adsabs/SciX_UAT_keywords huggingface dataset:
Kinematics and Metallicity of Red Giant Branch Stars in the Northeast Shelf of M31

We obtained Keck/DEIMOS spectra of 556 individual red giant branch stars in four spectroscopic fields spanning 13-31 projected kpc along the northeast (NE) shelf of M31. We present the first detection of a complete wedge pattern in the space of projected M31-centric radial distance versus line-of-sight velocity for this feature, which includes the returning stream component of the shelf. This wedge pattern agrees with expectations of a tidal shell formed in a radial merger and provides strong evidence in favor of predictions of Giant Stellar Stream (GSS) formation models in which the NE shelf originates from the second orbital wrap of the tidal debris. The observed concentric wedge patterns of the NE, west (W), and southeast (SE) shelves corroborate this interpretation independently of the models. We do not detect a kinematical signature in the NE shelf region corresponding to an intact progenitor core, favoring GSS formation models in which the progenitor is completely disrupted. The shelf's photometric metallicity ([Fe/H]phot) distribution implies that it is dominated by tidal material, as opposed to the phase-mixed stellar halo or the disk. The metallicity distribution ([Fe/H]phot = -0.42 ± 0.01) also matches the GSS, and consequently the W and SE shelves, further supporting a direct physical association between the tidal features. *The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

Keywords: galaxy structure; andromeda galaxy; galaxy mergers; galaxy dynamics
Evolution of AM CVn Binaries with White Dwarf Donors

The evolution and the stability of mass transfer of CO+He white dwarf (WD) binaries are not well understood. Observationally they may emerge as AM CVn binaries and are important gravitational wave (GW) emitters. In this work, we have modeled the evolution of double WD binaries with accretor masses of 0.50-1.30 M ⊙ and donor masses of 0.17-0.45 M ⊙ using the detailed stellar evolution code MESA. We find that the evolution of binaries with same donor masses but different accretor masses is very similar and binaries with same accretor masses but larger He donor masses have larger maximum mass transfer rates and smaller minimum orbital periods. We also demonstrate that the GW signal from AM CVn binaries can be detected by spaceborne GW observatories, such as LISA and TianQin. There is a linear relation between the donor mass and gravitational wave frequency during the mass transfer phase. In our calculation, all binaries can have dynamically stable mass transfer, which is very different from previous studies. The threshold donor mass of Eddington-limited mass transfer for a given accretor WD mass is lower than previous studies. Assuming that a binary may enter a common envelope if the mass transfer rate exceeds the maximum stable burning rate of He, we provide a new criterion for double WDs surviving mass transfer, which is below the threshold of the Eddington limit. Finally, we find that some systems with oxygen-neon (ONe) WDs in our calculation may evolve into detached binaries consisting of neutron stars and extremely low-mass He WDs, and further ultracompact X-ray binaries.

Keywords: close binary stars; white dwarf stars; am canum venaticorum stars; compact binary stars; gravitational wave sources
Blast Waves from Magnetar Flares and Fast Radio Bursts

Magnetars younger than one century are expected to be hyperactive: besides winds powered by rotation, they can generate frequent magnetic flares, which launch powerful blast waves into the wind. The resulting magnetized shocks act as masers producing bright radio emission. This theoretical picture predicts radio bursts with the following properties. (1) GHz radio emission occurs at radii $r\sim {10}^{14}\,\mathrm{cm}$ and lasts $\lesssim 1$ ms in the observer's time. (2) Induced scattering in the surrounding wind does not suppress the radio burst. (3) The emission has linear polarization set by the magnetar rotation axis. (4) The emission drifts to lower frequencies during the burst, and its duration broadens at lower frequencies. (5) Blast waves in inhomogeneous winds may emit variable radio bursts; periodicity might appear on submillisecond timescales if the magnetar rotates with a ∼1 s period. However, the observed burst structure is likely changed by lensing effects during propagation through the host galaxy. (6) The magnetar bursts should repeat, with rare ultrastrong events (possibly up to ∼1043 erg in radio waves) or more frequent weak bursts. (7) When a repeating magnetic flare strikes the wind bubble in the tail of a previous flare, the radio burst turns into a bright optical burst lasting $\lesssim 1\,{\rm{s}}$ . Locations of hyperactive magnetars in their host galaxies depend on how they form: magnetars created in supernova explosions will trace star formation regions, and magnetars formed in mergers of compact objects will be offset. The merger magnetars are expected to be particularly hyperactive.

