of a switchback are compelling questions posed by Parker Solar Probe (PSP) observations of velocity spikes and magnetic field reversals. By assuming interchange reconnection between coronal loop and open magnetic field, we show that this results in the generation of upward (into the heliosphere) and downward complex structures propagating at the fast magnetosonic speed (i.e., the Alfvén speed in the low plasma beta corona) that can have an arbitrary radial magnetic field deflection, including "S-shaped." We derive the evolution equation for the switchback radial magnetic field as it propagates through the inhomogeneous supersonic solar corona. An analytic solution for arbitrary initial conditions is used to investigate the properties of a switchback propagating from launch ∼6 to ∼35 R⊙ where PSP observed switchbacks during its first encounter. We provide a detailed comparison to an example event, showing that the magnetic field and plasma solutions are in accord with PSP observations. For a simple single switchback, the model predicts either a single or a double-humped structure; the former corresponding to PSP observing either the main body or the flanks of the switchback. The clustering of switchbacks and their sometimes complicated structure may be due to the formation of multiple closely spaced switchbacks created by interchange reconnection with numerous open and loop magnetic field lines over a short period. We show that their evolution yields a complex, aggregated group of switchbacks that includes "sheaths" with large-amplitude radial magnetic field and velocity fluctuations.

Keywords: active solar corona; solar coronal waves; interplanetary turbulence
Rapid Deceleration of Blast Waves Witnessed in Tycho's Supernova Remnant

In spite of their importance as standard candles in cosmology and as major major sites of nucleosynthesis in the universe, the kinds of progenitor systems that lead to Type Ia supernovae (SNe Ia) remains a subject of considerable debate in the literature. This is true even for the case of Tycho's supernova (SN) that exploded in 1572, although it has been deeply studied both observationally and theoretically. Analyzing X-ray data of Tycho's supernova remnant (SNR) obtained with Chandra in 2003, 2007, 2009, and 2015, we discover that the expansion before 2007 was substantially faster than radio measurements reported in the past decades and then rapidly decelerated during the last ∼15 yr. The result is well explained if the shock waves recently hit a wall of dense gas surrounding the SNR. In fact, such a gas structure is expected in the so-called single-degenerate scenario, in which the progenitor is a binary system consisting of a white dwarf and a stellar companion, whereas it is not generally predicted by a competing scenario, the double-degenerate scenario, which has a binary of two white dwarfs as the progenitor. Our result thus favors the former scenario. This work also demonstrates a novel technique to probe gas environments surrounding SNRs and thus disentangle the two progenitor scenarios for SNe Ia.

Keywords: type ia supernovae; supernova remnants; interstellar medium; x-ray sources; circumstellar gas; molecular clouds
CLEAR. II. Evidence for Early Formation of the Most Compact Quiescent Galaxies at High Redshift

The origin of the correlations between mass, morphology, quenched fraction, and formation history in galaxies is difficult to define, primarily due to the uncertainties in galaxy star formation histories (SFHs). SFHs are better constrained for higher redshift galaxies, observed closer to their formation and quenching epochs. Here we use "nonparametric" SFHs and a nested sampling method to derive constraints on the formation and quenching timescales of quiescent galaxies at $0.7\lt z\lt 2.5$ . We model deep HST grism spectroscopy and photometry from the CLEAR (CANDELS Lyα Emission at Reionization) survey. The galaxy formation redshifts, z50 (defined as the point where they had formed 50% of their stellar mass) range from ${z}_{50}\sim 2$ (shortly prior to the observed epoch) up to ${z}_{50}\simeq 5-8$ . We find that early formation redshifts are correlated with high stellar-mass surface densities, $\mathrm{log}{{\rm{\Sigma }}}_{1}/({M}_{\odot }\,{\mathrm{kpc}}^{-2})\gt 10.25$ , where Σ1 is the stellar mass within 1 pkpc (proper kpc). Quiescent galaxies with the highest stellar-mass surface density, $\mathrm{log}{{\rm{\Sigma }}}_{1}/({M}_{\odot }{\mathrm{kpc}}^{-2})\gt 10.25$ , show a minimum formation redshift: all such objects in our sample have ${z}_{50}\gt 2.9$ . Quiescent galaxies with lower surface density, $\mathrm{log}{{\rm{\Sigma }}}_{1}/({M}_{\odot }\,{\mathrm{kpc}}^{-2})=9.5-10.25$ , show a range of formation epochs ( ${z}_{50}\simeq 1.5\mbox{--}8$ ), implying these galaxies experienced a range of formation and assembly histories. We argue that the surface density threshold $\mathrm{log}{{\rm{\Sigma }}}_{1}/({M}_{\odot }\,{\mathrm{kpc}}^{-2})\gt 10.25$ uniquely identifies galaxies that formed in the first few Gyr after the big bang, and we discuss the implications this has for galaxy formation models.

