adsabs/SciX_UAT_keywords huggingface dataset:
Near L-edge Single and Multiple Photoionization of Doubly Charged Iron Ions

Using the photon-ion merged-beams technique at a synchrotron light source, we have measured relative cross sections for single and up to five-fold photoionization of Fe2+ ions in the energy range of 690-920 eV. This range contains thresholds and resonances associated with ionization and excitation of 2p and 2s electrons. Calculations were performed to simulate the total absorption spectra. The theoretical results show very good agreement with the experimental data, if overall energy shifts of up to 2.5 eV are applied to the calculated resonance positions and assumptions are made about the initial experimental population of the various levels of the Fe2+([Ar]3d6) ground configuration. Furthermore, we performed extensive calculations of the Auger cascades that result when an electron is removed from the 2p subshell of Fe2+. These computations lead to a better agreement with the measured product-charge-state distributions as compared to earlier work. We conclude that the L-shell absorption features of low-charged iron ions are useful for identifying gas-phase iron in the interstellar medium and for discriminating against the various forms of condensed-phase iron bound to composite interstellar dust grains.

Keywords: interstellar atomic gas; interstellar clouds; interstellar medium; interstellar absorption; line intensities; atomic spectroscopy; line positions; x-ray astronomy; laboratory astrophysics
Identifying Candidate Optical Variables Using Gaia Data Release 2

Gaia is undertaking a deep synoptic survey of the Galaxy, but photometry from individual epochs has, as of yet, only been released for a minimal number of sources. We show that it is possible to identify variable stars in Gaia Data Release 2 by selecting stars with unexpectedly large photometric uncertainties given their brightness and number of observations. By comparing our results to existing catalogs of variables, we show that information on the amplitude of variability is also implicitly present in the Gaia photometric uncertainties. We present a catalog of about 9.3 million candidate variable stars, and discuss its limitations and prospects for future tests and extensions.

Keywords: variable stars; sky surveys; catalogs
Different Fates of Young Star Clusters after Gas Expulsion

We identify structures of the young star cluster NGC 2232 in the solar neighborhood (323.0 pc) and a newly discovered star cluster, LP 2439 (289.1 pc). Member candidates are identified using the Gaia DR2 sky position, parallax, and proper-motion data by an unsupervised machine-learning method, STARGO. Member contamination from the Galactic disk is further removed using the color-magnitude diagram. The four identified groups (NGC 2232, LP 2439, and two filamentary structures) of stars are coeval with an age of 25 Myr and were likely formed in the same giant molecular cloud. We correct the distance asymmetry from the parallax error with a Bayesian method. The 3D morphology shows the two spherical distributions of clusters NGC 2232 and LP 2439. Two filamentary structures are spatially and kinematically connected to NGC 2232. Both NGC 2232 and LP 2439 are expanding. The expansion is more significant in LP 2439, generating a loose spatial distribution with shallow volume number and mass density profiles. The expansion is suggested to be mainly driven by gas expulsion. With 73% of the cluster mass bound, NGC 2232 is currently experiencing a process of revirialization, However, LP 2439, with 52% of the cluster mass unbound, may fully dissolve in the near future. The different survivability traces the different dynamical states of NGC 2232 and LP 2439 prior to the onset of gas expulsion. While NGC 2232 may have been substructured and subvirial, LP 2439 may have either been virial/supervirial or experienced a much faster rate of gas removal.

Keywords: star clusters; open star clusters
Monitoring Inner Regions in the RY Tau Jet

We present multiepoch observations of the RY Tau jet for Hα and [Fe II] 1.644 μm emission lines obtained with the Subaru Coronagraphic Extreme-AO and Visible Aperture Masking Polarimetric Imager for Resolved Exoplanetary Structures (VAMPIRES), Gemini Near-infrared Integral Field Spectrograph, and Keck/OSIRIS in 2019-2021. These data show a series of four knots within 1″ consistent with the proper motion of ~0.″3 yr-1, analogous to the jets associated with another few active T Tauri stars. However, the spatial intervals between the knots suggest the time intervals of the ejections of about 1.2, 0.7, and 0.7 yr, significantly shorter than those estimated for the other stars. These Hα images contrast with the archival Very Large Telescope Spectro-Polarimetric High-contrast Exoplanet Research and Zurich IMaging POLarimeter (ZIMPOL) observations from 2015, which showed only a single knot-like feature at ~0.″25. The difference between the 2015 and 2019-2021 epochs suggests an irregular ejection interval within the six-year range. Such variations of the jet ejection may be related to a short-term (<1 yr) variability of the mass accretion rate. We compared the peaks of the Hα emissions with the ZIMPOL data taken in 2015, showing the brighter profile at the base (<0.″3) than the 2020-2021 VAMPIRES profiles due to time-variable mass ejection rates or the heating-cooling balance in the jet. The observed jet knot structures may be alternatively attributed to stationary shocks, but a higher angular resolution is required to confirm its detailed origin.

