adsabs/SciX_UAT_keywords huggingface dataset:
Vertical Equilibrium Model for Accretion Flow: An Exact General Relativistic Self-consistent Analysis

We provide a complete general relativistic and fluid dynamical treatment to derive an expression for the relativistic accretion rate in curved spacetime. We exploit the Killing symmetry of stationary axisymmetric spacetime to obtain the conserved current, using which we find an integral relation between mass flux and height of the accretion disk. In comparison with the existing different models of thin- and slim-disk approaches our analysis is self-consistent and mathematically rigorous. We reviewed the previous work and surveyed the recent status in this field. Our formula is different from the existing expression in the literature. The difference arises solely due to the rotation parameter of the compact star. The effect becomes important for slim/thick disks and for rapidly spinning compact stars. Thus one needs to alter the formula $\dot{M}=2\pi {rH}\rho {u}^{r}$ for Newtonian or spherically symmetric spacetime while studying the accretion flows around a Kerr black hole. Our formula can also be used to connect the variability of the vertical structure of the accretion disk through the luminosity variation of X-ray binaries, and can thereby explain the origin of the hysteresis loop of the spectral states.

Keywords: accretion; black hole physics; relativistic fluid dynamics; hydrodynamics; relativity
Mixed Modes and Asteroseismic Surface Effects. II. Subgiant Systematics

Models of solar-like oscillators yield acoustic modes at different frequencies than would be seen in actual stars possessing identical interior structure, due to modeling error near the surface. This asteroseismic "surface term" must be corrected when mode frequencies are used to infer stellar structure. Subgiants exhibit oscillations of mixed acoustic (p-mode) and gravity (g-mode) character, which defy description by the traditional p-mode asymptotic relation. Since nonparametric diagnostics of the surface term rely on this description, they cannot be applied to subgiants directly. In Paper I, we generalized such nonparametric methods to mixed modes, and showed that traditional surface-term corrections only account for mixed-mode coupling to, at best, first order in a perturbative expansion. Here, we apply those results, modeling subgiants using asteroseismic data. We demonstrate that, for grid-based inference of subgiant properties using individual mode frequencies, neglecting higher-order effects of mode coupling in the surface term results in significant systematic differences in the inferred stellar masses, and measurable systematics in other fundamental properties. While these systematics are smaller than those resulting from other choices of model construction, they persist for both parametric and nonparametric formulations of the surface term. This suggests that mode coupling should be fully accounted for when correcting for the surface term in seismic modeling with mixed modes, irrespective of the choice of correction used. The inferred properties of subgiants, in particular masses and ages, also depend on the choice of surface-term correction, in a different manner from those of both main-sequence and red giant stars.

Keywords: asteroseismology; stellar oscillations; computational methods; subgiant stars
Magnetohydrodynamic Poynting Flux Vortices in the Solar Atmosphere and Their Role in Concentrating Energy

The nature of energy generation, transport, and effective dissipation responsible for maintaining a hot solar upper atmosphere is still elusive. The Poynting flux is a vital parameter for describing the direction and magnitude of the energy flow, which is mainly used in solar physics for estimating the upward energy generated by photospheric plasma motion. This study presents a pioneering 3D mapping of the magnetic energy transport within a numerically simulated solar atmosphere. By calculating the Finite Time Lyapunov Exponent of the energy velocity, defined as the ratio of the Poynting flux to the magnetic energy density, we precisely identify the sources and destinations of the magnetic energy flow throughout the solar atmosphere. This energy mapping reveals the presence of transport barriers in the lower atmosphere, restricting the amount of magnetic energy from the photosphere reaching the chromosphere and corona. Interacting kinematic and magnetic vortices create energy channels, breaking through these barriers and allowing three times more energy input from photospheric motions to reach the upper atmosphere than before the vortices formed. The vortex system also substantially alters the energy mapping, acting as a source and deposition of energy, leading to localized energy concentration. Furthermore, our results show that the energy is transported following a vortical motion: the Poynting flux vortex. In regions where these vortices coexist, they favor conditions for energy dissipation through ohmic and viscous heating, since they naturally create large gradients in the magnetic and velocity fields over small spatial scales. Hence, the vortex system promotes local plasma heating, leading to temperatures around a million Kelvins.

