adsabs/SciX_UAT_keywords huggingface dataset:
A Comparative Study of Mid-infrared Star Formation Rate Tracers and Their Metallicity Dependence

We present a comparative study of a set of star formation rate (SFR) tracers based on mid-infrared emission in the 12.81 μm [Ne II] line, the 15.56 μm [Ne III] line, and emission features from polycyclic aromatic hydrocarbons (PAHs) between 5.2 and 14.7 μm. We calibrate our tracers with the thermal component of the radio continuum emission at 33 GHz from 33 extranuclear star-forming regions observed in nearby galaxies. Correlations between mid-IR emission features and thermal 33 GHz SFRs show significant metallicity-dependent scatter and offsets. We find similar metallicity-dependent trends in commonly used SFR tracers such as Hα and 24 μm. As seen in previous studies, PAH emission alone is a poor SFR tracer owing to a strong metallicity dependence: lower-metallicity regions show decreased PAH emission relative to their SFR compared to higher-metallicity regions. We construct combinations of PAH bands, neon emission lines, and their respective ratios to minimize metallicity trends. The calibrations that most accurately trace SFR with minimal metallicity dependence involve the sum of the integrated intensities of the 12.81 μm [Ne II] line and the 15.56 μm [Ne III] line combined with any major PAH feature normalized by dust continuum emission. This mid-IR calibration is a useful tool for measuring SFR, as it is minimally sensitive to variations in metallicity and is composed of bright, ubiquitous emission features. The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope will detect these features from galaxies as far as redshift z ∼ 1. We also investigate the behavior of the PAH band ratios and find that subtracting the local background surrounding a star-forming region decreases the ratio of PAH 11.3 μm to 7.7 μm emission. This implies that PAHs are more ionized in star-forming regions relative to their surroundings.

Keywords: star formation; star forming regions; polycyclic aromatic hydrocarbons; metallicity; radio continuum emission
Emergent Spectral Fluxes of Hot Jupiters: An Abrupt Rise in Dayside Brightness Temperature Under Strong Irradiation

We study the emergent spectral fluxes of transiting hot Jupiters, using secondary eclipses from Spitzer. To achieve a large and uniform sample, we have reanalyzed all secondary eclipses for all hot Jupiters observed by Spitzer at 3.6 and/or 4.5 μm. Our sample comprises 457 eclipses of 122 planets, including eclipses of 13 planets not previously published. We use these eclipse depths to calculate the spectral fluxes emergent from the exoplanetary atmospheres, and thereby infer the temperatures and spectral properties of hot Jupiters. We find that an abrupt rise in brightness temperature, similar to a phase change, occurs on the dayside atmospheres of the population at an equilibrium temperature between 1714 and 1818 K (99% confidence limits). The amplitude of the rise is 291 ± 49 K, and two viable causes are the onset of magnetic drag that inhibits longitudinal heat redistribution, and/or the rapid dissipation of dayside clouds. We also study hot Jupiter spectral properties with respect to metallicity and temperature inversions. Models exhibiting 4.5 μm emission from temperature inversions reproduce our fluxes statistically for the hottest planets, but the transition to emission is gradual, not abrupt. The Spitzer fluxes are sensitive to metallicity for planets cooler than ~1200 K, and most of the hot Jupiter population falls between model tracks having solar to 30× solar metallicity.

Keywords: exoplanet astronomy; exoplanet atmospheres; hot jupiters; infrared astronomy
Dependence of the Continuum Intensities on the Magnetic Fields at Different Evolution Phases of Sunspots

The relationship between the continuum intensities and magnetic fields for stable and decaying sunspots is analyzed using the scattered-light-corrected data from the Helioseismic and Magnetic Imager. From our analysis, the main differences between stable and decaying sunspots are as follows. In the continuum intensity range from 0.35I qs to 0.65I qs, where I qs is the continuum intensity of the quiet solar surface, the relationship between continuum intensity and transverse magnetic field and the relationship between continuum intensity and inclination display a much higher scatter during the decaying phase of the sunspots. During and after the formation of the light bridge, the scatter plots show a bifurcation that indicates that the two umbrae separated by the light bridge have different thermodynamic properties. The continuum intensity of the umbra in a decaying sunspot is brighter than that of the stable sunspot, indicating that the temperatures in the umbra of decaying sunspots are higher. Furthermore, our results show that the mean continuum intensity of the umbra gradually increases during the decay of the sunspot, but the mean continuum intensity of the penumbra remains constant. Simultaneously, the vertical and transverse magnetic field strengths in the umbra gradually decrease, and the vertical magnetic field strengths in the penumbra gradually increase. The changes in the umbra occur earlier than the changes in the penumbra of the decaying sunspot, suggesting that the umbral and penumbral decay may be an interdependent process during the decay of the sunspot.

