adsabs/SciX_UAT_keywords huggingface dataset:
Be Star Disks: Powered by a Nonzero Central Torque

Be stars are rapidly rotating B stars with Balmer emission lines that indicate the presence of a Keplerian, rotationally supported, circumstellar gas disk. Current disk models, referred to as "decretion disks," make use of the zero-torque inner boundary condition typically applied to accretion disks, with the "decretion" modeled by adding mass to the disk at a radius of about 2% larger than the inner disk boundary. We point out that, in this model, the rates at which mass and energy need to be added to the disk are implausibly large. What is required is that the disk has not only a source of mass but also a continuing source of angular momentum. We argue that the disk evolution may be more physically modeled by application of the nonzero torque inner boundary condition of Nixon & Pringle, which determines the torque applied at the boundary as a fraction of the advected angular momentum flux there and approaches the accretion and decretion disk cases in the appropriate limits. We provide supporting arguments for the suggestion that the origin of the disk material is small-scale magnetic flaring events on the stellar surface, which, when combined with rapid rotation, can provide sufficient mass to form, and sufficient angular momentum to maintain, a Keplerian Be star disk. We discuss the origin of such small-scale magnetic fields in radiative stars with differential rotation. We conclude that small-scale magnetic fields on the stellar surface, may be able to provide the necessary mass flux and the necessary time-dependent torque on the disk inner regions to drive the observed disk evolution.

Keywords: accretion; stellar accretion disks; hydrodynamics; magnetic fields; emission line stars; stellar coronal mass ejections; solar coronal mass ejections; circumstellar disks
Arecibo S-band Radar Characterization of Local-scale Heterogeneities within Mercury's North Polar Deposits

Ground-based planetary radar observations first revealed deposits of potentially nearly pure water ice in some permanently shadowed regions (PSRs) on Mercury's poles. Later, the MESSENGER spacecraft confirmed the icy nature of the deposits, as well as their location within PSRs. Considering the geologic context provided by MESSENGER, we further characterized the north polar deposits by pairing spacecraft data with new Arecibo S-band radar observations. Here we show that some ice deposits within PSRs have a gradational pattern in their radar properties that is likely associated with differences in ice purity. Radar-bright features with a circular polarization ratio μ c > 1 can be characterized by water ice with ≳3% impurities by volume while those with μ c < 1 by ≳20% impurities. Furthermore, areas in PSRs with μ c < 1 typically surround locations of stronger radar backscatter with μ c > 1. Therefore, deposits of nearly pure water ice are likely surrounded by lower-purity material, such as water-ice-rich regolith, which could be the result of impact gardening or the crater's thermal environment. However, such deposits are not always colocated within large polar craters where ice should be the most stable, even at the surface. In fact, we found that there is no significant difference between the radar backscattering properties of deposits thought to have surficial ice and those with buried ice. Our results also help improve the identification of icy reservoirs elsewhere, such as the Moon. Indeed, we found that μ c is not an adequate diagnostic, but rather the radar backscatter in each circular polarization independently provides information to identify water-ice deposits.

Keywords: mercury (planet); radar astronomy; planetary polar regions; surface ices
The Influence of the Secular Perturbation of an Intermediate-mass Companion. II. Ejection of Hypervelocity Stars from the Galactic Center

