adsabs/SciX_UAT_keywords huggingface dataset:
In the Trenches of the Solar-Stellar Connection. III. The HST/COS Ecliptic-poles Stellar Survey (EclipSS)

The Ecliptic-poles Stellar Survey (EclipSS) collected far-ultraviolet (FUV: 1160-1420 Å) spectra of 49 nearby (d ≲ 100 pc) F3-K3 main-sequence stars, located at high ecliptic latitudes (north and south), using the Cosmic Origins Spectrograph of the Hubble Space Telescope. The ecliptic poles receive higher exposures from scanning missions like the Transiting Exoplanet Survey Satellite (high-precision optical photometry) and Extended Roentgen Survey with an Imaging Telescope Array (X-ray monitoring), which can deliver crucial contextual information, not otherwise easily secured. The objective was to support theoretical studies of stellar hot outer atmospheres—chromospheres (∼104 K) and coronae (≳1 MK)—which, among other things, can adversely impact exoplanets via host-star "space weather." Flux-flux diagrams (e.g., C II 1335 Å versus O I 1306 Å) were constructed for the EclipSS stars, solar Cycle 23/24 irradiances, and long-term FUV records of α Cen A (G2 V) and B (K1 V). The EclipSS cohort displays similar minimum ("basal") fluxes to the Sun and solar twin α Cen A, in chromospheric O I 1306 Å. In hotter C II 1335 Å, a downward slump of the basal fluxes—noted in previous, less controlled surveys—can now be explained as an effect of subsolar abundances. The consistent basal minima in chromospheric and higher temperature species at solar metallicity favor the idea that stellar analogs of the solar supergranulation network provide a baseline of high-energy emissions. The magnetic network is replenished by a "local dynamo" independently of the stellar spin. It can operate even when the starspot-spawning internal dynamo has ceased cycling, as during the Sun's 17th century Maunder Minimum.

Keywords: stellar activity; solar activity; spectroscopy; solar ultraviolet emission; ultraviolet sources; solar chromosphere; stellar chromospheres; sunspot cycle; maunder minimum
Probing M87 Globular Clusters for Flaring Ultraluminous X-Ray Sources

We present X-ray analysis of three short Chandra observations of M87. The basis of this analysis is the search for possible new ultraluminous X-ray sources (ULXs) in M87's globular clusters (GCs) and attempt to quantify possible variability within the observations. We searched Chandra ObsIDs 1808 (2000 July 30, 14 ks) Wilson & Yang, 3975 (2002 November 17, 5 ks) and 3977 (2003 February 4, 5 ks) Harris et al. and identified one previously discovered GC ULX, and two new GC ULX candidates. Analysis of the light-curves revealed no new evidence of variability in these sources for the duration of the observations.

Keywords: ultraluminous x-ray sources; globular star clusters
Constraining Annihilating Dark Matter Using the Multifrequency Radio Flux Profiles of the M33 Galaxy

Radio data can give stringent constraints for annihilating dark matter. In general, radio observations can detect very accurate radio flux density with high resolution and different frequencies for nearby galaxies. We are able to obtain the radio flux density as a function of distance from the galactic center and frequencies S(r, ν). In this article, we demonstrate a comprehensive radio analysis of the M33 galaxy, combining the radio flux density profile S(r) and the frequency spectrum S(ν) to get the constraints of dark matter annihilation parameters. By analyzing the archival radio data obtained from the Effelsberg telescope, we show that the dark matter annihilation contributing to the radio flux density might be insignificant in the disk region of the M33 galaxy. Moreover, by including the baryonic radio contribution, we constrain the 2σ conservative upper limits of the annihilation cross section, which can be complementary to the existing constraints based on neutrino, cosmic-ray, and gamma-ray observations. Our results indicate that analyzing the galactic multifrequency radio flux profiles can give useful and authentic constraints on dark matter for the leptophilic annihilation channels.