Keywords: solar coronal heating; quiet solar chromosphere; solar magnetic fields; magnetohydrodynamics; magnetohydrodynamical simulations; solar chromospheric heating; solar granulation
P/2019 LD2 (ATLAS): An Active Centaur in Imminent Transition to the Jupiter Family

The recently discovered object P/2019 LD2 (by the Asteroid Terrestrial-impact Last Alert System) was initially thought to be a Jupiter Trojan asteroid, until dynamical studies and the appearance of persistent cometary activity revealed that this object is actually an active Centaur. However, the dynamical history, thermal environment, and impact of such environments on the activity of 2019 LD2 are poorly understood. Here we conduct dynamical simulations to constrain its orbital history and resulting thermal environment over the past 3000 yr. We find that 2019 LD2 is currently in the vicinity of a dynamical "Gateway" that facilitates the majority of transitions from the Centaur population into the Jupiter Family of Comets (JFC population). Our calculations show that it is unlikely to have spent significant amounts of time in the inner solar system, suggesting that its nucleus is relatively pristine in terms of physical, chemical, and thermal processing through its history. This could explain its relatively high level of distant activity as a recently activated primordial body. Finally, we find that the median frequency of transition from the Gateway population into the JFC population varies from once every ∼3 yr to less than once every 70 yr, if 2019 LD2's nucleus is ∼1 km in radius or greater than 3 km in radius. Forward modeling of 2019 LD2 shows that it will transition into the JFC population in 2063, representing the first known opportunity to observe the evolution of an active Centaur nucleus as it experiences this population-defining transition.

Keywords: centaur group; short period comets; comets; comet interiors; orbits; astrophysical fluid dynamics
Massive Black Holes Regulated by Luminous Blue Variable Mass Loss and Magnetic Fields

We investigate the effects of mass loss during the main-sequence (MS) and post-MS phases of massive star evolution on black hole (BH) birth masses. We compute solar metallicity Geneva stellar evolution models of an 85 ${M}_{\odot }$ star with mass-loss rate ( $\dot{M}$ ) prescriptions for MS and post-MS phases and analyze under which conditions such models could lead to very massive BHs. Based on the observational constraints for $\dot{M}$ of luminous stars, we discuss two possible scenarios that could produce massive BHs at high metallicity. First, if a massive BH progenitor evolves from the observed population of massive MS stars known as WNh stars, we show that its average post-MS mass-loss rate has to be less than $1\times {10}^{-5}\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$ . However, this is lower than the typical observed mass-loss rates of luminous blue variables (LBV). Second, a massive BH progenitor could evolve from a yet undetected population of 80-85 ${M}_{\odot }$ stars with strong surface magnetic fields, which could quench mass loss during the evolution. In this case, the average mass-loss rate during the post-MS LBV phase has to be less than 5 × 10-5 ${M}_{\odot }\,{\mathrm{yr}}^{-1}$ to produce 70 ${M}_{\odot }$ BHs. We suggest that LBVs that explode as SNe have large envelopes and small cores that could be prone to explosion, possibly evolving from binary interaction (either mergers or mass gainers that do not fully mix). Conversely, LBVs that directly collapse to BHs could have evolved from massive single stars or binary-star mergers that fully mix, possessing large cores that would favor BH formation.

Keywords: massive stars; stellar evolutionary models; evolved stars; hypergiant stars; luminous blue variable stars
Softening of the Metagalactic Ionizing Background from Internal He II Absorption in Quasars

