adsabs/SciX_UAT_keywords huggingface dataset:
Hubble Space Telescope STIS Spectroscopy of Nova T Aurigae 1891

T Aurigae is an eclipsing old nova that exploded in 1891. At a Gaia EDR3 distance of 815-871 pc, it is a relatively nearby old nova. Through ultraviolet spectral modeling and using the new precise Gaia distance, we find that the HST/STIS spectrum of T Aurigae is consistent with an accretion disk with a mass-transfer rate $\dot{M}$ of the order of 10-8 M ⊙ yr-1, for a white-dwarf mass of M wd ≈ 0.7 ± 0.2M ⊙, an inclination of i ~ 60°, and a Gaia distance of of ${840}_{-25}^{+31}$ pc. The sharp absorption lines of metals cannot form in the disk and are likely forming in material above the disk (e.g., due stream disk overflow), in circumbinary material, and/or in material associated with the ejected shell from the 1891 nova explosion. The saturated hydrogen Lyα absorption feature is attributed to a large interstellar-medium hydrogen-column density of the order of 1021cm-2 toward T Aur, as corroborated by the value of its reddening E(B - V) = 0.42 ± 0.08.

Keywords: cataclysmic variable stars; classical novae
RW Aur A: SpeX Spectral Evidence for Differentiated Planetesimal Formation, Migration, and Destruction in an 3 Myr Old Excited CTTS System

We present 2007-2020 SpeX VISNIR spectral monitoring of the highly variable RW Aur A CTTS. We find direct evidence for a highly excited, IR-bright, asymmetric, and time-variable system. Comparison of the spectral and temporal trends found determines five different components: (1) a stable continuum from 0.7 to 1.3 μm, with color temperature ~4000 K, produced by the CTTS photospheric surface; (2) variable hydrogen emission lines emitted from hot excited hydrogen in the CTTS's protostellar atmosphere/accretion envelope; (3) hot CO gas in the CTTS's protostellar atmosphere/accretion envelope; (4) highly variable 1.8-5.0 μm thermal continuum emission with color temperature ranging from 1130 to 1650 K, due to a surrounding accretion disk that is spatially variable and has an inner wall at r ~ 0.04 au and T ~ 1650 K and outer edges at ~1200 K; and (5) transient, bifurcated signatures of abundant Fe II + associated S I, Si I, and Sr I in the system's jet structures. The bifurcated signatures first appeared in 2015, but these collapsed and disappeared into a small single-peaked protostellar atmosphere feature by late 2020. The temporal evolution of RW Aur A's spectral signatures is consistent with a dynamically excited CTTS system forming differentiated Vesta-sized planetesimals in an asymmetric accretion disk and migrating them inward to be destructively accreted. By contrast, nearby coeval binary companion RW Aur B evinces only a stable WTTS photospheric continuum from 0.7 to 1.3 μm + cold CO gas in absorption + stable 1.8-5.0 μm thermal disk continuum emission with color temperature ~1650 K.

Keywords: t tauri stars; irregular variable stars; spectroscopy; astronomical techniques; near infrared astronomy; planetary system formation; protoplanetary disks
Kilonovae and Optical Afterglows from Binary Neutron Star Mergers. II. Optimal Search Strategy for Serendipitous Observations and Target-of-opportunity Observations of Gravitational Wave Triggers

In the second work of this series, we explore the optimal search strategy for serendipitous and gravitational-wave-triggered target-of-opportunity (ToO) observations of kilonovae and optical short-duration gamma-ray burst (sGRB) afterglows from binary neutron star (BNS) mergers, assuming that cosmological kilonovae are AT2017gfo-like (but with viewing-angle dependence) and that the properties of afterglows are consistent with those of cosmological sGRB afterglows. A one-day cadence serendipitous search strategy with an exposure time of ~30 s can always achieve an optimal search strategy of kilonovae and afterglows for various survey projects. We show that the optimal detection rates of the kilonovae (afterglows) are ~0.3/0.6/1/20 yr-1 (~50/60/100/800 yr-1) for Zwicky the Transient Facility (ZTF)/Multi-channel Photometric Survey Telescope (Mephisto)/Wide Field Survey Telescope (WFST)/Large Synoptic Survey Telescope (LSST), respectively. A better search strategy for SiTian than the current design is to increase the exposure time. In principle, a fully built SiTian can detect ~7(2000) yr-1 kilonovae (afterglows). Population properties of electromagnetic (EM) signals detected by serendipitous observations are studied in detail. For ToO observations, we predict that one can detect ~11 yr-1 BNS gravitational wave (GW) events during the fourth observing run (O4) by considering an exact duty cycle of the third observing run. The median GW sky localization area is expected to be ~10 deg2 for detectable BNS GW events. For O4, we predict that ZTF/Mephisto/WFST/LSST can detect ~5/4/3/3 kilonovae (~1/1/1/1 afterglows) per year, respectively. The GW detection rates, GW population properties, GW sky localizations, and optimistic ToO detection rates of detectable EM counterparts for BNS GW events at the Advanced Plus, LIGO Voyager, and ET&CE eras are detailedly simulated in this paper.