Keywords: dark matter
Saturn's Seismic Rotation Revisited

Normal mode seismology is a promising means of measuring rotation in gas giant interiors, and ring seismology presents a singular opportunity to do so at Saturn. We calculate Saturn's normal modes of oscillation and zonal gravity field, using nonperturbative methods for normal modes in the rigidly rotating approximation, and perturbative methods for the shifts that Saturn's deep winds induce in the mode frequencies and zonal gravity harmonics. The latter are calculated by solving the thermogravitational wind equation in an oblate geometry. Comparing many such models to gravity data and the frequencies of ring patterns excited by Saturn's normal modes, we use statistical methods to estimate that Saturn's cloud-level winds extend inward along cylinders before decaying at a depth 0.125-0.138 times Saturn's equatorial radius, or 7530-8320 km, consistent with analyses of Cassini's gravity and magnetic field data. The seismology is especially useful for pinning down Saturn's poorly constrained deep rotation period, which we estimate at 2π/ΩS = 634.7 minutes (median) with a 5/95% quantile range of 633.8-635.5 minutes. Outstanding residuals in mode frequencies at low angular degree suggest a more complicated deep interior than has been considered to date. Smaller but still significant residuals at high angular degrees also show that our picture for the thermal, composition, and/or rotation profile in Saturn's envelope is not yet complete.

Keywords: solar system gas giant planets; non-radial pulsations; planetary science; planetary rings; orbital resonances
2 mm Observations and the Search for High-redshift Dusty Star-forming Galaxies

Finding high-redshift (z ≫ 4) dusty star-forming galaxies is extremely challenging. It has recently been suggested that millimeter selections may be the best approach since the negative K-correction makes galaxies at a given far-infrared luminosity brighter at z ≳ 4 than those at z = 2-3. Here we analyze this issue using a deep Atacama Large Millimeter/submillimeter Array (ALMA) 2 mm sample obtained by targeting ALMA 870 μm priors (these priors were the result of targeting SCUBA-2 850 μm sources) in the GOODS-S. We construct prior-based 2 mm galaxy number counts and compare them with published blank field-based 2 mm counts, finding good agreement down to 0.2 mJy. Only a fraction of the current 2 mm extragalactic background light is resolved, and we estimate what observational depths may be needed to resolve it fully. By complementing the 2 mm ALMA data with a deep SCUBA-2 450 μm sample, we exploit the steep gradient with a redshift of the 2 mm-450 μm flux density ratio to estimate redshifts for those galaxies without spectroscopic or robust optical/near-infrared photometric redshifts. Our observations measure galaxies with star formation rates in excess of 250 M ⊙ yr-1. For these galaxies, the star formation rate densities fall by a factor of 9 from z = 2-3 to z = 5-6.

Keywords: galaxies; starburst galaxies; millimeter astronomy; submillimeter astronomy
Evidence of Particle Acceleration in the Superbubble 30 Doradus C with NuSTAR

We present evidence of diffuse, non-thermal X-ray emission from the superbubble 30 Doradus C (30 Dor C) using hard X-ray images and spectra from NuSTAR observations. For this analysis, we utilize data from a 200 ks targeted observation of 30 Dor C as well as 2.8 Ms of serendipitous off-axis observations from the monitoring of nearby SN 1987A. The complete shell of 30 Dor C is detected up to 20 keV, and the young supernova remnant MCSNR J0536-6913 in the southeast of 30 Dor C is not detected above 8 keV. Additionally, six point sources identified in previous Chandra and XMM-Newton investigations have hard X-ray emission coincident with their locations. Joint spectral fits to the NuSTAR and XMM-Newton spectra across the 30 Dor C shell confirm the non-thermal nature of the diffuse emission. Given the best-fit rolloff frequencies of the X-ray spectra, we find maximum electron energies of ≈70-110 TeV (assuming a B-field strength of 4 μG), suggesting 30 Dor C is accelerating particles. Particles are either accelerated via diffusive shock acceleration at locations where the shocks have not stalled behind the Hα shell, or cosmic-rays are accelerated through repeated acceleration of low-energy particles via turbulence and magnetohydrodynamic waves in the bubble's interior.

Keywords: shocks; x-ray astronomy; interstellar medium; superbubbles
Discovery of ASKAP J173608.2-321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder

We report the discovery of a highly polarized, highly variable, steep-spectrum radio source, ASKAP J173608.2-321635, located ~4° from the Galactic Center in the Galactic plane. The source was detected six times between 2020 January and 2020 September as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients (ASKAP VAST) survey at 888 MHz. It exhibited a high degree (~25%) of circular polarization when it was visible. We monitored the source with the MeerKAT telescope from 2020 November to 2021 February on a 2-4 week cadence. The source was not detected with MeerKAT before 2021 February 7 when it appeared and reached a peak flux density of 5.6 mJy. The source was still highly circularly polarized, but also showed up to 80% linear polarization, and then faded rapidly with a timescale of one day. The rotation measure of the source varied significantly, from -11.8 ± 0.8 rad m-2 to -64.0 ± 1.5 rad m-2 over three days. No X-ray counterpart was found in follow-up Swift or Chandra observations about a week after the first MeerKAT detection, with upper limits of ~5.0 × 1031 erg s-1 (0.3-8 keV, assuming a distance ~10 kpc). No counterpart is seen in new or archival near-infrared observations down to J = 20.8 mag. We discuss possible identifications for ASKAP J173608.2-321635 including a low-mass star/substellar object with extremely low infrared luminosity, a pulsar with scatter-broadened pulses, a transient magnetar, or a Galactic Center radio transient: none of these fully explains the observations, which suggests that ASKAP J173608.2-321635 may represent part of a new class of objects being discovered through radio imaging surveys.

Keywords: radio transient sources; galactic radio sources; neutron stars; magnetars; galactic center
HST FUV Spectroscopy of Super Star Cluster A in the Green Pea Analog Mrk 71: Revealing the Presence of Very Massive Stars

Mrk 71 is a low-metallicity (Z = 0.16 Z ☉) starburst region in the local dwarf galaxy NGC 2366, hosting two super star clusters (SSCs A and B), and it is recognized as a Green Pea (GP) analog with SSC A responsible for the GP properties. We present STIS and FOS far-ultraviolet (FUV) spectra of the embedded SSC Mrk 71-A obtained with the Hubble Space Telescope. The STIS FUV spectrum shows the characteristic features of very massive stars (VMS; masses >100 M ⊙) and we derive an age of 1 ± 1 Myr by comparison with the Charlot & Bruzual suite of spectral population synthesis models with upper mass limits of 300 and 600 M ⊙. We compare the STIS spectrum with all known SSC spectra exhibiting VMS signatures: NGC 5253-5, R136a, NGC 3125-A1, and the z = 2.37 Sunburst cluster. We find that the cluster mass-loss rates and wind velocities, as characterized by the C IV P Cygni profiles and the He II emission line strengths, are very similar over Z = 0.16-0.4 Z☉. This agrees with predictions that the optically thick winds of VMS will be enhanced near the Eddington limit and show little metallicity dependence. We find very strong damped Lyα absorption with N(H I) =1022.2 cm-2 associated with Mrk 71-A. We discuss the natal environment of this young SSC in terms of radiatively driven winds, catastrophic cooling, and recent models where the cluster is surrounded by highly pressurized clouds with large neutral columns.

Keywords: dwarf irregular galaxies; starburst galaxies; young massive clusters; massive stars; h ii regions; spectroscopy
Contribution of γ-Ray Burst Afterglow Emissions to the Isotropic Diffuse γ-Ray Background

The isotropic diffuse γ-ray background (IGRB) serves as a fundamental probe of the evolution of the extreme Universe. Although various astrophysical sources have been proposed as potential contributors to the IGRB, the dominant population is still under debate. γ-ray bursts (GRBs) are among candidate contributors of IGRB, although they are not as frequently discussed as blazars or starburst galaxies. Recent observations of TeV emission from GRB afterglows have provided fresh insights into this subject. This work aims to investigate the potential contribution of GRB afterglows to the IGRB under the standard afterglow model. We carefully examine the influence of each microphysical parameter of the afterglow model on this contribution, with a particular emphasis on the significant role played by the initial kinematic energy. To determine the energy and quantify the contribution of GRB afterglow to IGRBs, we utilize the observed GRB afterglow energy emissions from the Swift X-ray Telescope and Fermi Large Area Telescope instruments. Our calculations, considering the synchrotron self-Compton emission, suggest that GRB afterglows make up less than 10% of the IGRBs. To enhance the precision of our findings, it is crucial to further constrain these parameters by conducting additional ground-based observations of GRB afterglows.

Keywords: gamma-ray bursts
Spectroscopic Signature of a Re-established Accretion Disk in Symbiotic-like Recurrent Nova RS Ophiuchi

A novel method is presented which can pin down the time the accretion disk re-established itself in the RS Oph system after it experienced a nova disruption. The method is based on the re-ionization of the ejecta by photoionization from the radiation released in the boundary layer from accretion.