There is a population of stars with velocities in excess of 500 km s-1 relative to the Galactic center. Many, perhaps most, of these hypervelocity stars (HVSs) are B stars similar to the disk and S stars in a nuclear cluster around a supermassive black hole (SMBH) near Sgr A⋆. In Paper I of this series, we showed that the eccentricity of the stars emerged from a hypothetical disk around the SMBH that can be rapidly excited by the secular perturbation of its intermediate-mass companion (IMC), and we suggested IRS 13E as a potential candidate for the IMC. Here we show that this process leads to an influx of stars on parabolic orbits to the proximity of Sgr A⋆ on a secular timescale of a few megayears. This timescale is much shorter than the diffusion timescale into the lost cone through either the classical or the resonant relaxation. Precession of the highly eccentric stars' longitude of periastron, relative to that of the IMC, brings them to its proximity within a few megayears. The IMC's gravitational perturbation scatters a fraction of the stars from nearly parabolic to hyperbolic orbits with respect to the SMBH. Their follow-up close encounters with the SMBH induce them to escape with hypervelocity. This scenario is a variant of the hypothesis proposed by Hills based on the anticipated breakup of some progenitor binary stars in the proximity of the SMBH, and its main objective is to account for the limited life span of the known HVSs. We generalize our previous numerical simulations of this process with a much wider range of orbital configurations. We demonstrate the robustness and evaluate the efficiency of this channel of HVS formation. From these numerical simulations, we infer observable kinematic properties for the HVSs.

Keywords: galactic center; ob stars; hypervelocity stars; intermediate-mass black holes; dynamical evolution; orbits
SDSS-IV MaNGA: Understanding Ionized Gas Turbulence Using Integral Field Spectroscopy of 4500 Star-forming Disk Galaxies

The Sloan Digital Sky Survey MaNGA program has now obtained integral field spectroscopy for over 10,000 galaxies in the nearby universe. We use the final MaNGA data release DR17 to study the correlation between ionized gas velocity dispersion and galactic star formation rate, finding a tight correlation in which σ Hα from galactic H II regions increases significantly from ~18-30 km s-1, broadly in keeping with previous studies. In contrast, σ Hα from diffuse ionized gas increases more rapidly from 20-60 km s-1. Using the statistical power of MaNGA, we investigate these correlations in greater detail using multiple emission lines and determine that the observed correlation of σ Hα with local star formation rate surface density is driven primarily by the global relation of increasing velocity dispersion at higher total star formation rate, as are apparent correlations with stellar mass. Assuming H II region models consistent with our finding that σ [O III] < σ Hα < σ [O I], we estimate the velocity dispersion of the molecular gas in which the individual H II regions are embedded, finding values σ Mol = 5-30 km s-1 consistent with ALMA observations in a similar mass range. Finally, we use variations in the relation with inclination and disk azimuthal angle to constrain the velocity dispersion ellipsoid of the ionized gas σ z /σ r = 0.84 ± 0.03 and σ ϕ /σ r = 0.91 ± 0.03, similar to that of young stars in the Galactic disk. Our results are most consistent with the theoretical models in which turbulence in modern galactic disks is driven primarily by star formation feedback.

Keywords: galaxy kinematics; disk galaxies
Galactic Cosmic-Ray Hydrogen Spectra in the 40-250 MeV Range Measured by the High-energy Particle Detector (HEPD) on board the CSES-01 Satellite between 2018 and 2020

The High-energy Particle Detector (HEPD) on board the China Seismo-Electromagnetic Satellite (CSES-01)—launched on 2018 February 2—is a light and compact payload suitable for measuring electrons (3-100 MeV), protons (30-250 MeV), and light nuclei (up to a few hundreds of MeV). The Sun-synchronous orbit and large acceptance allow HEPD to measure cosmic-ray particles near the ±65° latitude limit for a fair amount of time per day. In this work, three semiannual galactic hydrogen energy spectra between ∼40 and 250 MeV are presented, including a comparison with theoretical spectra from HelMod, a 2D Monte Carlo model developed to simulate the solar modulation of cosmic rays throughout the heliosphere. To our knowledge, these are the first hydrogen energy spectra below 250 MeV measured at 1 au between 2018 and 2020.