Keywords: gravitational waves; neutron stars; gamma-ray bursts
HD 41004: A Photometric Sheep in Wolf's Clothing?

HD 41004 is an unusual multiple star/planet system. Recent TESS photometry revealed a striking 31 hr period, far too short for the slowly rotating K dwarf primary. Instead, the modulation likely is from the diminutive dM secondary, spun-up to synchronous rotation by its close-in brown dwarf companion, and suffering intense magnetic activity as a result. If so, the red dwarf is perhaps destined to someday return the favor by cannibalizing its substellar partner.

Keywords: stellar activity; brown dwarfs; exoplanets
Constraints on Aquatic Photosynthesis for Terrestrial Planets around Other Stars

Aquatic photosynthesis plays a major role in carbon fixation and O2 production on Earth. In this Letter, we analyze the prospects for oxygenic photosynthesis in aquatic environments on modern Earth-analogs around F-, G-, K-, and M-type stars. Our analysis takes into account the spectral type of the host star, attenuation of light by aquatic organisms, and rates of respiration and photosynthesis. We study the compensation depth ( ${{ \mathcal Z }}_{\mathrm{CO}}$ ) and the critical depth ( ${{ \mathcal Z }}_{\mathrm{CR}}$ ), defined respectively as the locations where the net growth rates and vertically integrated net growth rates of photoautotrophs become zero. Our analysis suggests that ${{ \mathcal Z }}_{\mathrm{CO}}$ declines by more than an order of magnitude as one moves from the habitable zones around Sun-like stars to late-type M-dwarfs, but ${{ \mathcal Z }}_{\mathrm{CR}}$ decreases by only a modest amount (∼40%). For M-dwarf exoplanets, we propose that the photosynthetic red edge may constitute a more robust biosignature of aquatic photosynthesis compared to atmospheric O2.

Keywords: astrobiology; extrasolar rocky planets; habitable planets; exoplanet surface characteristics; exoplanet surface composition; biosignatures
Using Mg II Doublet to Predict the Lyman Continuum Escape Fraction from 14 HETDEX Galaxies

Indirect diagnostics of Lyman continuum (LyC) escape are needed to constrain which sources reionized the universe. We used Mg II to predict the LyC escape fraction ( ${f}_{\mathrm{esc}}^{\mathrm{LyC}}$ ) in 14 galaxies selected from the Hobby-Eberly Telescope Dark Energy Experiment solely based upon their Mg II properties. Using the Low Resolution Spectrograph on HET, we identified 7 and 5 possible LyC leakers depending on the method, with ${f}_{\mathrm{esc}}^{\mathrm{LyC}}$ ranging from 3% to 80%. Interestingly, our targets display diverse [O III]/[O II] ratios (O32), with strong inferred LyC candidates showing lower O32 values than previous confirmed LyC leaker samples. Additionally, a correlation between dust and ${f}_{\mathrm{esc}}^{\mathrm{LyC}}$ was identified. Upcoming Hubble Space Telescope/Cosmic Origins Spectrograph LyC observations of our sample will test if Mg II and dust are predictors of ${f}_{\mathrm{esc}}^{\mathrm{LyC}}$ , providing insights for future JWST studies of high-redshift galaxies.