Keywords: stellar jets; t tauri stars
The Scaling of Vortical Electron Acceleration in Thin-current Magnetic Reconnection and Its Implications in Solar Flares

To investigate how magnetic reconnection (MR) accelerates electrons to a power-law energy spectrum in solar flares, we explore the scaling of a kinetic model proposed by Che & Zank (CZ) and compare it to observations. Focusing on thin current sheet MR particle-in-cell (PIC) simulations, we analyze the impact of domain size on the evolution of the electron Kelvin-Helmholtz instability (EKHI). We find that the duration of the growth stage of the EKHI ( ${t}_{G}\sim {{\rm{\Omega }}}_{e}^{-1}$ ) is short and remains nearly unchanged because the electron gyrofrequency Ω e is independent of domain size. The quasi-steady stage of the EKHI (t MR) dominates the electron acceleration process and scales linearly with the size of the simulations as L/v A0, where v A0 is the Alfvén speed. We use the analytical results obtained by CZ to calculate the continuous temporal evolution of the electron energy spectra from PIC simulations and linearly scale them to solar flare observational scales. For the first time, an electron acceleration model predicts the sharp two-stage transition observed in typical soft-hard-harder electron energy spectra, implying that the electron acceleration model must be efficient with an acceleration timescale that is a small fraction of the duration of solar flares. Our results suggest that we can use PIC MR simulations to investigate the observational electron energy spectral evolution of solar flares if the ratio t MR/t G is sufficiently small, i.e., ≲10%.

Keywords: solar energetic particles; solar magnetic reconnection; solar flares
Cloud Formation by Supernova Implosion

The deposition of energy and momentum by supernova explosions has been subject to numerous studies in the past few decades. However, while there has been some work that focused on the transition from the adiabatic to the radiative stage of a supernova remnant (SNR), the late radiative stage and merging with the interstellar medium (ISM) have received little attention. Here, we use three-dimensional, hydrodynamic simulations, focusing on the evolution of SNRs during the radiative phase, considering a wide range of physical explosion parameters ( ${n}_{{\rm{H}},\mathrm{ISM}}\in \left[0.1,100\right]{\mathrm{cm}}^{-3}$ and ${E}_{\mathrm{SN}}\in \left[1,14\right]\times {10}^{51}\,\mathrm{erg}$ ). We find that the radiative phase can be subdivided in four stages: A pressure-driven snowplow phase, during which the hot overpressurized bubble gas is evacuated and pushed into the cold shell; a momentum-conserving snowplow phase that is accompanied by a broadening of the shell; an implosion phase where cold material from the back of the shell is flooding the central vacuum; and a final cloud phase, during which the imploding gas is settling as a central, compact overdensity. The launching timescale for the implosion ranges from a few 100 kyr to a few Myr, while the cloud formation timescale ranges from a few to about 10 Myr. The highly chemically enriched clouds can become massive (M cl ∼ 103–104 M ⊙) and self-gravitating within a few Myr after their formation, providing an attractive, novel pathway for supernova-induced star and planet formation in the ISM.