Keywords: solar magnetic fields; solar photosphere; sunspots
The Core Mass Function in the Orion Nebula Cluster Region: What Determines the Final Stellar Masses?

Applying dendrogram analysis to the CARMA-NRO C18O (J = 1-0) data having an angular resolution of ∼8″, we identified 692 dense cores in the Orion Nebula Cluster region. Using this core sample, we compare the core and initial stellar mass functions in the same area to quantify the step from cores to stars. About 22% of the identified cores are gravitationally bound. The derived core mass function (CMF) for starless cores has a slope similar to Salpeter's stellar initial mass function (IMF) for the mass range above 1 M⊙, consistent with previous studies. Our CMF has a peak at a subsolar mass of ∼0.1 M⊙, which is comparable to the peak mass of the IMF derived in the same area. We also find that the current star formation rate is consistent with the picture in which stars are born only from self-gravitating starless cores. However, the cores must gain additional gas from the surroundings to reproduce the current IMF (e.g., its slope and peak mass), because the core mass cannot be accreted onto the star with 100% efficiency. Thus, the mass accretion from the surroundings may play a crucial role in determining the final stellar masses of stars.

Keywords: star formation; interstellar medium; molecular clouds; protostars; radio observatories; co line emission; initial mass function
Variability Census of Legacy Catalogs. II. 6600+ New δ Scuti and γ Doradus Stars

From Phase II of a variability vetting census in BD, HD, SAO and PPM catalogs, I have identified a total of new variables of over 5000 δ Scuti, 3500 γ Doradus, 1200 Maia variables, 4 RR Lyrae stars, 420 eclipsing binaries, 5500 rotating variables (220+ ellipsoidal variable stars, 550+ ACV and several other subtypes including solar-like oscillators), and other types. Notably, more than 66 eclipsing binary systems have been identified with pulsating primary components of either DSCT or GDOR type, while 10 more binaries feature overlapping Maia-type pulsation. Moreover, over 960 of these variables exhibit hybrid δ Sct-γ Dor pulsations. *Dedicated to my wife Jingyun Zhang.

Keywords: pulsating variable stars; delta scuti variable stars; gamma doradus variable stars; rr lyrae variable stars; eclipsing binary stars
Long Characteristics versus Short Characteristics in 3D Radiative Transfer Simulations of Polarized Radiation

We compare maps of scattering polarization signals obtained from three-dimensional (3D) radiation transfer calculations in a magnetoconvection model of the solar atmosphere using formal solvers based on the "short characteristics" (SC) and the "long characteristics" (LC) methods. The SC method requires less computational work, but it is known to introduce spatial blurring in the emergent radiation for inclined lines of sight. For polarized radiation this effect is generally more severe due to it being a signed quantity and to the sensitivity of the scattering polarization to the model's inhomogeneities. We study the differences in the polarization signals of the emergent spectral line radiation calculated with such formal solvers. We take as a case study already published results of the scattering polarization in the Sr I 4607 Å line obtained with the SC method, demonstrating that in high-resolution grids it is accurate enough for that type of study. In general, the LC method is the preferred one for accurate calculations of the emergent radiation, which is the reason why it is now one of the options in the public version of the 3D radiative transfer code PORTA.

