adsabs/SciX_UAT_keywords huggingface dataset:
A Carbon/Oxygen-dominated Atmosphere Days after Explosion for the "Super-Chandrasekhar" Type Ia SN 2020esm

Seeing pristine material from the donor star in a type Ia supernova (SN Ia) explosion can reveal the nature of the binary system. In this paper, we present photometric and spectroscopic observations of SN 2020esm, one of the best-studied SNe of the class of "super-Chandrasekhar" SNe Ia (SC SNe Ia), with data obtained -12 to +360 days relative to peak brightness, obtained from a variety of ground- and space-based telescopes. Initially misclassified as a type II supernova, SN 2020esm peaked at M B = -19.9 mag, declined slowly (Δm 15(B) = 0.92 mag), and had particularly blue UV and optical colors at early times. Photometrically and spectroscopically, SN 2020esm evolved similarly to other SC SNe Ia, showing the usual low ejecta velocities, weak intermediate-mass elements, and the enhanced fading at late times, but its early spectra are unique. Our first few spectra (corresponding to a phase of ≳10 days before peak) reveal a nearly pure carbon/oxygen atmosphere during the first days after explosion. This composition can only be produced by pristine material, relatively unaffected by nuclear burning. The lack of H and He may further indicate that SN 2020esm is the outcome of the merger of two carbon/oxygen white dwarfs. Modeling its bolometric light curve, we find an 56Ni mass of ${1.23}_{-0.14}^{+0.14}$ M ⊙ and an ejecta mass of ${1.75}_{-0.20}^{+0.32}$ M ⊙, in excess of the Chandrasekhar mass. Finally, we discuss possible progenitor systems and explosion mechanisms of SN 2020esm and, in general, the SC SNe Ia class.

Keywords: supernovae; white dwarf stars
The Hills Mechanism and the Galactic Center S-stars

Our Galactic center contains young stars, including the few million year-old clockwise disk between 0.05 and 0.5 pc from the Galactic center and the S-star cluster of B-type stars at a galactocentric distance of 0.01 pc. Recent observations suggest the S-stars are remnants of tidally disrupted binaries from the clockwise disk. In particular, Koposov et al. discovered a hypervelocity star that was ejected from the Galactic center 5 Myr ago with a velocity vector consistent with the disk. We perform a detailed study of this binary disruption scenario. First, we quantify the plausible range of binary semimajor axes in the disk. Dynamical evaporation of such binaries is dominated by other disk stars rather than by the isotropic stellar population. For the expected range of semimajor axes in the disk, binary tidal disruptions can reproduce the observed S-star semimajor axis distribution. Reproducing the observed thermal eccentricity distribution of the S-stars requires an additional relaxation process. The flight time of the Koposov star suggests that this process must be effective within 10 Myr. We consider three possibilities: (I) scalar resonant relaxation from the observed isotropic star cluster, (II) torques from the clockwise disk, and (III) an intermediate-mass black hole. We conclude that the first and third mechanisms are fast enough to reproduce the observed S-star eccentricity distribution. Finally, we show that the primary star from an unequal-mass binary would be deposited at larger semimajor axes than the secondary, possibly explaining the dearth of O stars among the S-stars.

Keywords: galactic center; binary stars; hypervelocity stars
Magnetic Rayleigh-Taylor Instability in an Experiment Simulating a Solar Loop

A hoop force driven magnetic Rayleigh-Taylor instability (MRTI) is observed in a laboratory experiment that simulates a solar coronal loop. Increase of the axial wavelength λ is observed when the axial magnetic field increases. This scaling is consistent with the theoretical MRTI growth rate ${\gamma }^{2}={gk}-2{\left({\boldsymbol{k}}\cdot {{\boldsymbol{B}}}_{0}\right)}^{2}/{\mu }_{0}\rho $ , which implies that if ${\boldsymbol{k}}$ is parallel to ${{\boldsymbol{B}}}_{0}$ (I.e., undular mode), the fastest-growing mode has $\lambda =2\pi /k=8\pi {B}_{0}^{2}/{\mu }_{0}\rho g$ .

