adsabs/SciX_UAT_keywords huggingface dataset:
Streaming Instability in Turbulent Protoplanetary Disks

The streaming instability for solid particles in protoplanetary disks is reexamined assuming the familiar alpha (α) model for isotropic turbulence. Turbulence always reduces the growth rates of the streaming instability relative to values calculated for globally laminar disks. While for small values of the turbulence parameter, α < 10-5, the wavelengths of the fastest growing disturbances are small fractions of the local gas vertical scale height H, we find that for moderate values of the turbulence parameter, I.e., α ∼ 10-5-10-3, the length scales of maximally growing disturbances shift toward larger scales, approaching H. At these moderate turbulent intensities and for local particle to gas mass density ratios ɛ < 0.5, the vertical scales of the most unstable modes begin to exceed the corresponding radial scales so that the instability appears in the form of vertically oriented sheets extending well beyond the particle scale height. We find that for hydrodynamical turbulent disk models reported in the literature, with α = 4 × 10-5-5 × 10-4, together with state-of-the-art global evolution models of particle growth, the streaming instability is predicted to be viable within a narrow triangular patch of α-τs parameter space centered on Stokes numbers, τs ∼ 0.01 and α ∼ 4 × 10-5, and further, exhibits growth rates on the order of several hundreds to thousands of orbit times for disks with 1% (Z = 0.01) cosmic solids abundance or metallicity. Our results are consistent with, and place in context, published numerical studies of streaming instabilities.

Keywords: hydrodynamics; protoplanetary disks
Candidate Brown-dwarf Microlensing Events with Very Short Timescales and Small Angular Einstein Radii

Short-timescale microlensing events are likely to be produced by substellar brown dwarfs (BDs), but it is difficult to securely identify BD lenses based on only event timescales ${t}_{{\rm{E}}} because short-timescale events can also be produced by stellar lenses with high relative lens-source proper motions. In this paper, we report three strong candidate BD-lens events found from the search for lensing events not only with short timescales ( ${t}_{{\rm{E}}}\lesssim 6\,\mathrm{days} ) but also with very small angular Einstein radii ( ${\theta }_{{\rm{E}}}\lesssim 0.05\,\mathrm{mas} ) among the events that have been found in the 2016-2019 observing seasons. These events include MOA-2017-BLG-147, MOA-2017-BLG-241, and MOA-2019-BLG-256, in which the first two events are produced by single lenses and the last event is produced by a binary lens. From the Monte Carlo simulations of Galactic events conducted with the combined ${t}_{{\rm{E}}} and ${\theta }_{{\rm{E}}} constraint, it is estimated that the lens masses of the individual events are ${0.051}_{-0.027}^{+0.100}\,{M}_{\odot }, ${0.044}_{-0.023}^{+0.090}\,{M}_{\odot }, and ${0.046}_{-0.023}^{+0.067}\,{M}_{\odot }/{0.038}_{-0.019}^{+0.056}\,{M}_{\odot } and the probability of the lens mass smaller than the lower limit of stars is ∼80% for all events. We point out that routine lens mass measurements of short-timescale lensing events require survey-mode space-based observations.

Keywords: gravitational microlensing; brown dwarfs
Measuring Young Stars in Space and Time. II. The Pre-main-sequence Stellar Content of N44

The Hubble Space Telescope survey Measuring Young Stars in Space and Time (MYSST) entails some of the deepest photometric observations of extragalactic star formation, capturing even the lowest-mass stars of the active star-forming complex N44 in the Large Magellanic Cloud. We employ the new MYSST stellar catalog to identify and characterize the content of young pre-main-sequence (PMS) stars across N44 and analyze the PMS clustering structure. To distinguish PMS stars from more evolved line of sight contaminants, a non-trivial task due to several effects that alter photometry, we utilize a machine-learning classification approach. This consists of training a support vector machine (SVM) and a random forest (RF) on a carefully selected subset of the MYSST data and categorize all observed stars as PMS or non-PMS. Combining SVM and RF predictions to retrieve the most robust set of PMS sources, we find ∼26,700 candidates with a PMS probability above 95% across N44. Employing a clustering approach based on a nearest neighbor surface density estimate, we identify 16 prominent PMS structures at 1σ significance above the mean density with sub-clusters persisting up to and beyond 3σ significance. The most active star-forming center, located at the western edge of N44's bubble, is a subcluster with an effective radius of ∼5.6 pc entailing more than 1100 PMS candidates. Furthermore, we confirm that almost all identified clusters coincide with known H II regions and are close to or harbor massive young O stars or YSOs previously discovered by MUSE and Spitzer observations.