Keywords: supernova remnants; dense interstellar clouds; shocks; hydrodynamical simulations; interstellar medium; interstellar dynamics
A Database of Magnetic and Thermodynamic Properties of Confined and Eruptive Solar Flares

Solar flares sometimes lead to coronal mass ejections that directly affect Earth's environment. However, a large fraction of flares, including on solar-type stars, are confined flares. What are the differences in physical properties between confined and eruptive flares? For the first time, we quantify the thermodynamic and magnetic properties of hundreds of confined and eruptive flares of GOES class C5.0 and above, 480 flares in total. We first analyze large flares of GOES class M1.0 and above observed by the Solar Dynamics Observatory, 216 flares in total, including 103 eruptive and 113 confined flares, from 2010 until 2016 April; we then look at the entire data set of 480 flares above class C5.0. We compare GOES X-ray thermodynamic flare properties, including peak temperature and emission measure, and active-region (AR) and flare-ribbon magnetic field properties, including reconnected magnetic flux and peak reconnection rate. We find that for fixed peak X-ray flux, confined and eruptive flares have similar reconnection fluxes; however, for fixed peak X-ray flux confined flares have on average larger peak magnetic reconnection rates, are more compact, and occur in larger ARs than eruptive flares. These findings suggest that confined flares are caused by reconnection between more compact, stronger, lower-lying magnetic fields in larger ARs that reorganizes a smaller fraction of these regions' fields. This reconnection proceeds at faster rates and ends earlier, potentially leading to more efficient flare particle acceleration in confined flares.

Keywords: solar magnetic fields; solar surface; solar photosphere; the sun; solar flares; solar physics; stellar flares; solar coronal mass ejections; eruptive phenomena
Spatially Resolved C III] λ1909 Emission in Haro 11

The C III] 1909 (hereafter C III]) line is the strongest ultraviolet emission line after Lyα and is therefore of interest to high-redshift studies of star-forming (SF) galaxies near the epoch of reionization. It is thought that C III] emission is strongest in galaxies with subsolar metallicity and low mass; however, spectral observations of numerous such galaxies at high and low redshift produce inconclusive or even contradictory results. We present the first-ever C III] imaging, obtained with HST/STIS for the low-redshift SF galaxy Haro 11. Cluster parameters like stellar mass, dust fraction and attenuation, and ionization parameter, obtained through spectral energy distribution fitting, show no correlation with the C III] equivalent width (EW), which may be due to a combination of the limitation of the models and the age homogeneity of the cluster population. Comparing the ratio of C III] emission line flux from individual clusters to that of Hα, [O III], and [O II], we find that the clusters with the highest EW(C III]) can be reconciled only with Cloudy models with an extremely high C/O ratio of ≥1.4(C/O)⊙ for an ionizing population of single stars, binary stars, or a mixture of binary stars and active galactic nuclei. Given the pointlike nature of strong C III], the integrated total strength of EW(C III]) becomes dependent on the morphology of the galaxy, which would explain the large scatter in EW(C III]) strengths observed in galaxies with otherwise similar SF properties and of similarly low metallicity and stellar mass.

Keywords: starburst galaxies; interstellar medium; photoionization
Grain Alignment and Disruption by Radiative Torques in Dense Molecular Clouds and Implication for Polarization Holes

Dust polarization induced by aligned grains is widely used to study magnetic fields in various astrophysical environments. However, the question of to what optical depth grain alignment still exists in a dense molecular cloud (MC) is unclear. In this paper, we derive analytical formulae for the minimum size of aligned grains (aalign) and rotational disruption (adisr) by RAdiative Torques (RATs) as a function of the local physical parameters within MCs. We first find the analytical approximations for the radiation strength and mean wavelength of the attenuated radiation field in a dense MC with and without embedded stars, and then derive analytical formulae for aalign and adisr as functions of the visual extinction AV and gas density. We find that, within a starless core of density ${n}_{{\rm{H}}}\sim {10}^{5}\,{\mathrm{cm}}^{-3}$ , grains of size $a\lt 0.25\,\mu {\rm{m}}$ can be aligned at AV ∼ 5 by RATs, whereas micron-sized grains can still be aligned at ${A}_{V}\sim 50$ . The increase in aalign with AV can explain the presence of polarization holes observed toward starless cores. For MCs with an embedded protostar, the efficiency of both alignment and rotational disruption increases toward the protostar due to the increasing radiation strength. Such a disruption effect results in the decrease of the polarization degree with AV or emission intensity, reproducing the popular polarization holes observed toward the location of protostars. Finally, we derive the formula for the maximum AV where grain alignment still exists in a starless core, and we discuss its potential for constraining grain growth.