Keywords: magnetohydrodynamics; laboratory astrophysics; solar activity; solar corona; solar prominences; solar filaments; quiescent solar prominence; solar coronal plumes; solar coronal mass ejections; space plasmas
Aurora: A Generalized Retrieval Framework for Exoplanetary Transmission Spectra

Atmospheric retrievals of exoplanetary transmission spectra provide important constraints on various properties, such as chemical abundances, cloud/haze properties, and characteristic temperatures, at the day-night atmospheric terminator. To date, most spectra have been observed for giant exoplanets due to which retrievals typically assume hydrogen-rich atmospheres. However, recent observations of mini Neptunes/super-Earths, and the promise of upcoming facilities including the James Webb Space Telescope (JWST), call for a new generation of retrievals that can address a wide range of atmospheric compositions and related complexities. Here we report Aurora, a next-generation atmospheric retrieval framework that builds upon state-of-the-art architectures and incorporates the following key advancements: (a) a generalized compositional retrieval allowing for H-rich and H-poor atmospheres, (b) a generalized prescription for inhomogeneous clouds/hazes, (c) multiple Bayesian inference algorithms for high-dimensional retrievals, (d) modular considerations for refraction, forward scattering, and Mie scattering, and (e) noise modeling functionalities. We demonstrate Aurora on current and/or synthetic observations of the hot Jupiter HD 209458 b, mini Neptune K2-18b, and rocky exoplanet TRAPPIST-1 d. Using current HD 209458 b spectra, we demonstrate the robustness of our framework and cloud/haze prescription against assumptions of H-rich/H-poor atmospheres, improving on previous treatments. Using real and synthetic spectra of K2-18b, we demonstrate an agnostic approach to confidently constrain its bulk atmospheric composition and obtain precise abundance estimates. For TRAPPIST-1 d, 10 JWST-NIRSpec transits can enable identification of the main atmospheric component for cloud-free, CO2-rich, and N2-rich atmospheres and abundance constraints on trace gases, including initial indications of O3 if present at enhanced levels (∼10×-100× Earth levels).

Keywords: exoplanet atmospheric composition; exoplanets; hot jupiters; exoplanet atmospheres; mini neptunes; computational methods; super earths; astronomy data modeling; transmission spectroscopy
Dynamical Evolution of Newly Formed Structures After Magnetic Reconnection

Although extensive research on magnetic reconnection, e.g., current sheets, inflows/outflows, and plasma ejections, has been done, the dynamical evolution of newly formed structures during and after the reconnection between two sets of atmospheric structures is rarely studied. Here we investigate five reconnection events based on observations from the Solar Dynamics Observatory and the New Vacuum Solar Telescope. In each event, two independent atmospheric structures are involved. While they meet, a plasma sheet is detected. After exchanging topological connectivity, new structures are formed in the outflow regions. For each new structure, it is composed of a part of one original structure and that of the other one. In this Letter, for the first time, we find that there are two types of evolution patterns of the new structures. The first type is that the new structures move away from the reconnection position as a whole and then undergo a to-and-fro motion (an oscillation). The second is that the new structures display a "throwing whip" movement. We suggest that the evolution patterns are relevant to the topological configuration of the original structures and the position of the reconnection site.

Keywords: solar magnetic reconnection; solar activity; solar magnetic fields
X-Ray Superflares from Pre-main-sequence Stars: Flare Modeling

Getman et al. report the discovery, energetics, frequencies, and effects on environs of >1000 X-ray superflares with X-ray energies EX ~ 1034-1038 erg from pre-main-sequence (PMS) stars identified in the Chandra MYStIX and SFiNCs surveys. Here we perform detailed plasma evolution modeling of 55 bright MYStIX/SFiNCs superflares from these events. They constitute a large sample of the most powerful stellar flares analyzed in a uniform fashion. They are compared with published X-ray superflares from young stars in the Orion Nebula Cluster, older active stars, and the Sun. Several results emerge. First, the properties of PMS X-ray superflares are independent of the presence or absence of protoplanetary disks inferred from infrared photometry, supporting the solar-type model of PMS flaring magnetic loops with both footpoints anchored in the stellar surface. Second, most PMS superflares resemble solar long-duration events that are associated with coronal mass ejections. Slow-rise PMS superflares are an interesting exception. Third, strong correlations of superflare peak emission measure and plasma temperature with the stellar mass are similar to established correlations for the PMS X-ray emission composed of numerous smaller flares. Fourth, a new correlation of loop geometry is linked to stellar mass; more massive stars appear to have thicker flaring loops. Finally, the slope of a long-standing relationship between the X-ray luminosity and magnetic flux of various solar-stellar magnetic elements appears steeper in PMS superflares than for solar events.