Keywords: classical novae; recurrent novae; shocks
ELVES. IV. The Satellite Stellar-to-halo Mass Relation Beyond the Milky Way

Quantifying the connection between galaxies and their host dark matter halos has been key for testing cosmological models on various scales. Below M ⋆ ~ 109 M ⊙, such studies have primarily relied on the satellite galaxy population orbiting the Milky Way (MW). Here we present new constraints on the connection between satellite galaxies and their host dark matter subhalos using the largest sample of satellite galaxies in the Local Volume (D ≲ 12 Mpc) to date. We use 250 confirmed and 71 candidate dwarf satellites around 27 MW-like hosts from the Exploration of Local VolumE Satellites (ELVES) Survey and use the semianalytical SatGen model for predicting the population of dark matter subhalos expected in the same volume. Through a Bayesian model comparison of the observed and the forward-modeled satellite stellar mass functions (SSMFs), we infer the satellite stellar-to-halo mass relation. We find that the observed SSMF is best reproduced when subhalos at the low-mass end are populated by a relation of the form ${M}_{\star }\propto {M}_{\mathrm{peak}}^{\alpha }$ , with a moderate slope of ${\alpha }_{\mathrm{const}}=2.10\pm 0.01$ and a low scatter, constant as a function of the peak halo mass, of ${\sigma }_{\mathrm{const}}={0.06}_{-0.05}^{+0.07}$ . A model with a steeper slope (α grow = 2.39 ± 0.06) and a scatter that grows with decreasing M peak is also consistent with the observed SSMF but is not required. Our new model for the satellite-subhalo connection, based on hundreds of Local Volume satellite galaxies, is in line with what was previously derived using only MW satellites.

Keywords: observational astronomy; bayesian statistics; dwarf galaxies; dark matter; scaling relations; observational cosmology; galaxy groups; surveys
Changing Look of the Optical Spectrum of the MeV Blazar PKS 0446+112 (4FGL J0449.1+1121)

Following the high activity of the γ-ray Fermi source 4FGL J0449.1+1121 (PKS 0446+112), possibly associated with a IceCube neutrino event IC-240105A, we obtained optical spectroscopy with the Gran Telescopio Canarias of the counterpart. We detect a clear emission line at 3830 Å identified as Lyα that confirms the redshift of source at z = 2.153. Comparing with previous spectroscopy, we find an increase of the continuum by a factor ∼10, and a significant decrease of the C IV 1550 emission line flux by a factor ∼5. This produces a dramatic drop of the equivalent width from ∼20 to 0.8 Å, which is suggestive of a very high jet activity. The full width half maximum of the emission lines are midway (1000–2000 km s ‑1) between those typical of the broad and narrow regions of quasars. Based on this, the source classification is intermediate between flat spectrum radio quasar and BL Lac object.

Keywords: bl lacertae objects; redshifted; gamma-ray sources; neutrino astronomy
Identifying 46 New Open Cluster Candidates in Gaia EDR3 Using a Hybrid pyUPMASK and Random Forest Method

Open clusters (OCs) are regarded as tracers to understand stellar evolution theory and validate stellar models. In this study, we presented a robust approach to identifying OCs. A hybrid method consisting of pyUPMASK and the random forest (RF) algorithm is first used to remove field stars and determine more reliable members. An identification model based on the RF algorithm built based on 3714 OC samples from Gaia DR2 and EDR3 is then applied to identify OC candidates. The OC candidates are obtained after isochrone fitting, advanced stellar population synthesis model fitting, and visual inspection. Using the proposed approach, we revisited 868 candidates and preliminarily clustered them by the friends-of-friends algorithm in Gaia EDR3. Excluding OCs that have already been reported, we focused on the remaining 300 unknown candidates. From high to low fitting quality, these unrevealed candidates were further classified into Class A (59), Class B (21), and Class C (220). As a result, 46 new reliable OC candidates among Classes A and B are identified after visual inspection.

Keywords: open star clusters
Monitoring Hα Emission from the Wide-orbit Brown-dwarf Companion FU Tau B

Monitoring mass accretion onto substellar objects provides insights into the geometry of the accretion flows. We use the Lulin One-meter Telescope to monitor Hα emission from FU Tau B, a ~19 M Jup brown-dwarf companion at 5.″7 (719 au) from the host star, for six consecutive nights. This is the longest continuous Hα monitoring for a substellar companion near the deuterium-burning limit. We aim to investigate if accretion near the planetary regime could be rotationally modulated as suggested by magnetospheric accretion models. We find tentative evidence that Hα mildly varies on hourly and daily timescales, though our sensitivity is not sufficient to definitively establish any rotational modulation. No burst-like events are detected, implying that accretion onto FU Tau B is overall stable during the time baseline and sampling windows over which it was observed. The primary star FU Tau A also exhibits Hα variations over timescales from minutes to days. This program highlights the potential of monitoring accretion onto substellar objects with small telescopes.