Keywords: radio bursts; magnetars; shocks; neutron stars; optical bursts; plasma astrophysics
The Dust Sublimation Region of the Type 1 AGN NGC 4151 at a Hundred Microarcsecond Scale as Resolved by the CHARA Array Interferometer

The nuclear region of Type 1 active galactic nuclei (AGNs) has only been partially resolved so far in the near-infrared (IR), where we expect to see the dust sublimation region and the nucleus directly without obscuration. Here, we present the near-IR interferometric observation of the brightest Type 1 AGN NGC 4151 at long baselines of ~250 m using the CHARA Array, reaching structures at hundred microarcsecond scales. The squared visibilities decrease down to as low as ~0.25, definitely showing that the structure is resolved. Furthermore, combining with the previous visibility measurements at shorter baselines but at different position angles, we show that the structure is elongated perpendicular to the polar axis of the nucleus, as defined by optical polarization and a linear radio jet. A thin-ring fit gives a minor/major axis ratio of ~0.7 at a radius ~0.5 mas (~0.03 pc). This is consistent with the case where the sublimating dust grains are distributed preferentially in the equatorial plane in a ring-like geometry, viewed at an inclination angle of ~40°. The recent mid-IR interferometric finding of polar-elongated geometry at a pc scale, together with a larger-scale polar outflow as spectrally resolved by the Hubble Space Telescope, would generally suggest a dusty, conical and hollow outflow being launched, presumably in the dust sublimation region. This might potentially lead to a polar-elongated morphology in the near-IR, as opposed to the results here. We discuss a possible scenario where an episodic, one-off anisotropic acceleration formed a polar-fast and equatorially slow velocity distribution, having led to an effectively flaring geometry as we observe.

Keywords: black holes; long baseline interferometry; active galaxies
The Spitzer Coverage of HSC-Deep with IRAC for Z studies (SHIRAZ). I. IRAC Mosaics

We present new Spitzer Infrared Array Camera (IRAC) 3.6 and 4.5 μm mosaics of three fields, E-COSMOS, DEEP2-F3, and ELAIS-N1. Our mosaics include both new IRAC observations as well as reprocessed archival data in these fields. These fields are part of the HSC-Deep grizy survey and have a wealth of additional ancillary data. The addition of these new IRAC mosaics is critical in allowing for improved photometric redshifts and stellar population parameters at cosmic noon and earlier epochs. The total area mapped by this work is ~17 deg2 with a mean integration time of ≈1200s, providing a median 5σ depth of 23.7(23.3) at 3.6(4.5) μm in AB. We perform SExtractor photometry both on the combined mosaics as well as the single-epoch mosaics taken ≈6 months apart. The resultant IRAC number counts show good agreement with previous studies. In combination with the wealth of existing and upcoming spectrophotometric data in these fields, our IRAC mosaics will enable a wide range of galactic evolution and AGN studies. With that goal in mind, we make the combined IRAC mosaics and coverage maps of these three fields publicly available.

Keywords: astronomy data analysis; astronomy data acquisition
The Great Inequality and the Dynamical Disintegration of the Outer Solar System

Using an ensemble of N-body simulations, this paper considers the fate of the outer gas giants (Jupiter, Saturn, Uranus, and Neptune) after the Sun leaves the main sequence and completes its stellar evolution. Due to solar mass loss—which is expected to remove roughly half of the star's mass—the orbits of the giant planets expand. This adiabatic process maintains the orbital period ratios, but the mutual interactions between planets and the width of mean-motion resonances (MMR) increase, leading to the capture of Jupiter and Saturn into a stable 5:2 resonant configuration. The expanded orbits, coupled with the large-amplitude librations of the critical MMR angle, make the system more susceptible to perturbations from stellar flyby interactions. Accordingly, within about 30 Gyr, stellar encounters perturb the planets onto the chaotic subdomain of the 5:2 resonance, triggering a large-scale instability, which culminates in the ejections of all but one planet over the subsequent ∼10 Gyr. After an additional ∼50 Gyr, a close stellar encounter (with a perihelion distance less than ∼200 au) liberates the final planet. Through this sequence of events, the characteristic timescale over which the solar system will be completely dissolved is roughly 100 Gyr. Our analysis thus indicates that the expected dynamical lifetime of the solar system is much longer than the current age of the universe, but is significantly shorter than previous estimates.