Keywords: pre-main sequence stars; stellar x-ray flares; x-ray stars; protoplanetary disks; solar flares; stellar flares
Vertical Structure and Color of Jovian Latitudinal Cloud Bands during the Juno Era

The identity of the coloring agent(s) in Jupiter's atmosphere and the exact structure of Jupiter's uppermost cloud deck are yet to be conclusively understood. The Crème Brûlée model of Jupiter's tropospheric clouds, originally proposed by Baines et al. and expanded upon by Sromovsky et al. and Baines et al., presumes that the chromophore measured by Carlson et al. is the singular coloring agent in Jupiter's troposphere. In this work, we test the validity of the Crème Brûlée model of Jupiter's uppermost cloud deck using spectra measured during the Juno spacecraft's fifth perijove pass in 2017 March. These data were obtained as part of an international ground-based observing campaign in support of the Juno mission using the New Mexico State University Acousto-optic Imaging Camera at the 3.5 m telescope at Apache Point Observatory in Sunspot, NM, USA. We find that the Crème Brûlée model cloud-layering scheme can reproduce Jupiter's visible spectrum both with the Carlson et al. chromophore and with modifications to its imaginary index of refraction spectrum. While the Crème Brûlée model provides reasonable results for regions of Jupiter's cloud bands such as the North Equatorial Belt and Equatorial Zone, we find that it is not a safe assumption for unique weather events, such as the 2016-2017 Southern Equatorial Belt outbreak that was captured by our measurements.

Keywords: jupiter; radiative transfer
The Hazardous km-sized NEOs of the Next Thousands of Years

The catalog of km-sized near-Earth objects (NEOs) is nearly complete. Typical impact monitoring analyses search for possible impacts over the next 100 yr and none of the km-sized objects represent an impact threat over that time interval. Assessing the impact risk over longer timescales is a challenge since orbital uncertainties grow. To overcome this limitation we analyze the evolution of the minimum orbit intersection distance (MOID), which bounds the closest possible encounters between the asteroid and the Earth. The evolution of the MOID highlights NEOs that are in the vicinity of the Earth for longer periods of time, and we propose a method to estimate the probability of a deep Earth encounter during these periods. This metric is used to rank the km-sized catalog in terms of their long-term impact hazard to identify targets of potential interest for additional observation and exploration.

Keywords: asteroids; near-earth objects; astrodynamics; asteroid dynamics; close encounters
Electron Spectrum for the Prompt Emission of Gamma-Ray Bursts in the Synchrotron Radiation Scenario

Growing evidence indicates that the synchrotron radiation mechanism may be responsible for the prompt emission of gamma-ray bursts (GRBs). In the synchrotron radiation scenario, the electron energy spectrum of the prompt emission is diverse in theoretical works and has not been estimated from observations in a general way (i.e., without specifying a certain physical model for the electron spectrum). In this paper, we creatively propose a method to directly estimate the electron spectrum for the prompt emission, without specifying a certain physical model for the electron spectrum in the synchrotron radiation scenario. In this method, an empirical function (i.e., a four-order Bézier curve jointed with a linear function at high energy) is applied to describe the electron spectrum in log-log coordinates. It is found that our empirical function can well mimic the electron spectra obtained in many numerical calculations or simulations. Then, our method can figure out the electron spectrum for the prompt emission without specifying a model. By employing our method on observations, taking GRB 180720B and GRB 160509A as examples, it is found that the obtained electron spectra are generally different from that in the standard fast-cooling scenario and even a broken power law. Moreover, the morphology of electron spectra in its low-energy regime varies with time in a burst and even in a pulse. Our proposed method provides a valuable way to confront the synchrotron radiation mechanism with observations.