Keywords: radiative transfer; the sun; quiet sun; solar physics; solar atmosphere
The Swan: Data-driven Inference of Stellar Surface Gravities for Cool Stars from Photometric Light Curves

Stellar light curves are well known to encode physical stellar properties. Precise, automated, and computationally inexpensive methods to derive physical parameters from light curves are needed to cope with the large influx of these data from space-based missions such as Kepler and TESS. Here we present a new methodology that we call "The Swan," a fast, generalizable, and effective approach for deriving stellar surface gravity ( $\mathrm{log}g$ ) for main-sequence, subgiant, and red giant stars from Kepler light curves using local linear regression on the full frequency content of Kepler long-cadence power spectra. With this inexpensive data-driven approach, we recover $\mathrm{log}g$ to a precision of ∼0.02 dex for 13,822 stars with seismic $\mathrm{log}g$ values between 0.2 and 4.4 dex and ∼0.11 dex for 4646 stars with Gaia-derived $\mathrm{log}g$ values between 2.3 and 4.6 dex. We further develop a signal-to-noise metric and find that granulation is difficult to detect in many cool main-sequence stars (Teff ≲ 5500 K), in particular K dwarfs. By combining our $\mathrm{log}g$ measurements with Gaia radii, we derive empirical masses for 4646 subgiant and main-sequence stars with a median precision of ∼7%. Finally, we demonstrate that our method can be used to recover $\mathrm{log}g$ to a similar mean absolute deviation precision for a TESS baseline of 27 days. Our methodology can be readily applied to photometric time series observations to infer stellar surface gravities to high precision across evolutionary states.

Keywords: asteroseismology; time domain astronomy; linear regression; photometry; light curves; stellar oscillations; astronomy data modeling; stellar astronomy; stellar granulation
The Hg II Line at 5677.10 Å: A New Proxy to Derive Abundances of Mercury in Late B-type Stars?

Abundance determinations of mercury in late B-type stars in the optical range usually rely on the use of one absorption line of Hg II at 3983.93 Å. A proposal is made to use another but weaker line of Hg II located at 5677.10 Å. This line is present in the spectra of two mercury rich stars, κ Cnc and HD 58661, and absent in 53 Tau whose abundance of mercury is less than the solar value. The synthesis of the 5677.10 Hg II line, using the current atomic parameters available in NIST, confirms that mercury is very abundant in κ Cnc and HD 58661. The abundances derived with 5677.10 Å are currently less than these derived from 3983.93 Å.

Keywords: chemically peculiar stars; mercury-manganese stars
Long-term Evolution of Tightly Packed Stellar Black Holes in AGN Disks: Formation of Merging Black Hole Binaries via Close Encounters

We study the long-term evolution of two or more stellar black holes (BHs) on initially separated but unstable circular orbits around a supermassive BH (SMBH). Such a close-packed orbital configuration can naturally arise from BH migrations in the AGN disk. Dynamical instability of the orbits leads to recurring close encounters between two BHs, during which the BH separation r p becomes less than the Hill radius R H. In rare very close encounters, a tight merging BH binary can form with the help of gravitational wave emission. We use N-body simulations to study the time evolution of close encounters of various degrees of closeness. For a typical "SMBH+2BH" system, the averaged cumulative number of close encounters (with r p ≲ R H) scales approximately as ∝ t 0.5. The minimum encounter separation r p follows a cumulative distribution P(<r p) ∝ r p for r p ≪ R H. We obtain a semi-analytical expression for the averaged rate of binary captures that lead to BH mergers. Our results suggest that close-packed BHs in AGN disks may take a long time (≳107 orbits around the SMBH) to experience a sufficiently close encounter and form a bound binary. This time can be shorter if the initial BH orbits are highly aligned. The BH binary mergers produced in this scenario have high eccentricities when entering the LIGO band and broad distribution of orbital inclinations relative to the original AGN disk. We explore the effects of the gas disk and find that simple gas drags on the BHs do not necessarily lead to an enhanced BH binary capture rate.

Keywords: star clusters; black holes; galaxy accretion disks; gravitational wave sources; n-body simulations
The RS Oph Outburst of 2021 Monitored in X-Rays with NICER

The 2021 outburst of the symbiotic recurrent nova RS Oph was monitored with the Neutron Star Interior Composition Explorer Mission (NICER) in the 0.2-12 keV range from day one after the optical maximum, until day 88, producing an unprecedented, detailed view of the outburst development. The X-ray flux preceding the supersoft X-ray phase peaked almost 5 days after optical maximum and originated only in shocked ejecta for 21-25 days. The emission was thermal; in the first 5 days, only a non-collisional-ionization equilibrium model fits the spectrum, and a transition to equilibrium occurred between days 6 and 12. The ratio of peak X-ray flux measured in the NICER range to that measured with Fermi in the 60 MeV-500 GeV range was about 0.1, and the ratio to the peak flux measured with H.E.S.S. in the 250 GeV-2.5 TeV range was about 100. The central supersoft X-ray source (SSS), namely the shell hydrogen burning white dwarf (WD), became visible in the fourth week, initially with short flares. A huge increase in flux occurred on day 41, but the SSS flux remained variable. A quasi-periodic oscillation every ≃35 s was always observed during the SSS phase, with variations in amplitude and a period drift that appeared to decrease in the end. The SSS has characteristics of a WD of mass >1 M ⊙. Thermonuclear burning switched off shortly after day 75, earlier than in the 2006 outburst. We discuss implications for the nova physics.