Keywords: heliosphere; cosmic rays; cosmic ray detectors
Variation in Cosmic-Ray Intensity Lags Sunspot Number: Implications of Late Opening of Solar Magnetic Field

Galactic cosmic rays (GCRs), the highly energetic particles that may raise critical health issues for astronauts in space, are modulated by solar activity, with their intensity lagging behind the variation in sunspot number (SSN) by about one year. Previously, this lag has been attributed to the combined effect of outward convecting solar wind and inward propagating GCRs. However, the lag's amplitude and its solar-cycle dependence are still not fully understood. By investigating the solar surface magnetic field, we find that the source of heliospheric magnetic field-the open magnetic flux on the Sun-already lags behind SSN before it convects into the heliosphere along with the solar wind. The delay during odd cycles is longer than that during sequential even cycles. Thus, we propose that the GCR lag is primarily due to the very late opening of the solar magnetic field with respect to SSN, though solar wind convection and particle transport in the heliosphere also matter. We further investigate the origin of the open flux from different latitudes of the Sun and find that the total open flux is significantly contributed by that from low latitudes, where coronal mass ejections frequently occur and also show an odd-even cyclic pattern. Our findings challenge existing theories, and may serve as the physical basis of long-term forecasts of radiation dose estimates for manned deep-space exploration missions.

Keywords: cosmic rays; solar magnetic fields; solar cycle; heliosphere
On the Origin of Fast-rotating Stars. I. Photometric Calibration and Results of AO-assisted BVRI+Hα Imaging of NGC 330 with SAMI/SOAR

Hα emission is a clear indicator of circumstellar activity in Be stars, historically employed to assess the classical Be star (CBe) population in young open clusters (YOCs). The YOC NGC 330 in the Small Magellanic Cloud exhibits a large known fraction of CBe stars and was selected for a pilot study to establish a comprehensive methodology for identifying Hα emitters in the Magellanic Clouds, encompassing the entire B-type spectral range. Using the SOAR Adaptive Module Imager (SAMI), we investigated the stellar population of NGC 330 using multiband BVRI+Hα imaging. We identified Hα emitters within the entire V-band range covered by SAMI/SOAR observations (V ≲ 22), comprising the complete B-type stellar population and offering a unique opportunity to explore the Be phenomenon across all spectral subclasses. The stellar radial distribution shows a clear bimodal pattern between the most massive (B5 or earlier) and the lower mass main-sequence objects (later than B6) within the cluster. The former is concentrated toward the cluster center (showing a dispersion of σ = 4.26 ± 0.20 pc), whereas the latter extends across larger radii (σ = 10.83 ± 0.65 pc), indicating mass stratification within NGC 330. The total fraction of emitters is 4.4% ± 0.5%, notably smaller than previous estimates from flux- or seeing-limited observations. However, a higher fraction of Hα emitters is observed among higher mass stars (32.8% ± 3.4%) than within lower mass (4.4% ± 0.9%). Consequently, the putative CBe population exhibits distinct dynamical characteristics compared to the bulk of the stellar population in NGC 330. These findings highlight the significance of the current observations in providing a complete picture of the CBe population in NGC 330.

Keywords: be stars; magellanic clouds; open star clusters
A Cosmic Zevatron Based on Cyclotron Auto-resonance

A Zevatron is an accelerator scheme envisaged to accelerate particles to ZeV energies (1 ZeV = 1021 eV). Schemes, most notably the internal shock model, have been proposed to explain the acceleration of ultra-high-energy cosmic-ray particles that have been sporadically detected reaching Earth since 1962. Here, the cyclotron auto-resonance acceleration (CARA) mechanism is tailored and used to demonstrate the possible acceleration of particles ejected as a result of violent astrophysical processes such as the merger of a binary system or a supernova explosion. Such events result in emission of highly energetic particles and ultra-intense beamed radiation. In the simultaneous presence of a super-strong magnetic field, the condition for cyclotron auto-resonance may be met. Thus CARA can act like a booster for particles pre-accelerated inside their progenitor by shock waves, possibly among other means. As examples, it is shown that nuclei of hydrogen, helium, and iron-56, may reach ZeV energies by CARA, under which conditions the particles, while gyrating around the lines of an ultra-strong magnetic field, also surf on the waves of a super-intense radiation field. When radiation-reaction is taken into account, it is shown that the ZeV energy gained by a particle can fall off by less than an order of -magnitude if the resonance condition is missed by roughly less than 20%.