Keywords: interstellar medium; astrophysical dust processes; interstellar dust; interstellar dust extinction; dust continuum emission; starlight polarization; interstellar magnetic fields
Herschel Observations of Disks around Late-type Stars

A set of twenty late-type (K5-M5) stars were observed with the Herschel Space Observatory at 100 and 160 microns with the goal of searching for far-infrared excesses indicative of the presence of circumstellar disks. Out of this sample, four stars (TYC 7443-1102-1, TYC 9340-437-1, GJ 784 and GJ 707) have infrared excesses above their stellar photospheres at either 100 or 160 μm or both. At 100 microns TYC 9340-437-1 is spatially resolved with a shape that suggests it is surrounded by a face-on disk. The 100 μm excess flux associated with GJ 707 is marginal at around 3σ. The excess flux associated with GJ 784 is most likely due to a background galaxy as the dust radius estimated from the spectral energy fit implies that any associated dust disk should have been resolved in the Herschel images but is not. TYC 7443-1102-1 has been observed with ALMA which resolves the emission at its location into two distinct sources making the Herschel excess most likely also due to a background galaxy. It is worth noting that this star is in the 23 Myr old β Pic association. With a disk luminosity on the order of 10-3 L*, this system is an ideal follow-up target for high-contrast imaging and ALMA.

Keywords: late-type stars; circumstellar disks
Pre- and Post-entry, Descent and Landing Assessment of the Martian Atmosphere for the Mars 2020 Rover

This review provides an analysis of activities undertaken by the Mars 2020 Council of Atmospheres (CoA) in support of the entry, descent, and landing (EDL) of the Mars 2020 rover Perseverance in Jezero crater, Mars. The activities of the CoA were designed to evaluate the safety of early-stage landing site candidates and, later, to constrain the range of plausible conditions expected at Jezero crater during the early northern spring season of EDL, following the successful blueprint of similar councils for prior landed Mars missions. The multiyear effort of the CoA involved using a combination of numerical modeling of the local Martian atmosphere with limited-domain mesoscale models and atmospheric reanalysis using data assimilation techniques, along with atmospheric observations from multiple orbiting assets, to generate an atmospheric "forecast" for the day of landing. Here we present an overview of these activities, focusing in greater detail on those elements that depart from prior CoA activities as performed for Mars Phoenix, Mars Science Laboratory, and the InSight lander. Following the successful landing of Perseverance on 2021 February 18, reconstruction and reassessment activities were performed and are presented here, comparing prelanding predictions with actual, as-flown conditions.

Keywords: mars; planetary climates; planetary atmospheres; atmospheric science; planetary science
Carnegie Supernova Project. II. Near-infrared Spectral Diversity and Template of Type Ia Supernovae

We present the largest and most homogeneous collection of near-infrared (NIR) spectra of Type Ia supernovae (SNe Ia): 339 spectra of 98 individual SNe obtained as part of the Carnegie Supernova Project-II. These spectra, obtained with the FIRE spectrograph on the 6.5 m Magellan Baade telescope, have a spectral range of 0.8-2.5 μm. Using this sample, we explore the NIR spectral diversity of SNe Ia and construct a template of spectral time series as a function of the light-curve-shape parameter, color stretch s BV . Principal component analysis is applied to characterize the diversity of the spectral features and reduce data dimensionality to a smaller subspace. Gaussian process regression is then used to model the subspace dependence on phase and light-curve shape and the associated uncertainty. Our template is able to predict spectral variations that are correlated with s BV , such as the hallmark NIR features: Mg II at early times and the H-band break after peak. Using this template reduces the systematic uncertainties in K-corrections by ~90% compared to those from the Hsiao template. These uncertainties, defined as the mean K-correction differences computed with the color-matched template and observed spectra, are on the level of 4 × 10-4 mag on average. This template can serve as the baseline spectral energy distribution for light-curve fitters and can identify peculiar spectral features that might point to compelling physics. The results presented here will substantially improve future SN Ia cosmological experiments, for both nearby and distant samples.