Keywords: reionization; galaxies; starburst galaxies
Line Profile Studies of Coronal Active Regions in Fe XII λ195.12 Using Hinode/EIS

Coronal active regions are studied using Hinode/EIS observations in the EUV line Fe XII λ195.12 by analyzing their line profiles from 2006 December to 2019 December. The period covers the last 2 yr of solar cycle 23 and solar cycle 24 fully. Active regions are the main source of magnetic field in the solar atmosphere, important in its heating and dynamics. Line profiles were obtained from various active regions spread across the Sun on a monthly basis from which we obtained the intensity, line width, Doppler velocity, and centroid and examined their variation during the solar cycle. The histograms of the Doppler velocity and centroid show that they behave in six different ways with respect to the position of rest wavelength. In addition, the shifts in the centroid were found to be more compared to the Doppler velocity. The variation of the line width with respect to the Doppler velocity or the centroid mostly follows a second-degree polynomial. A multicomponent line profile is simulated to explain the difference in the behavior of the Doppler velocity and the centroid with respect to the line width. We also find that the intensity and the line width of the different data sets show a global dependence on the solar cycle with a good correlation. The implications of the results for the coronal heating and dynamics are pointed out.

Keywords: solar atmosphere; solar corona; solar active regions
ANTARES and IceCube Combined Search for Neutrino Point-like and Extended Sources in the Southern Sky

A search for point-like and extended sources of cosmic neutrinos using data collected by the ANTARES and IceCube neutrino telescopes is presented. The data set consists of all the track-like and shower-like events pointing in the direction of the Southern Sky included in the nine-year ANTARES point-source analysis, combined with the throughgoing track-like events used in the seven-year IceCube point-source search. The advantageous field of view of ANTARES and the large size of IceCube are exploited to improve the sensitivity in the Southern Sky by a factor of ∼2 compared to both individual analyses. In this work, the Southern Sky is scanned for possible excesses of spatial clustering, and the positions of preselected candidate sources are investigated. In addition, special focus is given to the region around the Galactic Center, whereby a dedicated search at the location of SgrA* is performed, and to the location of the supernova remnant RXJ 1713.7-3946. No significant evidence for cosmic neutrino sources is found, and upper limits on the flux from the various searches are presented.

Keywords: neutrino astronomy
The Active Fraction of Massive Black Holes in Dwarf Galaxies

The population of massive black holes (MBHs) in dwarf galaxies is elusive, but fundamentally important to understand the coevolution of black holes with their hosts and the formation of the first collapsed objects in the universe. While some progress was made in determining the X-ray detected fraction of MBHs in dwarfs, with typical values ranging from 0%-6%, their overall active fraction, ${ \mathcal A }$ , is still largely unconstrained. Here, we develop a theoretical model to predict the multiwavelength active fraction of MBHs in dwarf galaxies starting from first principles and based on the physical properties of the host, namely, its stellar mass and angular momentum content. We find multiwavelength active fractions for MBHs, accreting at typically low rates, ranging from 5%-22%, and increasing with the stellar mass of the host as ${ \mathcal A }$ $\sim \,{\left({\mathrm{log}}_{10}{M}_{\star }\right)}^{4.5}$ . If dwarfs are characterized by low-metallicity environments, the active fraction may reach ~30% for the most massive hosts. For galaxies with stellar mass in the range of 107 < M⋆[ M⊙] < 1010, our predictions are in agreement with occupation fractions derived from simulations and semi-analytical models. Additionally, we provide a fitting formula to predict the probability of finding an active MBH in a dwarf galaxy from observationally derived data. This model will be instrumental to guide future observational efforts to find MBHs in dwarfs. The James Webb Space Telescope, in particular, will play a crucial role in detecting MBHs in dwarfs, possibly uncovering active fractions ~3 times larger than current X-ray surveys.

Keywords: black holes; intermediate-mass black holes; supermassive black holes; dwarf galaxies; low-luminosity active galactic nuclei; active galactic nuclei; galaxy evolution
Long-period High-amplitude Red Variables in the KELT Survey

We present a sample of 4132 Mira-like variables (red variables with long periods and high amplitudes) in the Kilodegree Extremely Little Telescope (KELT) survey. Of these, 376 are new Mira-like detections. We used Two Micron All Sky Survey (2MASS) colors to identify candidate asymptotic giant branch stars. We searched for photometric variability among the candidate asymptotic giant branch stars and identified stars that show periodic variability. We selected variables with high amplitudes and strong periodic behavior using a Random Forest classifier. Of the sample of 4132 Mira-like variables, we estimate that 70% are Miras and 30% are semiregular (SR) variables. We also adopt the method of using (WRP-WK_s) versus (J-{Ks) colors in distinguishing between O-rich and C-rich Miras and find it to be an improvement over 2MASS colors.