Keywords: gamma-ray bursts; relativistic jets
Pulsar and Magnetar Navigation with Fermi/GBM and GECAM

The determination of the absolute and relative position of a spacecraft is critical for its operation, observations, data analysis, scientific studies, as well as deep-space exploration in general. A spacecraft that can determine its own absolute position autonomously may perform better than those that must rely on transmission solutions. In this work, we report an absolute navigation accuracy of ~20 km using 16 day Crab pulsar data observed with Fermi's Gamma-ray Burst Monitor (GBM). In addition, we propose a new method with the inverse process of the triangulation for joint navigation using repeated bursts like those from the magnetar SGR J1935+2154 observed by the Gravitational-wave High-energy Electromagnetic Counterpart All-sky Monitor and GBM.

Keywords: pulsars; magnetars; astronomical techniques
Free-form Lens Model and Mass Estimation of the High-redshift Galaxy Cluster ACT-CL J0102-4915, "El Gordo"

We examine the massive colliding cluster El Gordo, one of the most massive clusters at high redshift. We use a free-form lensing reconstruction method that avoids making assumptions about the mass distribution. We use data from the RELICS program and identify new multiply lensed system candidates. The new set of constraints and free-form method provide a new independent mass estimate of this intriguing colliding cluster. Our results are found to be consistent with earlier parametric models, indirectly confirming the assumptions made in earlier work. By fitting a double gNFW profile to the lens model and extrapolating to the virial radius, we infer a total mass for the cluster of ${M}_{200c}=({1.08}_{-0.12}^{+0.65})\times {10}^{15}$ M ${}_{\odot }$ . We estimate the uncertainty in the mass due to errors in the photometric redshifts and discuss the uncertainty in the inferred virial mass due to the extrapolation from the lens model. We also find in our lens map a mass overdensity corresponding to the large cometary tail of hot gas, reinforcing its interpretation as a large tidal feature predicted by hydrodynamical simulations that mimic El Gordo. Finally, we discuss the observed relation between the plasma and the mass map, finding that the peak in the projected mass map may be associated with a large concentration of colder gas exhibiting possible star formation. El Gordo is one of the first clusters that will be observed with JWST, which is expected to unveil new high-redshift lensed galaxies around this interesting cluster and provide a more accurate estimation of its mass.

Keywords: gravitational lensing; dark matter; high-redshift galaxy clusters; rich galaxy clusters
The Discovery of a Hidden Broad-line AGN in a Bulgeless Galaxy: Keck NIR Spectroscopic Observations of SDSS J085153.64+392611.76

We report the discovery of a buried, active supermassive black hole (SMBH) in SDSS J085153.64+392611.76, a bulgeless Seyfert 2 (Sy2) galaxy. Keck near-infrared observations reveal a hidden broad-line region, allowing for the rare case where strong constraints can be placed on both the BH mass and bulge component. Using virial mass estimators, we obtain a BH mass of log(MBH/M⊙) = 6.78 ± 0.50. This is one of the only Sy2 active galactic nuclei (AGNs) hosted in a bulgeless galaxy with a virial BH mass estimate and could provide important constraints on the formation scenarios of the BH seed population. The lack of a bulge component suggests that the SMBH has grown quiescently, likely caused by secular processes independent of major mergers. In the absence of a detectable bulge component, we find the MBH-Mstellar relation to be more reliable than the MBH-Mbulge relation. In addition, we detect extended narrow Paα emission that allows us to create a rotation curve where we see counterrotating gas within the central kiloparsec. Possible causes of this counterrotation include a galactic bar or disruption of the inner gas by a recent fly-by of a companion galaxy. This in turn could have triggered accretion onto the central SMBH in the current AGN phase.

Keywords: active galactic nuclei; near infrared astronomy; galaxy bulges; seyfert galaxies; supermassive black holes
'Oumuamua as a Cometary Fractal Aggregate: The "Dust Bunny" Model

The first known interstellar object, 1I/2017 U1 'Oumuamua, displayed such unusual properties that its origin remains a subject of much debate. We propose that 'Oumuamua's properties could be explained as those of a fractal dust aggregate (a "dust bunny") formed in the inner coma of a fragmenting exo-Oort cloud comet. Such fragments could serve as accretion sites by accumulating dust particles, resulting in the formation of a fractal aggregate. The fractal aggregate eventually breaks off from the fragment due to hydrodynamic stress. With their low density and tenuously bound orbits, most of these cometary fractal aggregates are then ejected into interstellar space by radiation pressure.