Keywords: gamma-ray bursts; cosmic ray sources; ultra-high-energy cosmic radiation; interstellar cyclotron emission; compact radiation sources; compact objects
Model Dependence of Bayesian Gravitational-wave Background Statistics for Pulsar Timing Arrays

Pulsar timing array (PTA) searches for a gravitational-wave background (GWB) typically include time-correlated "red" noise models intrinsic to each pulsar. Using a simple simulated PTA data set with an injected GWB signal we show that the details of the red noise models used, including the choice of amplitude priors and even which pulsars have red noise, have a striking impact on the GWB statistics, including both upper limits and estimates of the GWB amplitude. We find that the standard use of uniform priors on the red noise amplitude leads to 95% upper limits, as calculated from one-sided Bayesian credible intervals, that are less than the injected GWB amplitude 50% of the time. In addition, amplitude estimates of the GWB are systematically lower than the injected value by 10%-40%, depending on which models and priors are chosen for the intrinsic red noise. We tally the effects of model and prior choice and demonstrate how a "dropout" model, which allows flexible use of red noise models in a Bayesian approach, can improve GWB estimates throughout.

Keywords: gravitational waves; gravitational wave astronomy; gravitational wave detectors; bayesian statistics; prior distribution; posterior distribution; astrostatistics strategies; pulsar timing method; red noise; time series analysis; millisecond pulsars
The Dependence of the Structure of Planet-opened Gaps in Protoplanetary Disks on Radiative Cooling

Planets can excite density waves and open annular gas gaps in protoplanetary disks. The depth of gaps is influenced by the evolving angular momentum carried by density waves. While the impact of radiative cooling on the evolution of density waves has been studied, a quantitative correlation to connect gap depth with the cooling timescale is lacking. To address this knowledge gap, we employ the grid-based code Athena++ to simulate disk-planet interactions, treating cooling as a thermal relaxation process. We establish quantitative dependencies of steady-state gap depth (Equation 36) and width (Equation 41) on planetary mass, Shakura-Sunyaev viscosity, disk scale height, and thermal relaxation timescale (β). We confirm previous results that gap opening is the weakest when the thermal relaxation timescale is comparable to the local dynamical timescale. Significant variations in gap depth, up to an order of magnitude, are found with different β. In terms of width, a gap is at its narrowest around β = 1, approximately 10%-20% narrower compared to the isothermal case. When β ~ 100, it can be ~20% wider, and higher viscosity enhances this effect. We derive possible masses of the gas gap-opening planets in AS 209, HD 163296, MWC 480, and HL Tau, accounting for the uncertainties in the local thermal relaxation timescale.

Keywords: protoplanetary disks; planetary system formation; planetary-disk interactions; planet formation
Potential Impact of Fast Flavor Oscillations on Neutrino-driven Winds and Their Nucleosynthesis

The wind driven by the intense neutrino emission from a protoneutron star (PNS) is an important site for producing nuclei heavier than the Fe group. Because of certain features in the neutrino angular distributions, the so-called fast flavor oscillations may occur very close to the PNS surface, effectively resetting the neutrino luminosities and energy spectra that drive the wind. Using the unoscillated neutrino emission characteristics from two core-collapse supernova simulations representative of relevant progenitors at the lower and higher mass end, we study the potential effects of fast flavor oscillations on neutrino-driven winds and their nucleosynthesis. We find that such oscillations can increase the total mass loss by factors up to ∼1.5-1.7 and lead to significantly more proton-rich conditions. The latter effect can greatly enhance the production of 64Zn and the so-called light p-nuclei 74Se, 78Kr, and 84Sr. Implications for abundances in metal-poor stars, Galactic chemical evolution in general, and isotopic anomalies in meteorites are discussed.