Quasars and other active galactic nuclei (AGN) are significant contributors to the metagalactic ionizing background at redshifts z < 3. Recent Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS) composite spectra of AGN find a harder flux distribution in the Lyman continuum, ${F}_{\nu }\propto {\nu }^{-{\alpha }_{s}}$ (αs = 1.41 ± 0.15), compared to previous studies. This index appears to be inconsistent with observed He II/H I absorption ratios (η) in the Lyα forest. We explore the effects of internal AGN absorption in the He II (4 ryd) continuum using an analytic source-function model of the ionizing background in which the emissivity (jν) arises from quasars, reprocessed by the opacity (κν) of the intervening Lyα forest and distinct AGN escape fractions ${f}_{\mathrm{esc}}^{({\rm{H}}\,{\rm{\small{I}}})}$ and ${f}_{\mathrm{esc}}^{(\mathrm{He}\,{\rm{\small{II}}})}$ at 1 ryd and 4 ryd, respectively. We also examine H I and He II photoelectric heating from the reprocessed background, whose spectral index (αb > αs) depends on αs, the H I column density slope β, and the ratio $R={f}_{\mathrm{esc}}^{({\rm{H}}\,{\rm{\small{I}}})}/{f}_{\mathrm{esc}}^{(\mathrm{He}\,{\rm{\small{II}}})}$. We compare the model to Lyα absorption lines of He II and H I toward the quasar HE 2347-4342. Internal AGN absorption with ${f}_{\mathrm{esc}}^{(\mathrm{He}\,{\rm{\small{II}}})}\approx 0.6\mbox{--}0.8$ but ${f}_{\mathrm{esc}}^{({\rm{H}}\,{\rm{\small{I}}})}\approx 1$ would increase the index by Δαb ≈ 0.3-1.0, corresponding to η = 60-200 for β ≈ 1.5-1.6, in agreement with HST/COS observations at z ≈ 2.5-2.9. The observed range of ratios, η < 200, constrains αb < 3.4 and ${f}_{\mathrm{esc}}^{(\mathrm{He}\,{\rm{\small{II}}})}\gt 0.4$. Individual AGN with softer spectra, αs > 1.7, and more internal He II absorption could produce a few absorbers with η > 300, in proximity to AGN transverse to the sight line.

Keywords: radio quiet quasars; intergalactic medium
Mid-infrared and Maser Flux Variability Correlation in Massive Young Stellar Object G036.70+00.09

We present the discovery of the simultaneous flux variation of a massive young stellar object (MYSO) G036.70+00.09 (G036.70) both in the maser emission and mid-infrared (MIR; λ = 3-5 μm) bands. Using the ALLWISE and NEOWISE archival databases that cover a long time span of approximately 10 yr with a cadence of 6 months, we confirm that G036.70 indicates a stochastic year-long MIR variability with no signs of a WISE band color change of W1 (3.4 μm) -W2 (4.6 μm). Cross-matching the MIR data set with the high-cadence 6.7 GHz class II methanol maser flux using the Hitachi 32 m radio telescope that discovered its periodicity in the methanol maser of 53.0-53.2 days, we also determine the flux correlations between the two bands at two different timescales, year-long and day-long, both of which have never been reported in MYSOs, except when they are in the accretion burst phase. The results of our study support the scenario that a class II methanol maser is pumped up by infrared emission from accreting disks of MYSOs. We also discuss the possible origins of MIR and maser variability. To explain the two observed phenomena, a stochastic year-long MIR variability with no signs of significant color change and maser-MIR variability correlation or a change in mass accretion rate and line-of-sight extinction because of the nonaxisymmetric dust density distribution in a rotating accretion disk are possible origins. Observations through spectroscopic monitoring of accretion-related emission lines are essential for determining the origin of the observed variability in G036.70.

Keywords: variable stars; time domain astronomy; young stellar objects; massive stars; infrared astronomy; circumstellar masers
Stereophotoclinometry on the OSIRIS-REx Mission: Mathematics and Methods

Stereophotoclinometry (SPC) makes it possible to extract the shapes of surfaces by combining information from images, namely stereo parallax data and surface shading from slopes, with knowledge of the location of a spacecraft. This technique has been used extensively in the past few decades to describe the shapes of planets and small bodies, such as asteroids and comets. It has also been used to carefully navigate spacecraft around very small bodies, as in the case of the OSIRIS-REx mission to the ~500 m diameter asteroid (101955) Bennu. This paper describes the mathematical foundation of SPC, with examples from the OSIRIS-REx mission.

Keywords: photogrammetry; asteroids
Bayesian Rotation Inversion of KIC 11145123

A scheme of Bayesian rotation inversion, which allows us to compute the probability of a model of a stellar rotational profile, is developed. The validation of the scheme with simple rotational profiles and the corresponding sets of artificially generated rotational shifts has been successfully carried out, and we can correctly distinguish the (right) rotational model, prepared beforehand for generating the artificial rotational shifts, from the other (wrong) rotational model. The Bayesian scheme is applied to a γ Dor-δ Sct-type hybrid star, KIC 11145123, leading to a result that the convective core of the star might be rotating much faster (~10 times faster) than the other regions of the star. The result is consistent with that previously suggested by Hatta et al. based on a three-zone modeling, further strengthening their argument from a Bayesian point of view.