Keywords: accretion; brown dwarfs; stellar accretion; time series analysis; lomb-scargle periodogram
Immediate Origin of the Moon as a Post-impact Satellite

The Moon is traditionally thought to have coalesced from the debris ejected by a giant impact onto the early Earth. However, such models struggle to explain the similar isotopic compositions of Earth and lunar rocks at the same time as the system's angular momentum, and the details of potential impact scenarios are hotly debated. Above a high resolution threshold for simulations, we find that giant impacts can immediately place a satellite with similar mass and iron content to the Moon into orbit far outside Earth's Roche limit. Even satellites that initially pass within the Roche limit can reliably and predictably survive, by being partially stripped and then torqued onto wider, stable orbits. Furthermore, the outer layers of these directly formed satellites are molten over cooler interiors and are composed of around 60% proto-Earth material. This could alleviate the tension between the Moon's Earth-like isotopic composition and the different signature expected for the impactor. Immediate formation opens up new options for the Moon's early orbit and evolution, including the possibility of a highly tilted orbit to explain the lunar inclination, and offers a simpler, single-stage scenario for the origin of the Moon.

Keywords: lunar origin; impact phenomena; earth-moon system; hydrodynamical simulations
How Do We Optimally Sample Model Grids of Exoplanet Spectra?

The construction and implementation of atmospheric model grids is a popular tool in exoplanet characterization. These typically vary a number of parameters linearly, containing one model for every combination of parameter values. Here we investigate alternative methods of sampling parameters, including random sampling and Latin hypercube (LH) sampling, and how these compare to linearly sampled grids. We use a random forest to analyze the performance of these grids for two different models, as well as investigate the information content of the particular model grid from Goyal et al. (2019). We also use nested sampling to implement mock atmospheric retrievals on simulated James Webb Space Telescope transmission spectra by interpolating on linearly sampled model grids. Our results show that random or LH sampling outperforms linear sampling in parameter predictability for our higher-dimensional models, requiring fewer models in the grid, and thus allowing for more computationally intensive forward models to be used. We also found that using a traditional retrieval with interpolation on a linear grid can produce biased posterior distributions, especially for parameters with nonlinear effects on the spectrum. In particular, we advise caution when performing linear interpolation on the C/O ratio, cloud properties, and metallicity. Finally, we found that the information content analysis of the grid from Goyal et al. (2019) was able to highlight key areas of the spectra where the presence or absence of certain molecules can be detected, providing good indicators for parameters such as temperature and C/O ratio.

Keywords: exoplanets; exoplanet atmospheres
Solar Energetic Electron Events Associated with Hard X-Ray Flares

We investigate 16 solar energetic electron (SEE) events measured by WIND/3DP with a double-power-law spectrum and the associated western hard X-ray (HXR) flares measured by RHESSI with good count statistics, from 2002 February to 2016 December. In all the 16 cases, the presence of an SEE power-law spectrum extending down to ≤5 keV at 1 au implies that the SEE source would be high in the corona, at a heliocentric distance of ≥1.3 solar radii, while the footpoint or footpoint-like emissions shown in HXR images suggest that the observed HXRs are likely produced mainly by HXR-producing electrons via thick-target bremsstrahlung processes very low in the corona. We find that for all the 16 cases, the estimated power-law spectral index of HXR-producing electrons is no less than the observed high-energy spectral index of SEEs, and it shows a positive correlation with the high-energy spectral index of SEEs. In addition, the estimated number of SEEs is only ∼10-4-10-2 of the estimated number of HXR-producing electrons at energies above 30 keV, but with a positive correlation between the two numbers. These results suggest that in these cases, SEEs are likely formed by upward-traveling electrons from an acceleration source high in the corona, while their downward-traveling counterparts may undergo a secondary acceleration before producing HXRs via thick-target bremsstrahlung processes. In addition, the associated 3He/4He ratio is positively correlated with the observed high-energy spectral index of SEEs, indicating a possible relation of the 3He ion acceleration with high-energy SEEs.

Keywords: solar energetic particles; solar flares