Keywords: compact galaxies; galaxy evolution; quenched galaxies; galaxy quenching
Period Bouncer Cataclysmic Variable EZ Lyn in Quiescence

We report the study of the accretion disk structure of the period bouncer cataclysmic variable EZ Lyn (SDSS J080434.20+510349.2) in quiescence based on our new time-resolved photometric and spectroscopic observations and data extracted from archives. The object magnitude now is V = 17.95(5), close to its brightest before the first superoutburst in 2006. We confirmed the presence of the small eclipse in the optical light curve. The spectra obtained in quiescence at different epochs look similar. However, the contribution of the disk and intensities of emission lines are strongly varied. We singled out pure accretion disk spectra and found that the Balmer decrement Hα:Hβ:Hγ:Hδ = 1.61:1.0:0.76:0.59 is comparable with one at bright accretion disks in longer period cataclysmic variables. The decrement suggests that emission lines are excited collisionally in an optical thin part of the disk with average density and temperature of logN0 = 12.5(2) and T = (10-15) × 103 K. Based on the photometric data and our modeling techniques, we redetermined the mass of MWD = 0.85(1) M⊙ and the current effective temperature TWD,eff = 11,250(50) K of the white dwarf. The secondary has mass of M2 = 0.042(14) M⊙. The system inclination is 79.0°(2). The mass accretion rate is about $\dot{M}\approx (0.3\mbox{--}3.0)\times {10}^{-12}\ {M}_{\odot }$ yr-1. The disk luminosity, together with results from light-curve modeling, suggests a low effective temperature ~2500 K of the continuum emitting region, where also the spiral arm pattern is hosted.

Keywords: dwarf novae; wz sagittae stars; cataclysmic variable stars
Small-amplitude Red Giants Elucidate the Nature of the Tip of the Red Giant Branch as a Standard Candle

The tip of the red giant branch (TRGB) is an important standard candle for determining luminosity distances. Although several 105 small-amplitude red giant stars (SARGs) have been discovered, variability was previously considered irrelevant for the TRGB as a standard candle. Here, we show that all stars near the TRGB are SARGs that follow several period–luminosity sequences, of which sequence A is younger than sequence B as predicted by stellar evolution. We measure apparent TRGB magnitudes, m TRGB, in the Large Magellanic Cloud (LMC) using Sobel filters applied to photometry from the Optical Gravitational Lensing Experiment and the ESA Gaia mission, and we identify several weaknesses in a recent LMC-based TRGB calibration used to measure the Hubble constant. We consider four samples: all red giants (RGs), SARGs, and sequences A and B. The B sequence is best suited for measuring distances to old RG populations, with M F814W,0 = ‑4.025 ± 0.014(stat.) ± 0.033(syst.) mag assuming the LMC's geometric distance. Control of systematics is demonstrated using detailed simulations. Population diversity affects m TRGB at a level exceeding the stated precision: the SARG and A-sequence samples yield 0.039 and 0.085 mag fainter (at 5σ significance) m TRGB values, respectively. Ensuring equivalent RG populations is crucial to measuring accurate TRGB distances. Additionally, luminosity function smoothing (∼0.02 mag) and edge detection response weighting (as much as ‑0.06 mag) can further bias TRGB measurements, with the latter introducing a tip-contrast relation. We are optimistic that variable RGs will enable further improvements to the TRGB as a standard candle.