Keywords: dynamical evolution; solar system
Observation of Cassini's Entry into Saturn: No Detection, and Lessons Learned

The mission of the 2000 kg Cassini spacecraft concluded on 2017 September 15, by its deliberate entry into Saturn's atmosphere at some 31.1 km s-1. Observations, using Hubble and groundbased observatories, to attempt optical detection of this 0.25 kT "artificial meteor" are summarized. No signatures were identified. A challenge with observing the event is that due to atmospheric drag, its timing was not completely deterministic months or even days in advance, a particular problem for space observatories. While imaging observations needed no geometric specification more than "Saturn," observations with spectrometers required pointing the instrument aperture or slit at the specific impact site. Since giant planet longitude systems are not always familiar, distribution of an unambiguous "finder chart" showing the location of the predicted entry site on the disk is essential, as is clarity on whether stated times are spacecraft event time, or Earth received time (light-travel time, 83 minutes, later).

Keywords: space vehicles; meteors; meteoroids
Energy Spectrum of Solar Energetic Electron Events over 25 Years

We investigate the peak flux energy spectrum of 458 solar energetic electron (SEE) events with a clear velocity dispersion detected at energies from ≤4.2 to ≥108 keV by Wind/3DP from 1994 December through 2019 December, utilizing a pan-spectrum fitting method. According to the fitted spectral parameters, these 458 events are self-consistently classified into five spectral types: 304 downward double-power-law (DDPL) events, 32 upward double-power-law (UDPL) events, 23 single-power-law (SPL) events, 44 Ellison-Ramaty (ER) events, and 55 logarithmic-parabola (LP) events. The DDPL events can be further divided into two types: 231 ${\mathrm{DDPL}}_{{E}_{B}\geqslant 20\mathrm{keV}}$ events and 73 ${\mathrm{DDPL}}_{{E}_{B}\lt 20\mathrm{keV}}$ events, since their break energy E B exhibits a double-peak distribution separated by a dip at ~20 keV. The ${\mathrm{DDPL}}_{{E}_{B}\lt 20\mathrm{keV}}$ ( ${\mathrm{DDPL}}_{{E}_{B}\geqslant 20\mathrm{keV}}$ ) events show a power-law spectral index of ${2.0}_{-0.2}^{+0.2}$ ( ${2.1}_{-0.3}^{+0.3}$ ) at energies below ${E}_{B}={5.6}_{-2.4}^{+2.3}$ ( ${61}_{-12}^{+23}$ ) keV and an index of ${3.3}_{-0.3}^{+0.5}$ ( ${3.9}_{-0.7}^{+0.6}$ ) at energies above. The UDPL events have a spectral index of ${3.0}_{-0.3}^{+0.3}$ at energies below ${E}_{B}={5.1}_{-1.8}^{+4.2}$ keV and an index of ${2.2}_{-0.3}^{+0.2}$ at energies above. The SPL events exhibit a spectral index of ${2.8}_{-0.2}^{+0.5}$ . The ER events show a spectral index of ${1.9}_{-0.3}^{+0.3}$ at energies below ${E}_{c}={31}_{-11}^{+19}$ keV. The LP events are characterized by a spectral slope of ${1.8}_{-0.3}^{+0.4}$ ( ${3.6}_{-0.5}^{+0.7}$ ) at 2.8 keV (108 keV). The six spectral types also behave differently in the relationship between spectral parameters and in solar cycle variations. The spectral shape of most SEE events appears to be unrelated to the estimated electron path lengths. These results suggest that the formation of SEE events can involve complex processes/sources.