Keywords: novae; recurrent novae; high energy astrophysics; x-ray transient sources; x-ray binary stars; transient sources
The Minimal Helicity of Solar Coronal Magnetic Fields

Potential field extrapolations are widely used as minimum-energy models for the Sun's coronal magnetic field. As the reference to which other magnetic fields are compared, they have—by any reasonable definition—no global (signed) magnetic helicity. Here we investigate the internal topological structure that is not captured by the global helicity integral, by splitting it into individual field line helicities. These are computed using potential field extrapolations from magnetogram observations over Solar Cycle 24, as well as for a simple illustrative model of a single bipolar region in a dipolar background. We find that localized patches of field line helicity arise primarily from linking between strong active regions and their overlying field, so that the total unsigned helicity correlates with the product of photospheric and open fluxes. Within each active region, positive and negative helicity may be unbalanced, but the signed helicity is only around a tenth of the unsigned helicity. Interestingly, in Cycle 24, there is a notable peak in unsigned helicity caused by a single large active region. On average, the total unsigned helicity at the resolution considered is approximately twice the typical signed helicity of a single real active region, according to non-potential models in the literature.

Keywords: solar corona; solar magnetic fields; solar cycle
Systematic Variations of CO J = 2-1/1-0 Ratio and Their Implications in The Nearby Barred Spiral Galaxy M83

We present spatial variations of the CO J = 2-1/1-0 line ratio ( ${R}_{21/10}$ ) in the barred spiral galaxy M83 using Total Power Array (single-dish telescopes) data from the Atacama Large Millimeter/submillimeter Array. While the intensities of these two lines correlate tightly, ${R}_{21/10}$ varies over the disk, with a disk average ratio of 0.69, and shows the galactic center and a two-arm spiral pattern. It is high (≳0.7) in regions of high molecular gas surface density (Σmol), but ranges from low to high ratios in regions of low Σmol. The ratio correlates well with the spatial distributions and intensities of far-ultraviolet (FUV) and infrared (IR) emissions, with FUV being the best correlated. It also correlates better with the ratio of specific intensities at 70 and 350 μm, a proxy for dust temperature, than with the IR intensities. Taken together, these results suggest either a direct or indirect link between the dust heating by the interstellar radiation field and the condition of giant molecular clouds (GMCs), even though no efficient mechanism is known for a thermal coupling of dust and bulk gas in GMCs. We speculate that the large spread of ${R}_{21/10}$ in low Σmol regions, mostly at the downstream sides of spiral arms, may be due to the evolution of massive stars after spiral arm passage. Having in a late phase escaped from the spiral arms and their parental clouds, they may contribute to the dust heating by FUV and gas heating by cosmic rays produced by supernovae.

Keywords: molecular gas; spiral galaxies; interstellar medium
FOSSIL. III. Lightcurves of 371 Trans-Neptunian Objects

From the first phase of the high-cadence Formation of the Outer Solar System: an Icy Legacy (FOSSIL) survey, we analyzed lightcurves, ranging from one to four nights in length, of 371 trans-Neptunian objects (TNOs) for periodicity. We found 29 TNOs with periodic lightcurves, one of which is a good candidate for a close/contact binary. Another of the periodic FOSSIL TNOs could potentially have the fastest of all known TNO spin rates, with a period of 1.3 hr. We do not have total confidence in the period and thus plan to obtain a more detailed lightcurve for confirmation. The periodic TNOs have an average rotation period of 11.2 hr, close to the value obtained by Alexandersen et al., which had similar cadence, but different from other surveys. In regards to contention in the literature about whether smaller TNOs are more irregular in shape and thus have larger lightcurve amplitudes, we found that there is a weak correlation between absolute magnitude and lightcurve amplitude in a subset of 194 FOSSIL TNOs, even when using the more appropriate brightest (minimum) absolute magnitude instead of the time-averaged value.