Keywords: nucleosynthesis; neutrino oscillations; supernovae; explosive nucleosynthesis
A Two-zone Photohadronic Interpretation of the EHBL-like Behavior of the 2016 Multi-TeV Flares of 1ES 1959+650

The high-energy-peaked blazar 1ES 1959+650 is a well-known and well-studied nearby blazar that has undergone several episodes of multi-TeV flaring. In 2002 for the first time an orphan TeV flare was observed from this blazar. During a multiwavelength campaign from 2016 April 29 to November 21, MAGIC telescopes observed multi-TeV flarings during the nights of 2016 June 13 and 14 and July 1 when the position of the synchrotron peak was found to be above 1017 Hz. Also observed was that the second peak of the spectral energy distribution shifted toward higher energy and exhibited extreme HBL-like behavior. The photohadronic model, which is very successful at explaining the multi-TeV flaring from many high energy blazars, including 1ES 1959+650, is applied to study the flaring events of 2016. It is observed that the photohadronic model is unable to explain the observed spectra. Here, we use a two-zone photohadronic model to explain the observed spectra. We clearly demonstrate that the low energy regime (zone-1) of the spectra corresponds to the standard flaring events of the high energy blazar and the high energy regime (zone-2) of the spectra is solely due to the extreme nature of the flaring events. Our two-zone photohadronic model explains the multi-TeV flaring events observed by MAGIC telescopes very well.

Keywords: high energy astrophysics; blazars; relativistic jets; bl lacertae objects; gamma-rays
Adiabatic Mass Loss in Binary Stars. III. From the Base of the Red Giant Branch to the Tip of the Asymptotic Giant Branch

The distinguishing feature of the evolution of close binary stars is the role played by the mass exchange between the component stars. Whether or not the mass transfer is dynamically stable is one of the essential questions in binary evolution. In the limit of extremely rapid mass transfer, the response of a donor star in an interacting binary becomes asymptotically one of adiabatic expansion. We use the adiabatic mass-loss model to systematically survey the thresholds for dynamical timescale mass transfer over the entire span of possible donor star evolutionary states. We also simulate mass-loss process with isentropic envelopes, the specific entropy of which is fixed to be that at the base of the convective envelope, to artificially mimic the effect of such mass loss in superadiabatic surface convection regions, where the adiabatic approximation fails. We illustrate the general adiabatic response of $3.2\,{M}_{\odot }$ donor stars at different evolutionary stages. We extend our study to a grid of donor stars with different masses (from 0.1 to 100 M⊙ with Z = 0.02) and at different evolutionary stages. We proceed to present our criteria for dynamically unstable mass transfer in both tabular and graphical forms. For red giant branch (RGB) and asymptotic giant branch (AGB) donors in systems with such mass ratios, they may have convective envelopes deep enough to evolve into common envelopes on a thermal timescale, if the donor star overfills its outer Lagrangian radius. Our results show that the RGB and AGB stars tend to be more stable than previously believed, and this may be helpful to explain the abundance of observed post-AGB binary stars with an orbital period of around 1000 days.

Keywords: stellar evolution; stellar mass loss; close binary stars; common envelope evolution; stellar interiors
Best Practices for Data Publication in the Astronomical Literature

We present an overview of best practices for publishing data in astronomy and astrophysics journals. These recommendations are intended as a reference for authors to help prepare and publish data in a way that will better represent and support science results, enable better data sharing, improve reproducibility, and enhance the reusability of data. Observance of these guidelines will also help to streamline the extraction, preservation, integration and cross-linking of valuable data from astrophysics literature into major astronomical databases, and consequently facilitate new modes of science discovery that will better exploit the vast quantities of panchromatic and multidimensional data associated with the literature. We encourage authors, journal editors, referees, and publishers to implement the best practices reviewed here, as well as related recommendations from international astronomical organizations such as the International Astronomical Union for publication of nomenclature, data, and metadata. A convenient Checklist of Recommendations for Publishing Data in the Literature (Appendix A) is included for authors to consult before the submission of the final version of their journal articles and associated data files. We recommend that publishers of journals in astronomy and astrophysics incorporate a link to this document in their Instructions to Authors.

Keywords: astronomy databases; astronomical reference materials; astronomical object identification; astrometry; astronomical coordinate systems; photometry; observatories; telescopes