Keywords: solar energetic particles; solar particle emission
Light-curve Test of GRB 090717 as a Gravitationally Lensed Echo

Light curve comparison is a simple but statistically powerful test for the presence of gravitational lensing in gamma-ray bursts (GRBs). Since gravitational lensing does not change relative timing internal to images, all source images created by gravitational lensing should show the same light curve to within a scale factor in amplitude. Applied to the recent claim that the two pulses in GRB 090717 are lensed images of the same pulse, it was found that the light curves for these pulses actually differ at about the 5.0σ confidence level (99.999956%) in a χ2 comparison test. Under the given assumptions, GRB 090717 therefore does not present a compelling example of gravitational lensing.

Keywords: gravitational lensing; gamma-ray bursts; chi-squared statistic; gamma-ray astronomy
Revisiting the Space Weather Environment of Proxima Centauri b

Close-in planets orbiting around low-mass stars are exposed to intense energetic photon and particle radiation and harsh space weather. We have modeled such conditions for Proxima Centauri b, a rocky planet orbiting in the habitable zone of our closest neighboring star, finding a stellar wind pressure 3 orders of magnitude higher than the solar wind pressure on Earth. At that time, no Zeeman-Doppler observations of the surface magnetic field distribution of Proxima Cen were available and a proxy from a star with a similar Rossby number to Proxima was used to drive the MHD model. Recently, the first Zeeman-Doppler imaging (ZDI) observation of Proxima Cen became available. We have modeled Proxima b's space weather using this map and compared it with the results from the proxy magnetogram. We also computed models for a high-resolution synthetic magnetogram for Proxima b generated by a state-of-the-art dynamo model. The resulting space weather conditions for these three scenarios are similar with only small differences found between the models based on the ZDI observed magnetogram and the proxy. We conclude that our proxy magnetogram prescription based on the Rossby number is valid, and provides a simple way to estimate stellar magnetic flux distributions when no direct observations are available. Comparisons with models based on the synthetic magnetogram show that the exact magnetogram details are not important for predicting global space weather conditions of planets, reinforcing earlier conclusions that the large-scale (low-order) field dominates, and that the small-scale field does not have much influence on the ambient stellar wind.

Keywords: stellar activity; stellar winds; space weather
The First Radio Spectrum of a Rapidly Rotating A-type Star

The radio spectra of main-sequence stars remain largely unconstrained due to the lack of observational data to inform stellar atmosphere models. As such, the dominant emission mechanisms at long wavelengths, how they vary with spectral type, and how much they contribute to the expected brightness at a given radio wavelength are still relatively unknown for most spectral types. We present radio continuum observations of Altair, a rapidly rotating A-type star. We observed Altair with NOEMA in 2018 and 2019 at 1.34, 2.09, and 3.22 mm and with the Very Large Array in 2019 at 6.7 and 9.1 mm. In the radio spectra, we see a brightness temperature minimum at millimeter wavelengths followed by a steep rise to temperatures larger than the optical photosphere, behavior that is unexpected for A-type stars. We use these data to produce the first submillimeter to centimeter spectrum of a rapidly rotating A-type star informed by observations. We generated both PHOENIX and KINICH-PAKAL model atmospheres and determine the KINICH-PAKAL model better reproduces Altair's radio spectrum. The synthetic spectrum shows a millimeter brightness temperature minimum followed by significant emission over that of the photosphere at centimeter wavelengths. Together, these data and models show how the radio spectrum of an A-type star can reveal the presence of a chromosphere, likely induced by rapid rotation, and that a Rayleigh Jean's extrapolation of the stellar photosphere is not an adequate representation of a star's radio spectrum.

Keywords: radio continuum emission; stellar atmospheres; circumstellar matter; submillimeter astronomy; millimeter astronomy
Evidence for H2 Dissociation and Recombination Heat Transport in the Atmosphere of KELT-9b