Keywords: high energy astrophysics; time domain astronomy; infrared spectroscopy; type ia supernovae
Black Hole Images as Tests of General Relativity: Effects of Spacetime Geometry

The images of supermassive black holes surrounded by optically thin, radiatively inefficient accretion flows, like those observed with the Event Horizon Telescope, are characterized by a bright ring of emission surrounding the black hole shadow. In the Kerr spacetime, this bright ring, when narrow, closely traces the boundary of the shadow and can, with appropriate calibration, serve as its proxy. The present paper expands the validity of this statement by considering two particular spacetime geometries: a solution to the field equations of a modified gravity theory and another that parametrically deviates from Kerr but recovers the Kerr spacetime when its deviation parameters vanish. A covariant, axisymmetric analytic model of the accretion flow based on conservation laws and spanning a broad range of plasma conditions is utilized to calculate synthetic non-Kerr black hole images, which are then analyzed and characterized. We find that in all spacetimes: (i) it is the gravitationally lensed unstable photon orbit that plays the critical role in establishing the diameter of the rings observed in black hole images, not the event horizon or the innermost stable circular orbit, (ii) bright rings in these images scale in size with, and encompass, the boundaries of the black hole shadows, even when deviating significantly from Kerr, and (iii) uncertainties in the physical properties of the accreting plasma introduce subdominant corrections to the relation between the diameter of the image and the diameter of the black hole shadow. These results provide important new theoretical justification for using black hole images to probe and test the spacetimes of supermassive black holes.

Keywords: high energy astrophysics
14N/15N Isotopic Ratio in CH3CN of Titan's Atmosphere Measured with ALMA

The nitriles present in the atmosphere of Titan can be expected to exhibit different ${}^{14}{\rm{N}}/{}^{15}{\rm{N}}$ values depending on their production processes, primarily because of the various ${{\rm{N}}}_{2}$ dissociation processes induced by different sources such as ultraviolet radiation, magnetospheric electrons, and Galactic cosmic rays. For ${\mathrm{CH}}_{3}\mathrm{CN}$ , one photochemical model predicted a 14N/15N value of 120-130 in the lower stratosphere. This is much higher than that for HCN and ${\mathrm{HC}}_{3}{\rm{N}}$ , ∼67-94. By analyzing archival data obtained by the Atacama Large Millimeter/submillimeter Array, we successfully detected submillimeter rotational transitions of ${\mathrm{CH}}_{3}{{\rm{C}}}^{15}{\rm{N}}$ (J = 19-18) located in the 338 GHz band in Titan's atmospheric spectra. By comparing those observations with the simultaneously observed ${\mathrm{CH}}_{3}\mathrm{CN}$ (J = 19-18) lines in the 349 GHz band, which probe from 160 to ∼400 km altitude, we then derived 14N/15N in ${\mathrm{CH}}_{3}\mathrm{CN}$ as 125 ${}_{-44}^{+145}$ . Although the range of the derived value shows insufficient accuracy due to limitations on data quality, the best-fit value suggests that 14N/15N for ${\mathrm{CH}}_{3}\mathrm{CN}$ is higher than values that have previously been observed and theoretically predicted for HCN and ${\mathrm{HC}}_{3}{\rm{N}}$ . This may be explained by the different ${{\rm{N}}}_{2}$ dissociation sources according to altitude, as suggested by a recent photochemical model.

Keywords: planetary atmospheres; submillimeter astronomy
Final Masses of Giant Planets. III. Effect of Photoevaporation and a New Planetary Migration Model

We herein develop a new simple model for giant planet formation that predicts the final mass of a giant planet born in a given disk by adding the disk mass loss due to photoevaporation and a new type II migration formula to our previous model. The proposed model provides some interesting results. First, it gives universal evolution tracks in the diagram of planetary mass and orbital radius, which clarifies how giant planets migrate at growth in the runaway gas accretion stage. Giant planets with a few Jupiter masses or less suffer only a slight radial migration in the runaway gas accretion stage. Second, the final mass of giant planets is approximately given as a function of only three parameters: the initial disk mass at the starting time of runaway gas accretion onto the planet, the mass-loss rate due to photoevaporation, and the starting time. On the other hand, the final planet mass is almost independent of the disk radius, viscosity, and planetary orbital radius. The obtained final planet mass is ≲10% of the initial disk mass. Third, the proposed model successfully explains properties in the mass distribution of giant exoplanets with the mass distribution of observed protoplanetary disks for a reasonable range of the mass-loss rate due to photoevaporation.

Keywords: exoplanet formation; exoplanet dynamics; extrasolar gaseous giant planets