Keywords: standard candles; stellar distance; red giant tip; pulsating variable stars; giant stars; hubble constant
Chandra Imaging of the Western Hotspot in the Radio Galaxy Pictor A: Image Deconvolution and Variability Analysis

Here we present an analysis of the X-ray morphology and flux variability of the particularly bright and extended western hotspot in the nearest powerful (FR II-type) radio galaxy, Pictor A, based on data obtained with the Chandra X-ray Observatory. The hotspot marks the position where the relativistic jet, which originates in the active nucleus of the system, interacts with the intergalactic medium, at hundreds-of-kiloparsec distances from the host galaxy, forming a termination shock that converts jet bulk kinetic energy to internal energy of the plasma. The hotspot is bright in X-rays due to the synchrotron emission of electrons accelerated to ultrarelativistic energies at the shock front. In our analysis, we make use of several Chandra observations targeting the hotspot over the last decades with various exposures and off-axis angles. For each pointing, we study in detail the point-spread function, which allows us to perform the image deconvolution, and to resolve the hotspot structure. In particular, the brightest segment of the X-ray hotspot is observed to be extended in the direction perpendicular to the jet, forming a thin, ∼3 kpc long, feature that we identify with the front of the reverse shock. The position of this feature agrees well with the position of the optical intensity peak of the hotspot, but is clearly offset from the position of the radio intensity peak, located ∼1 kpc further downstream. In addition, we measure the net count rate on the deconvolved images, finding a gradual flux decrease by about 30% over the 15 yr timescale of the monitoring.

Keywords: non-thermal radiation sources; active galaxies; radio galaxies; relativistic jets; x-ray sources; x-ray photometry; x-ray quasars; shocks; cosmic rays
Diagnosing a Solar Flaring Core with Bidirectional Quasi-periodic Fast Propagating Magnetoacoustic Waves

Quasi-periodic fast propagating (QFP) waves are often excited by solar flares, and could be trapped in the coronal structure with low Alfvén speed, so they could be used as a tool for diagnosing both the flaring core and magnetic waveguide. As the periodicity of a QFP wave could originate from a periodic source or be dispersively waveguided, it is a key parameter for diagnosing the flaring core and waveguide. In this paper, we study two QFP waves excited by a Geostationary Operational Environmental Satellite-class C1.3 solar flare occurring at active region NOAA 12734 on 2019 March 8. Two QFP waves were guided by two oppositely oriented coronal funnels. The periods of two QFP waves were identical and were roughly equal to the period of the oscillatory signal in the X-ray and 17 GHz radio emission released by the flaring core. It is very likely that the two QFP waves could be periodically excited by the flaring core. Many features of this QFP wave event are consistent with the magnetic tuning fork model. We also investigated the seismological application with QFP waves, and found that the magnetic field inferred with magnetohydrodynamic seismology was consistent with that obtained in the magnetic extrapolation model. Our study suggests that the QFP wave is a good tool for diagnosing both the flaring core and the magnetic waveguide.

Keywords: active solar corona; solar flares; solar oscillations; solar coronal waves; solar coronal seismology; magnetohydrodynamics; solar coronal heating
Fits for the Convective Envelope Mass in Massive Stars

We explore the evolution of massive stars (>8 M ⊙) with 1D models and present analytical fits to the masses and binding energies of the convective portions of their envelopes. These fits are given as functions of total mass, metallicity, and surface temperature (used as a proxy for evolutionary phase). They enable the application of the two-stage common envelope (CE) formalism in rapid binary population synthesis frameworks. We estimate that the degree of orbital hardening following CE ejection spans 6 orders of magnitude and is a very strong function of the accretor mass, and, to a lesser extent, donor evolutionary phase.