Keywords: asteroseismology; delta scuti variable stars; stellar interiors; stellar rotation; bayesian statistics; model selection
Long-term 4.6 μm Variability in Brown Dwarfs and a New Technique for Identifying Brown Dwarf Binary Candidates

Using a sample of 361 nearby brown dwarfs, we have searched for 4.6 μm variability, indicative of large-scale rotational modulations or large-scale, long-term changes on timescales of over 10 yr. Our findings show no statistically significant variability in Spitzer's Infrared Array Camera (IRAC) channel 2 (ch2) or Wide-field Infrared Survey Explorer W2 photometry. For Spitzer the ch2 1σ limits are ~8 mmag for objects at 11.5 mag and ~22 mmag for objects at 16 mag. This corresponds to no variability above 4.5% at 11.5 mag and 12.5% at 16 mag. We conclude that highly variable brown dwarfs, at least two previously published examples of which have been shown to have 4.6 μm variability above 80 mmag, are very rare. While analyzing the data, we also developed a new technique for identifying brown dwarf binary candidates in Spitzer data. We find that known binaries have IRAC ch2 point response function (PRF) flux measurements that are consistently dimmer than aperture flux measurements. We have identified 59 objects that exhibit such PRF versus aperture flux differences and are thus excellent binary brown dwarf candidates.

Keywords: brown dwarfs; low mass stars; atmospheric variability; binary stars; l subdwarfs
Hydrogen and Helium Shock Phenomena during Rising Light in RR Lyrae Fundamental Mode Pulsators

We present measurements of H and He emission and absorption lines produced in RRab fundamental mode pulsators during primary light rise. The lines define universal progressions of rise and decay in metal-poor RRab stars. Such a progression cannot be constructed for He in metal-rich RRab (those with [Fe/H] > -0.8) because weak He I emission is detected in only two of the six metal-rich RRab in our survey. Great variety exists in the phase variations of the blue- and red-shifted absorption components of the 5876 Å line during pre- and post-emission phases. Detection of measurable He II 4686 Å emission in eight RRab, three of them Blazhko variables, provides an additional constraint on ionization of helium.

Keywords: stellar spectral lines; horizontal branch stars; rr lyrae variable stars; pulsating variable stars
Hi-C 2.1 Observations of Small-scale Miniature-filament-eruption-like Cool Ejections in an Active Region Plage

We examine 172 Å ultra-high-resolution images of a solar plage region from the High-Resolution Coronal Imager, version 2.1 (Hi-C 2.1, or Hi-C) rocket flight of 2018 May 29. Over its five minute flight, Hi-C resolved a plethora of small-scale dynamic features that appear near noise level in concurrent Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) 171 Å images. For 10 selected events, comparisons with AIA images at other wavelengths and with Interface Region Imaging Spectrograph (IRIS) images indicate that these features are cool (compared to the corona) ejections. Combining Hi-C 172 Å, AIA 171 Å, IRIS 1400 Å, and Hα, we see that these 10 cool ejections are similar to the Hα "dynamic fibrils" and Ca II "anemone jets" found in earlier studies. The front of some of our cool ejections are likely heated, showing emission in IRIS 1400 Å. On average, these cool ejections have approximate widths 3"2 ± 2"1, (projected) maximum heights and velocities 4"3 ± 2"5 and 23 ± 6 km s-1, and lifetimes 6.5 ± 2.4 min. We consider whether these Hi-C features might result from eruptions of sub-minifilaments (smaller than the minifilaments that erupt to produce coronal jets). Comparisons with SDO's Helioseismic and Magnetic Imager (HMI) magnetograms do not show magnetic mixed-polarity neutral lines at these events' bases, as would be expected for true scaled-down versions of solar filaments/minifilaments. But the features' bases are all close to single-polarity strong-flux-edge locations, suggesting possible local opposite-polarity flux unresolved by HMI. Or it may be that our Hi-C ejections instead operate via the shock-wave mechanism that is suggested to drive dynamic fibrils and the so-called type I spicules.