Keywords: pre-main sequence stars; young star clusters; low mass stars; large magellanic cloud; support vector machine; random forests
Quantifying Chemical and Kinematical Properties of Galactic Disks

We aim to quantify the chemical and kinematical properties of Galactic disks with a sample of 119,558 giant stars having abundances and 3D velocities taken or derived from the APOGEE DR17 and Gaia EDR3 catalogs. A Gaussian mixture model is employed to distinguish the high-α and low-α sequences along the metallicity by simultaneously using chemical and kinematical data. Four disk components are identified and quantified; they are named the hαmp, hαmr, lαmp, and lαmr disks and correspond to the high-α or low-α, and metal-poor or metal-rich properties. Combined with the spatial and stellar-age information, we confirm that they are well interpreted by the two-infall formation model. The first infall of turbulent gas quickly forms the hot and thick hαmp disk with consequent thinner hαmr and lαmr disks. Then the second gas accretion forms a thinner and outermost lαmp disk. We find that the inside-out and upside-down scenario does not only satisfy the overall Galactic disk formation of these two major episodes but is also presented in the formation sequence of the three inner disks. Importantly, we reveal the inverse age-[M/H] trend of the lαmr disk, which means its younger stars are more metal-poor, indicating that the rejuvenated gas from the second accretion gradually dominates later star formation. Meanwhile, the recently formed stars converge to [M/H] ~ -0.1 dex, demonstrating a sufficient mixture of gas from two infalls.

Keywords: milky way galaxy; milky way disk; milky way evolution; milky way formation; milky way dynamics; galaxy structure; galaxy kinematics; astronomy data analysis; stellar abundances
Geometry and Kinematics of a Dancing Milky Way: Unveiling the Precession and Inclination Variation across the Galactic Plane via Open Clusters

This Letter presents a study of the geometry and motion of the Galactic disk using open clusters in the Gaia era. The findings suggest that the inclination θ i of the Galactic disk increases gradually from the inner to the outer disk, with a shift in orientation at the Galactocentric radius of approximately 6 ± 1 kpc. Furthermore, this study brings forth the revelation that the mid-plane of the Milky Way may not possess a stationary or fixed position. A plausible explanation is that the inclined orbits of celestial bodies within our Galaxy exhibit a consistent pattern of elliptical shapes, deviating from perfect circularity; however, more observations are needed to confirm this. An analysis of the vertical motion along the Galactocentric radius reveals that the disk has warped with precession and that the line of node shifts at different radii, aligning with the results from the classical Cepheids. Although there is uncertainty for precession/peculiar motion in solar orbit, after considering the uncertainty, the study derives a median value of ${\dot{\phi }}_{\mathrm{LON}}$ = 6.8 km s-1 kpc-1 in the Galaxy. This value for the derived precession in the outer disk is lower than those in the literature due to the systematic motion in solar orbit (θ i = 0.°6). The study also finds that the inclinational variation of the disk is significant and can cause systematic motion, with the variation rate ${\dot{\theta }}_{i}$ decreasing along the Galactic radius with a slope of -8.9 μas yr-1 kpc-1. Moreover, the derived ${\dot{\theta }}_{i}$ in solar orbit is 59.1 ± 11.2sample ± 7.7 ${}_{{V}_{Z\odot }}$ μas yr-1, which makes it observable for high-precision astrometry.

Keywords: milky way galaxy; open star clusters; galaxies; galaxy disks; galaxy structure
Vetting: A Stand-alone Tool for Finding Centroid Offsets in NASA Kepler, K2, and TESS, Alerting the Presence of Exoplanet False Positives

We present a simple, standalone implementation of a "centroid test" for NASA's Kepler, K2, and TESS missions. This test identifies significant changes in the centroid of a star during a transit or eclipse. A significant centroid shift can be used as evidence that a transiting signal is a "false positive", due to a background eclipsing source. This test produces a likelihood of a significant centroid offset during transit, and the spacial limit to which a contaminating object can be ruled out by the data. The tool to perform this test is packaged in vetting, a pip installable, open source Python package. In this note we discuss the procedure for creating a centroid test.