Keywords: mira variable stars; long period variable stars; time series analysis
Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. XIII. Ultraviolet Time Lag of Hβ Emission in Mrk 142

We performed a rigorous reverberation-mapping analysis of the broad-line region (BLR) in a highly accreting (L/L Edd = 0.74-3.4) active galactic nucleus, Markarian 142 (Mrk 142), for the first time using concurrent observations of the inner accretion disk and the BLR to determine a time lag for the Hβ λ4861 emission relative to the ultraviolet (UV) continuum variations. We used continuum data taken with the Niel Gehrels Swift Observatory in the UVW2 band, and the Las Cumbres Observatory, Dan Zowada Memorial Observatory, and Liverpool Telescope in the g band, as part of the broader Mrk 142 multiwavelength monitoring campaign in 2019. We obtained new spectroscopic observations covering the Hβ broad emission line in the optical from the Gemini North Telescope and the Lijiang 2.4 m Telescope for a total of 102 epochs (over a period of 8 months) contemporaneous to the continuum data. Our primary result states a UV-to-Hβ time lag of ${8.68}_{-0.72}^{+0.75}$ days in Mrk 142 obtained from light-curve analysis with a Python-based running optimal average algorithm. We placed our new measurements for Mrk 142 on the optical and UV radius-luminosity relations for NGC 5548 to understand the nature of the continuum driver. The positions of Mrk 142 on the scaling relations suggest that UV is closer to the "true" driving continuum than the optical. Furthermore, we obtain $\mathrm{log}({M}_{\bullet }/{M}_{\odot })$ = 6.32 ± 0.29 assuming UV as the primary driving continuum.

Keywords: active galactic nuclei; accretion; spectroscopy
A Strategy to Search for an Inner Binary Black Hole from the Motion of the Tertiary Star

There are several ongoing projects to search for stars orbiting around an invisible companion. A fraction of such candidates may be a triple, instead of a binary, consisting of an inner binary black hole (BBH) and an outer orbiting star. In this paper, we propose a methodology to search for a signature of such an inner BBH, possibly a progenitor of gravitational-wave sources discovered by LIGO, from the precise radial velocity (RV) follow-up of the outer star. We first describe a methodology using an existing approximate RV formula for coplanar circular triples. We apply this method and constrain the parameters of a possible inner binary object in 2M05215658+4359220, which consists of a red giant and an unseen companion. Next we consider coplanar but non-circular triples. We compute numerically the RV variation of a tertiary star orbiting around an inner BBH, generate mock RV curves, and examine the feasibility of detection of the BBH for our fiducial models. We conclude that short-cadence RV monitoring of a star-BH binary provides an interesting and realistic method to constrain and/or search for possible inner BBHs. Indeed a recent discovery of the star-BH binary system LB-1 may imply that there are a large number of such unknown objects in our Galaxy, which are ideal targets for the methodology proposed here.

Keywords: radial velocity; celestial mechanics; three-body problem; multiple stars; black holes; compact binary stars
Reaching Submillisecond Accuracy in Stellar Occultations and Artificial Satellite Tracking

In recent years, there appeared a need for astronomical observations timed with submillisecond accuracy. These include, e.g., timing stellar occultations by small, subkilometer, or fast near-Earth asteroids and tracking artificial satellites in low-Earth orbit using optical sensors. Precise astrometry of fast-moving satellites and accurate timing of stellar occultations have parallel needs, requiring a reliable time source and good knowledge of camera delays. Thus, there is a need for an external device that would enable equipment and camera testing to check if they reach the required accuracy in time. We designed, constructed, and thoroughly tested a New EXposure Timing Analyser (NEXTA), a Global Navigation Satellite System-based precise timer allowing us to reach an accuracy of 0.1 ms, which is an order of magnitude better than in previously available tools. The device is a simple strip of blinking diodes to be imaged with a camera and compare the imaged time with the internal camera time stamp. Our tests spanned a range of scientific cameras widely used for stellar occultations and ground-based satellite tracking. The results revealed high reliability of both NEXTA and most of the tested cameras but also pointed out that practically all cameras had internal time biases of various levels. NEXTA can serve the community, being easily reproducible with inexpensive components. We provide all the necessary schemes and usage instructions.