Keywords: common envelope evolution; common envelope binary stars; stellar evolution
The Impact of Cometary "Impacts" on the Chemistry, Climate, and Spectra of Hot Jupiter Atmospheres

Impacts from icy and rocky bodies have helped shape the composition of Solar System objects; for example, the Earth–Moon system, or the recent impact of comet Shoemaker–Levy 9 with Jupiter. It is likely that such impacts also shape the composition of exoplanetary systems. Here, we investigate how cometary impacts might affect the atmospheric composition/chemistry of hot Jupiters, which are prime targets for characterization. We introduce a parameterized cometary impact model that includes thermal ablation and pressure driven breakup, which we couple with the 1D "radiative-convective" atmospheric model ATMO, including disequilibrium chemistry. We use this model to investigate a wide range of impactor masses and compositions, including those based on observations of Solar System comets, and interstellar ices (with JWST). We find that even a small impactor (R = 2.5 km) can lead to significant short-term changes in the atmospheric chemistry, including a factor >10 enhancement in H2O, CO, and CO2 abundances, as well as atmospheric opacity more generally, and the near-complete removal of observable hydrocarbons, such as CH4, from the upper atmosphere. These effects scale with the change in atmospheric C/O ratio and metallicity. Potentially observable changes are possible for a body that has undergone significant/continuous bombardment, such that the global atmospheric chemistry has been impacted. Our works reveals that cometary impacts can significantly alter or pollute the atmospheric composition/chemistry of hot Jupiters. These changes have the potential to mute/break the proposed link between atmospheric C/O ratio and planet formation location relative to key snowlines in the natal protoplanetary disk.

Keywords: exoplanet atmospheres; atmospheric composition; exoplanet atmospheric composition; astrochemistry; computational astronomy; comets
Kindling the First Stars. I. Dependence of Detectability of the First Stars with JWST on the Population III Stellar Masses

The first Population III (Pop III) stars formed out of primordial, metal-free gas, in minihalos at z > 20, and kickstarted the cosmic processes of reionization and enrichment. While these stars are likely more massive than their enriched counterparts, the current unknowns of their astrophysics include when the first Pop III stars ignited, how massive they were, and when and how the era of the first stars ended. Investigating these questions requires an exploration of a multidimensional parameter space, including the slope of the Pop III stellar initial mass function (IMF) and the strength of the nonionizing UV background. In this work, we present a novel model which treats both the slope and maximum mass of Pop III stars as truly free parameters while including the physics of the fragmentation of primordial gas. Our results also hint at a nonuniversal Pop III IMF which is dependent on the efficiency of primordial gas fragmentation. Our relatively simple model reproduces the results from hydrodynamic simulations, but with a computational efficiency which allows us to investigate the observable differences between a wide range of potential Pop III IMFs. In addition, the evolution of the number density of Pop III stars may provide insight into the evolution of the H2 dissociating background. While the slope of the Pop III IMF does not significantly affect the predicted number density of the first stars, more top-heavy IMFs produce Pop III star clusters which are 2–3 magnitudes brighter than their more bottom-heavy counterparts. While the Pop III star clusters are too dim for direct detection by JWST, we find they are within the reach of gravitational lensing.

Keywords: population iii stars; n-body simulations; theoretical models
Comparison Between TeV and Non-TeV Fermi-detected BL Lacertae Objects