Keywords: active solar chromosphere; solar active region magnetic fields; solar atmospheric motions
Internal and External Alignment of Carbonaceous Grains within the Radiative Torque Paradigm

We study the internal and external alignment of carbonaceous grains in the interstellar medium (ISM) within the Radiative Torque (RAT) paradigm. For internal alignment (IA), we find that hydrogenated amorphous carbon (HAC) grains having nuclear paramagnetism due to hydrogen protons have efficient nuclear relaxation, whereas both HAC and graphite grains can have efficient inelastic relaxation at both low-J and high-J attractors. For external alignment, HAC and pure graphite grains can align with the radiation direction (k-RAT) at low-J attractors but cannot have stable alignment at high-J attractors due to the suppression of radiative precession. However, HAC grains can align with the magnetic field (B-RAT) at high-J attractors due to fast Larmor precession compared to gas collisions. For HAC grains drifting through the ISM, they can align along the induced electric field (E-RAT) at low-J attractors due to the fast electric precession and only small HAC grains can align at high-J attractors. Nuclear paramagnetic relaxation is inefficient for HAC due to the suppression of nuclear susceptibility. We then study the alignment of carbon dust in the envelope of a C-rich Asymptotic Giant Branch star (IRC+10216) and find that grains aligned at low-J attractors may occur via k-RAT with the wrong IA in the inner region but via B-RAT in the outer region. However, grains aligned at high-J attractors have the right IA alignment via k-RAT due to efficient inelastic relaxation. The polarization pattern observed toward IRC+10216 by SOFIA/HAWC+ can be reproduced when only grains at low-J attractors are present due to the removal of grains at high-J attractors by the RAT disruption.

Keywords: astrophysical dust processes; circumstellar dust; interstellar dust processes; dust composition; starlight polarization; evolved stars
What is Exceptional about Solar Activity in the Early Phase of Cycle 25?

Solar Cycle 25 began in 2019 December and has been progressing nominally since. However, a closely associated pair of strong active regions, NOAA 12786 and 12785, emerged in 2020 November. The greater, northern component, 12786, attained a maximum sunspot area of 1000 μHemi. The sudden, uncharacteristic emergence of such a large concentration of intense magnetic flux in the early phase of the solar cycle has not been seen in previous cycles. Although the active region pair survived for two Carrington rotations, it did not produce any X- or M-class flares. Here we remark on the evolution of NOAA 12786 and 12785, first in the Sun's invisible and subsequently the visible hemispheres, and compare the irradiance and other characteristic profiles it manifested in the early ascending phase of cycle 25 with those of previous solar cycles.

Keywords: solar activity; solar active regions; solar flares; solar oscillations; solar active region velocity fields
The Large Far-ultraviolet Flux Deficiency of α Doradus (B8 IIIp Si)

The far-UV spectral energy distribution of α Dor recorded by IUE is compared to that of the constant and normal star ζ Dra, which has similar fundamental parameters. α Dor has much less flux than ζ Dra shortwards of 1800 Å, due to the accumulation of very strong absorption lines of Si II, in particular autoionization lines around 1400 Å. Two spectra of α Dor separated by 9 hr (i.e., much less than the rotational period) show evidence of modest variability shortwards of 1800 Å.

Keywords: bp stars
Magneto-convective Modeling of Low-mass Pre-main Sequence Stars: Postponing the Depletion of Lithium

Standard evolutionary models of pre-main sequence (PMS) low-mass stars often predict radii R and Li abundances A(7Li) which are too small to agree with empirical data, while effective temperatures T eff are predicted to be too large. There are also discrepancies in replicating the stellar age. Discrepancies in R, T eff, and A(7Li) are reported by Stassun et al. for both components (A/B) of the eclipsing binary 2M1222-57 belonging to the young Lower Centaurus Crux association. Using the output of a magneto-convective stellar evolution model which we applied in an early study (in 2010) of PMS stars in a different young association, we show that the discrepancies reported by Stassun et al. in the 2M1222-57 system are alleviated. The models are instructive in showing why larger lithium abundances occur in magnetic PMS stars: it takes longer for the central gas in such a star to become hot enough to destroy lithium.

Keywords: magnetic variable stars; m dwarf stars