Keywords: occultation; astrometry; artificial satellites; time domain astronomy
On the Origin of the X-Ray Emission in Heavily Obscured Compact Radio Sources

X-ray continuum emission of active galactic nuclei (AGNs) may be reflected by circumnuclear dusty tori, producing prominent fluorescence iron lines at X-ray frequencies. Here, we discuss the broadband emission of three radio-loud AGNs belonging to the class of compact symmetric objects (CSOs), with detected narrow Fe Kα lines. CSOs have newly born radio jets, forming compact radio lobes with projected linear sizes of the order of a few to hundreds of parsecs. We model the radio-to-γ-ray spectra of compact lobes in J1407+2827, J1511+0518, and J2022+6137, which are among the nearest and the youngest CSOs known to date, and are characterized by an intrinsic X-ray absorbing column density of N H > 1023 cm‑2. In addition to the archival data, we analyze the newly acquired Chandra X-ray Observatory and Submillimeter Array (SMA) observations, and also refine the γ-ray upper limits from Fermi Large Area Telescope monitoring. The new Chandra data exclude the presence of the extended X-ray emission components on scales larger than 1.″5. The SMA data unveil a correlation between the spectral index of the electron distribution in the lobes and N H, which can explain the γ-ray quietness of heavily obscured CSOs. Based on our modeling, we argue that the inverse-Compton emission of compact radio lobes may account for the intrinsic X-ray continuum in all these sources. Furthermore, we propose that the observed iron lines may be produced by a reflection of the lobes' continuum from the surrounding cold dust.

Keywords: active galactic nuclei; active galaxies; radio lobes; jets; high energy astrophysics; gamma-ray astronomy; x-ray astronomy; x-ray active galactic nuclei; radio galaxies
Methane Formation in Cold Regions from Carbon Atoms and Molecular Hydrogen

Methane is typically thought to be formed in the solid state on top of cold interstellar icy grain mantles via the successive atomic hydrogenation of a carbon atom. In the current work we investigate the role of molecular hydrogen in the CH4 reaction network. We make use of an ultrahigh vacuum cryogenic setup combining an atomic carbon atom beam with atomic and/or molecular beams of hydrogen and deuterium on a water ice. These experiments lead to the formation of methane isotopologues detected in situ through reflection absorption infrared spectroscopy. Most notably, CH4 is experimentally formed by combining C atoms with only H2 on amorphous solid water, albeit more slowly than in experiments where H atoms are also present. Furthermore, CH2D2 is detected in an experiment involving C atoms with H2 and D2 on H2O ice. CD4, however, is only formed when D atoms are present in the experiment. These findings have been rationalized by means of computational and theoretical chemical insights. This leads to the following conclusions: (a) the reaction C + H2 → CH2 takes place, although it is not barrierless for all binding sites on water, (b) the reaction CH + H2 → CH3 is barrierless, but has not yet been included in astrochemical models, (c) the reactions CH2 + H2 → CH3 + H and CH3 + H2 → CH4 + H can take place only via a tunneling mechanism, and (d) molecular hydrogen possibly plays a more important role in the solid-state formation of methane than assumed so far.

Keywords: astrochemistry; interstellar molecules; molecular spectroscopy; molecular clouds; laboratory astrophysics; computational methods
Generating Transverse Loop Oscillations through a Steady-flow Driver

In recent years, the decay-less regime of standing transverse oscillations in coronal loops has been the topic of many observational and numerical studies, focusing on their physical characteristics, as well as their importance for coronal seismology and wave heating. However, no definitive answer has yet been given on the driving mechanism behind these oscillations, with most studies focusing on the use of periodic footpoint drivers as a means to excite them. In this paper, our goal is to explore the concept of these standing waves being self-sustained oscillations, driven by a constant background flow. To that end, we use the PLUTO code, to perform 3D magnetohydrodynamic simulations of a gravitationally stratified straight flux tube in a coronal environment, in the presence of a weak flow around the loop, perpendicular to its axis. Once this flow is firmly set up, a transverse oscillation is initiated, dominated by the fundamental kink mode of a standing wave, while the existence of a second harmonic is revealed, with a frequency ratio to the fundamental mode near the observed ones in decay-less oscillations. The presence of vortex shedding is also established in our simulations, which is connected to the "slippery" interaction between the oscillator and its surrounding plasma. We thus present a proof-of-concept of a self-oscillation in a coronal loop, and we propose it as a mechanism that could interpret the observed decay-less transverse oscillations of coronal loops.

Keywords: solar coronal loops; solar coronal waves; magnetohydrodynamics