In this paper, we compiled a sample of 410 Fermi-detected BL Lacs, including 42 TeV-detected BL Lacs (TBLs) and 368 non-TeV-detected BL Lacs (non-TBLs) with corresponding mid-infrared (mid-IR), TeV and Fermi γ-ray data, and calculated some important parameters including monochromatic luminosities (mid-IR, GeV and TeV bands) and mid-IR spectral indices. Based on those parameters, we discussed the relationship between the mid-IR and the TeV bands and that between the mid-IR and the GeV bands. Main conclusions are drawn as follows: (1) In the color-color and color-magnitude diagrams, our sample forms a WISE-Gamma Strip in the [3.4]-[4.6]-[12] μm color-color diagram, and TBLs occupy the brighter region than the non-TBLs for the similar color-index in the color-magnititue diagram; (2) The mid-IR luminosity of the TBLs is on average higher than that of non-TBLs, while the average mid-IR spectral index of TBLs is smaller than the non-TBLs, suggesting that TBLs are brighter and hold a more flat spectrum than do the non-TBLs in the mid-IR band. Besides, HBLs have a more flat mid-IR spectrum than LBLs and IBLs; (3) The mid-IR luminosity is positively correlated with the GeV luminosity and the intrinsic TeV luminosity. A positive correlation exists between the mid-IR spectral index and the observed TeV spectral index, which is consistent with the expectations of the synchrotron self-Compton mechanism. We suggest that the HBLs with extreme relativistic electrons might scatter the mid-IR photons up to the TeV band.

Keywords: active galactic nuclei; blazars; bl lacertae objects; gamma-rays
Zero and Extremely Low-metallicity Rotating Massive Stars: Evolution, Explosion, and Nucleosynthesis Up to the Heaviest Nuclei

We present the evolution and the explosion of two massive stars, 15 and 25 M ⊙, spanning a wide range of initial rotation velocities (from 0 to 800 km s‑1) and three initial metallicities: Z = 0 ([Fe/H] = ‑∞), 3.236 × 10‑7 ([Fe/H] = ‑5), and 3.236 × 10‑6 ([Fe/H] = ‑4). A very large nuclear network of 524 nuclear species extending up to Bi has been adopted. Our main findings may be summarized as follows: (a) rotating models above Z = 0 are able to produce nuclei up to the neutron closure shell N = 50, and in a few cases up to N = 82; (b) rotation drastically inhibits the penetration of the He convective shell in the H-rich mantle, a phenomenon often found in zero metallicity nonrotating massive stars; (c) vice versa, rotation favors the penetration of the O convective shell in the C-rich layers with the consequence of significantly altering the yields of the products of the C, Ne, and O burning; (d) none of the models that reach the critical velocity while in H burning lose more the 1 M ⊙ in this phase; (e) conversely, almost all models able to reach their Hayashi track exceed the Eddington luminosity and dynamically lose almost all their H-rich mantle. These models suggest that rotating massive stars may have contributed significantly to the synthesis of the heavy nuclei in the first phase of enrichment of the interstellar medium, i.e., at early times.

Keywords: s-process; stellar evolution; stellar rotation; massive stars; core-collapse supernovae; nucleosynthesis
The Decoupled Kinematics of High-z QSO Host Galaxies and Their Lyα Halos

We present a comparison of the interstellar medium traced by [C II] (Atacama Large Millimeter/submillimeter Array), and ionized halo gas traced by Lyα (Multi Unit Spectroscopic Explorer), in and around QSO host galaxies at z ~ 6. To date, 18 QSOs at this redshift have been studied with both MUSE and high-resolution ALMA imaging; of these, 8 objects display a Lyα halo. Using data cubes matched in velocity resolution, we compare and contrast the spatial and kinematic information of the Lyα halos and the host galaxies' [C II] (and dust-continuum) emission. We find that the Lyα halos extend typically 3-30 times beyond the interstellar medium of the host galaxies. The majority of the Lyα halos do not show ordered motion in their velocity fields, whereas most of the [C II] velocity fields do. In those cases where a velocity gradient can be measured in Lyα, the kinematics do not align with those derived from the [C II] emission. This implies that the Lyα emission is not tracing the outskirts of a large rotating disk, which is a simple extension of the central galaxy seen in [C II] emission. It rather suggests that the kinematics of the halo gas are decoupled from those of the central galaxy. Given the scattering nature of Lyα, these results need to be confirmed with James Webb Space Telescope Integral Field Unit observations that can constrain the halo kinematics further using the nonresonant Hα line.

Keywords: quasars; supermassive black holes; lyman-alpha galaxies