Keywords: time series analysis; exoplanets; astronomy data analysis
High-resolution ALMA Study of CO J = 2-1 Line and Dust Continuum Emissions in Cluster Galaxies at z = 1.46

We present new Atacama Large Millimeter/submillimeter Array (ALMA) results obtained from spatially resolved CO J = 2-1 line (0.″4 resolution) and 870 μm continuum (0.″2 resolution) observations of cluster galaxies in XMMXCS J2215.9-1738 at z = 1.46. Our sample comprises 17 galaxies within ~0.5 Mpc (0.6R 200) of the cluster center, all of which have previously been detected in the CO J = 2-1 line at a lower resolution. The effective radii of both the CO J = 2-1 line and 870 μm dust continuum emissions are robustly measured for nine galaxies by modeling the visibilities. We find that the CO J = 2-1 line emission in all of the nine galaxies is more extended than the dust continuum emission by a factor of 2.8 ± 1.4. We investigate the spatially resolved Kennicutt-Schmidt (KS) relation in two regions within the interstellar medium of the galaxies. The relation for our sample reveals that the central region (0 < r < R e,870μm) of galaxies tends to have a shorter gas depletion timescale, i.e., a higher star formation efficiency, compared to the extended region (R e,870μm < r < R e,CO). Overall, our result suggests that star formation activities are concentrated inside the extended gas reservoir, possibly resulting in the formation of a bulge structure. We find consistency between the ALMA 870 μm radii of star-forming members and the Hubble Space Telescope/1.6 μm radii of passive members in a mass-size distribution, which suggests a transition from star-forming to passive members within ~0.5 Gyr. In addition, no clear differences in the KS relation nor in the sizes are found between galaxies with and without a close companion.

Keywords: galaxy evolution; high-redshift galaxies; interstellar medium; star formation; galaxy structure; high-redshift galaxy clusters
Detection of the Irregular Shape of the Southern Limb of Menoetius from Observations of the 2017-2018 Patroclus-Menoetius Mutual Events

This work presents the analysis of seven mutual events of the Patroclus-Menoetius system (PMS) observed during the last season of mutual events, in 2017-2018. We compare the obtained light curves with those predicted using Grundy et al. and discuss the differences in the timing of the events and the drop of magnitude. Based on models of these observations, we present a collection of orbital parameters that provide the best fit for the observed events and compare the new solutions for the orbit of the system with solutions provided in the literature. Furthermore, we also discuss an interesting finding in the light curve of the only superior event in our collection. This light curve (the one with the best signal-to-noise ratio in our data set) shows the imprint of a possible crater in the south pole of Menoetius as deep as a fourth of its radius.

Keywords: jupiter trojans; photometry; surface photometry; planetary science; solar system astronomy; solar system; small solar system bodies; light curves; orbital elements; orbit determination; craters; impact phenomena
A Census of Photometrically Selected Little Red Dots at 4 < z < 9 in JWST Blank Fields

Observations with the James Webb Space Telescope (JWST) have uncovered numerous faint active galactic nuclei (AGN) at z ∼ 5 and beyond. These objects are key to our understanding of the formation of supermassive black holes (SMBHs), their coevolution with host galaxies, as well as the role of AGN in cosmic reionization. Using photometric colors and size measurements, we perform a search for compact red objects in an array of blank deep JWST/NIRCam fields totaling ∼640 arcmin2. Our careful selection yields 260 reddened AGN candidates at 4 < z phot < 9, dominated by a point-source-like central component (<r eff> < 130 pc) and displaying a dichotomy in their rest-frame colors (blue UV and red optical slopes). Quasar model fitting reveals our objects to be moderately dust-extincted (A V ∼ 1.6), which is reflected in their inferred bolometric luminosities of L bol = 1044–47 erg s‑1 and fainter UV magnitudes M UV ≃ ‑17 to ‑22. Thanks to the large areas explored, we extend the existing dusty AGN luminosity functions to both fainter and brighter magnitudes, estimating their number densities to be ×100 higher than for UV-selected quasars of similar magnitudes. At the same time, they constitute only a small fraction of all UV-selected galaxies at similar redshifts, but this percentage rises to ∼10% for M UV ∼ ‑ 22 at z ∼ 7. Finally, assuming a conservative case of accretion at the Eddington rate, we place a lower limit on the SMBH mass function at z ∼ 5, finding it to be consistent with both theory and previous JWST observations.