Keywords: comets; coma dust; oort cloud; long period comets
Nonlinear Outcome of Coagulation Instability in Protoplanetary Disks. II. Dust-ring Formation Mediated by Backreaction and Fragmentation

In our previous work (Paper I), we demonstrated that coagulation instability results in dust concentration against depletion due to the radial drift and accelerates dust growth locally. In this work (Paper II), we perform numerical simulations of coagulation instability taking into account effects of backreaction to gas and collisional fragmentation of dust grains. We find that the slowdown of the dust drift due to backreaction regulates dust concentration in the nonlinear growth phase of coagulation instability. The dust-to-gas surface density ratio increases from 10-3 up to ~10-2. Each resulting dust ring tends to have a mass of ≃0.5 M ⊕ - 1.5 M ⊕ in our disk model. In contrast to Paper I, the dust surface density profile shows a local plateau structure at each dust ring. In spite of the regulation at the nonlinear growth, the efficient dust concentration reduces their collision velocity. As a result, dust grains can grow beyond the fragmentation barrier, and the dimensionless stopping time reaches unity, as in Paper I. The necessary condition for the efficient dust growth is (1) weak turbulence of α < 1 × 10-3 and (2) a large critical velocity for dust fragmentation (>1 m s-1). The efficient dust concentration in outer regions will reduce the inward pebble flux and is expected to decelerate the planet formation via the pebble accretion. We also find that the resulting rings can be unstable to secular gravitational instability (GI). The subsequent secular GI promotes planetesimal formation. We thus expect that a combination of these instabilities is a promising mechanism for dust-ring and planetesimal formation.

Keywords: protoplanetary disks; circumstellar disks; planetesimals
Possible X-Ray Quasi-periodic Eruptions in a Tidal Disruption Event Candidate

X-ray quasi-periodic eruptions (QPEs) are a recently discovered phenomenon associated with supermassive black holes at the centers of galaxies. They are high-amplitude soft X-ray flares that recur on timescales of hours, but what causes these flares remains uncertain. In the two years since their original discovery, four known QPE-hosting galaxies have been found, with varying properties and levels of activity. We have conducted a blind algorithm-assisted search of the XMM-Newton Source Catalog and found a fifth QPE candidate, XMMSL1 J024916.6-041244. This is a star-forming galaxy hosting a relatively low-mass nuclear black hole, and has previously been identified as a tidal disruption event candidate. An XMM-Newton pointed observation of the source in 2006 exhibited nearly two QPE-like flares in soft X-rays, and unlike in other QPE sources, there are hints of corresponding dips in the UV light curves. Afterwards, a series of Swift observations observed the rapid dimming of the source; thereafter, in 2021 August, we triggered a second XMM-Newton observation, which revealed that the source is detected, but the QPEs are no longer present. Here we report on (I) the strategy we used to systematically search through XMM-Newton archival data; (II) the properties of J0249 and its QPE flares; and (III) the relative behaviors and properties of the QPE sample to date, now five members large.

Keywords: x-ray active galactic nuclei; variable radiation sources; galaxy luminosities
Dynamical Evidence of a Spiral Arm-driving Planet in the MWC 758 Protoplanetary Disk

More than a dozen young stars host spiral arms in their surrounding protoplanetary disks. The excitation mechanisms of such arms are under debate. The two leading hypotheses—companion-disk interaction and gravitational instability (GI)—predict distinct motion for spirals. By imaging the MWC 758 spiral arm system at two epochs spanning ∼5 yr using the SPHERE instrument on the Very Large Telescope, we test the two hypotheses for the first time. We find that the pattern speeds of the spirals are not consistent with the GI origin. Our measurements further evince the existence of a faint "missing planet" driving the disk arms. The average spiral pattern speed is 0°22 ± 0°03 yr-1, pointing to a driver at ${172}_{-14}^{+18}$ au around a 1.9 M⊙ central star if it is on a circular orbit. In addition, we witness time-varying shadowing effects on a global scale that are likely originating from an inner disk.

Keywords: protoplanetary disks; coronagraphic imaging; planetary system formation; orbital motion