Keywords: trans-neptunian objects; light curves; sky surveys
Universal Relation between the Variances of Distortions of Gravitational Waves owing to Gravitational Lensing

Gravitational waves from the distant sources are gravitationally lensed during their propagation through the intervening matter inhomogeneities before arriving at detectors. It has been proposed in the literature that the variance of the lensed waveform can be used to extract information of the matter power spectrum at very small scales and of low-mass dark halos. In this Letter, we show that the variance of the amplitude fluctuation and that of the phase fluctuation of the lensed waveform obey a simple relation irrespective of the shape of the matter power spectrum. We study conditions under which this relation can be violated and discuss some potential applications of the relation. This relation may be used to confirm the robustness of claimed observations of gravitational lensing of gravitational waves and the subsequent reconstruction of the matter power spectrum.

Keywords: gravitational lensing; gravitational waves
Modeling X-Ray and Gamma-Ray Emission from Redback Pulsar Binaries

We investigated the multiband emission from the pulsar binaries XSS J12270‑4859, PSR J2039‑5617, and PSR J2339‑0533, which exhibit orbital modulation in the X-ray and gamma-ray bands. We constructed the sources' broadband spectral energy distributions and multiband orbital light curves by supplementing our X-ray measurements with published gamma-ray results, and we modeled the data using intrabinary shock (IBS) scenarios. While the X-ray data were well explained by synchrotron emission from electrons/positrons in the IBS, the gamma-ray data were difficult to explain with the IBS components alone. Therefore, we explored other scenarios that had been suggested for gamma-ray emission from pulsar binaries: (1) inverse-Compton emission in the upstream unshocked wind zone and (2) synchrotron radiation from electrons/positrons interacting with the kilogauss magnetic field of the companion. Scenario (1) requires that the bulk motion of the wind substantially decelerates to ∼1000 km s‑1 before reaching the IBS for increased residence time, in which case the formation of a strong shock is untenable, inconsistent with the X-ray phenomenology. Scenario (2) can explain the data if we assume the presence of electrons/positrons with a Lorentz factor ∼ 108 (∼0.1 PeV) that pass through the IBS and tap a substantial portion of the pulsar voltage drop. These findings raise the possibility that the orbitally modulating gamma-ray signals from pulsar binaries can provide insights into the flow structure and energy conversion within pulsar winds and particle acceleration nearing PeV energies in pulsars. These signals may also yield greater understanding of kilogauss magnetic fields potentially hosted by the low-mass stars in these systems.

Keywords: millisecond pulsars; gamma-ray sources; theoretical models; x-ray sources; non-thermal radiation sources; spectral energy distribution; pulsars; termination shock
A Mathematical Treatment of the Offset Microlensing Degeneracy

The offset microlensing degeneracy, recently proposed by Zhang et al., has been shown to generalize the close-wide and inner-outer caustic degeneracies into a unified regime of magnification degeneracy in the interpretation of two-body planetary microlensing observations. While the inner-outer degeneracy expects the source trajectory to pass equidistant to the planetary caustics of the degenerate lens configurations, the offset degeneracy states that the same mathematical expression applies to any combination of the close, wide, and resonant caustic topologies, where the projected star-planet separations differ by an offset (s A ≠ s B) that depends on where the source trajectory crosses the lens axis. An important implication is that the s A = 1/s B solution of the close-wide degeneracy never strictly manifests in observations except when the source crosses a singular point near the primary. Nevertheless, the offset degeneracy was proposed upon numerical calculations, and no theoretical justification was given. Here, we provide a theoretical treatment of the offset degeneracy, which demonstrates its nature as a mathematical degeneracy. From first principles, we show that the offset degeneracy formalism is exact to zeroth order in the mass ratio (q) for two cases: when the source crosses the lens axis inside of caustics, and for ${({s}_{{\rm{A}}}-{s}_{{\rm{B}}})}^{6}\ll 1$ when crossing outside of caustics. The extent to which the offset degeneracy persists in oblique source trajectories is explored numerically. Finally, it is shown that the superposition principle allows for a straightforward generalization to N-body microlenses with N - 1 planetary lens components (q ≪ 1), which results in a 2 N-1-fold degeneracy.

Keywords: gravitational microlensing; binary lens microlensing