Phase curve observations provide an opportunity to study the energy budgets of exoplanets by quantifying the amount of heat redistributed from their daysides to their nightsides. Theories of phase curves for hot Jupiters have focused on the balance between radiation and dynamics as the primary parameter controlling heat redistribution. However, recent phase curves have shown deviations from the trends that emerge from this theory, which has led to work on additional processes that may affect hot Jupiter energy budgets. One such process, molecular hydrogen dissociation and recombination, can enhance energy redistribution on ultra-hot Jupiters with temperatures above ∼2000 K. In order to study the impact of H2 dissociation on ultra-hot Jupiters, we present a phase curve of KELT-9b observed with the Spitzer Space Telescope at 4.5 μm. KELT-9b is the hottest known transiting planet, with a 4.5 μm dayside brightness temperature of {4566}-136+140 {{K}} and a nightside temperature of {2556}-97+101 {{K}}. We observe a phase curve amplitude of 0.609 ± 0.020 and an offset of {18.7}-2.3+2.1^\circ . The observed amplitude is too small to be explained by a simple balance between radiation and advection. General circulation models (GCMs) and an energy balance model that include the effects of H2 dissociation and recombination provide a better match to the data. The GCMs, however, predict a maximum phase offset of 5°, which disagrees with our observations at >5σ confidence. This discrepancy may be due to magnetic effects in the planet’s highly ionized atmosphere.

Keywords: hot jupiters; exoplanet atmospheres
NuSTAR Observations of a Heavily X-Ray-obscured AGN in the Dwarf Galaxy J144013+024744

We present a multiwavelength analysis of the dwarf Seyfert 2 galaxy J144013+024744, a candidate obscured active galactic nucleus (AGN) thought to be powered by an intermediate-mass black hole (IMBH, M • ≈ 104-106 M ⊙) of mass M • ~ 105.2 M ⊙. To study its X-ray properties, we targeted J144013+024744 with NuSTAR for ≈100 ks. The X-ray spectrum was fitted with an absorbed power law, Pexmon, and a physical model (RXTorus). A Bayesian X-ray analysis was performed to estimate the posteriors. The phenomenological and the physical models suggest the AGN to be heavily obscured by a column density of N H = (3.4-7.0) × 1023 cm-2. In particular, the RXTorus model with a subsolar metallicity suggests the obscuring column to be almost Compton-thick. We compared the 2-10 keV intrinsic X-ray luminosity with the inferred X-ray luminosities based on empirical scaling relations for unobscured AGNs using L [OIV] 25.89 μm, L [OIII] λ5007, and L 6μm and found that the high-excitation [OIV] line provides a better estimate of the intrinsic 2-10 keV X-ray luminosity ( ${L}_{2\mbox{--}10}^{\mathrm{int}}\sim {10}^{41.41}$ erg s-1). Our results suggest that J144013+024744 is the first type 2 dwarf galaxy that shows X-ray spectroscopic evidence for obscuration. The column density that we estimated is among the highest measured to date for IMBH-powered AGNs, implying that a typical AGN torus geometry might extend to the low-mass end. This work has implications for constraining the BH occupation fraction in dwarf galaxies using X-ray observations.

Keywords: active galactic nuclei
Periodic Variable Star Classification with Deep Learning: Handling Data Imbalance in an Ensemble Augmentation Way

Time-domain astronomy is progressing rapidly with the ongoing and upcoming large-scale photometric sky surveys led by the Vera C. Rubin Observatory project (LSST). Billions of variable sources call for better automatic classification algorithms for light curves. Among them, periodic variable stars are frequently studied. Different categories of periodic variable stars have a high degree of class imbalance and pose a challenge to algorithms including deep learning methods. We design two kinds of architectures of neural networks for the classification of periodic variable stars in the Catalina Survey's Data Release 2: a multi-input recurrent neural network (RNN) and a compound network combing the RNN and the convolutional neural network (CNN). To deal with class imbalance, we apply Gaussian Process to generate synthetic light curves with artificial uncertainties for data augmentation. For better performance, we organize the augmentation and training process in a "bagging-like" ensemble learning scheme. The experimental results show that the better approach is the compound network combing RNN and CNN, which reaches the best result of 86.2% on the overall balanced accuracy and 0.75 on the macro F1 score. We develop the ensemble augmentation method to solve the data imbalance when classifying variable stars and prove the effectiveness of combining different representations of light curves in a single model. The proposed methods would help build better classification algorithms of periodic time series data for future sky surveys (e.g., LSST).

Keywords: periodic variable stars; light curve classification; neural networks; time domain astronomy; algorithms