Keywords: galaxies; high-redshift galaxies; active galaxies; active galactic nuclei
Significant H I and Metal Differences around the z = 0.83 Lens Galaxy toward the Doubly Lensed Quasar SBS 0909+532

We report a large difference in neutral hydrogen (H I) and metal column densities between the two sight lines probing opposite sides of the lensing galaxy at zlens = 0.83 toward the doubly lensed quasar SBS 0909+532. Using archival HST-STIS and Keck HIRES spectra of the lensed quasar images, we measure log NH I = 18.77 ± 0.12 cm-2 toward the brighter image (A) at an impact parameter of rA = 3.15 kpc and log NH I = 20.38 ± 0.20 cm-2 toward the fainter image (B) at an impact parameter of rB = 5.74 kpc. This difference by a factor of ∼41 is the highest difference between sight lines for a lens galaxy in which H I has been measured, suggesting patchiness and/or anisotropy on these scales. We estimate an average Fe abundance gradient between the sight lines to be ≥+0.35 dex kpc-1. The NFe II/NMg II ratios for the individual components detected in the Keck HIRES spectra have supersolar values for all components in sight line A and for 11 out of 18 components in sight line B, suggesting that Type Ia supernovae may have contributed to the chemical enrichment of the galaxy's environment. Additionally, these observations provide complementary information to detections of cold gas in early-type galaxies and the tension between these and some models of cloud survival.

Keywords: elliptical galaxies; quasar absorption line spectroscopy; gravitational lensing
Information Content of JWST NIRSpec Transmission Spectra of Warm Neptunes

Warm Neptunes offer a rich opportunity for understanding exo-atmospheric chemistry. With the upcoming James Webb Space Telescope (JWST), there is a need to elucidate the balance between investments in telescope time versus scientific yield. We use the supervised machine-learning method of the random forest to perform an information content (IC) analysis on a 11-parameter model of transmission spectra from the various NIRSpec modes. The three bluest medium-resolution NIRSpec modes (0.7-1.27 μm, 0.97-1.84 μm, 1.66-3.07 μm) are insensitive to the presence of CO. The reddest medium-resolution mode (2.87-5.10 μm) is sensitive to all of the molecules assumed in our model: CO, CO2, CH4, C2H2, H2O, HCN, and NH3. It competes effectively with the three bluest modes on the information encoded on cloud abundance and particle size. It is also competitive with the low-resolution prism mode (0.6-5.3 μm) on the inference of every parameter except for the temperature and ammonia abundance. We recommend astronomers to use the reddest medium-resolution NIRSpec mode for studying the atmospheric chemistry of 800-1200 K warm Neptunes; its corresponding high-resolution counterpart offers diminishing returns. We compare our findings to previous JWST IC analyses that favor the blue orders and suggest that the reliance on chemical equilibrium could lead to biased outcomes if this assumption does not apply. A simple, pressure-independent diagnostic for identifying chemical disequilibrium is proposed based on measuring the abundances of H2O, CO, and CO2.

Keywords: exoplanet atmospheres; exoplanets
Early-stage Coronal Hard X-Ray Source in Solar Flares in the Collapsing Trap Model

A bright hard X-ray coronal source observed at the early stage of solar flares is considered. The plasma density in a quiet corona is not enough to explain the hard X-ray bremsstrahlung radiation. The generally accepted concept of increasing plasma density in the looptop is associated with the effect of evaporation of hot chromosphere plasma. We discuss the increase in plasma density at the looptop at the early stage of a flare, due to magnetic loop contraction during the relaxation of the magnetic field (the so-called collapsing trap model). In this case, the increase in the plasma density at the looptop occurs on a timescale of seconds-tens of seconds, while the process of plasma evaporation increases the plasma density for much longer. The Fokker-Planck kinetic equation for accelerated electrons with a betatron and Fermi terms is solved numerically. We calculate increases in the energy of the accelerated electrons, the energy spectrum, and the pitch-angle anisotropy due to betatron and Fermi first-order acceleration. For a collapse time of 8 s, the total energy of the accelerated electrons increases by ~20%-200%, depending on the model parameters. The ratio of the looptop/total hard X-ray flux at 29-58 keV increases by 15%-30% in the collapsing trap model. It is shown that this model can explain the appearance of bright coronal hard X-ray sources in the first seconds-tens of seconds after the hard X-ray flux starts growing.

Keywords: active solar corona; solar flares; solar activity; solar active regions
Dark Comets? Unexpectedly Large Nongravitational Accelerations on a Sample of Small Asteroids

We report statistically significant detections of nonradial, nongravitational accelerations based on astrometric data in the photometrically inactive objects 1998 KY26, 2005 VL1, 2016 NJ33, 2010 VL65, 2016 RH120, and 2010 RF12. The magnitudes of the nongravitational accelerations are greater than those typically induced by the Yarkovsky effect, and there is no radiation-based, nonradial effect that can be so large. Therefore, we hypothesize that the accelerations are driven by outgassing and calculate implied H2O production rates for each object. We attempt to reconcile outgassing-induced acceleration with the lack of visible comae or photometric activity via the absence of surface dust and low levels of gas production. Although these objects are small, and some are rapidly rotating, the surface cohesive forces are stronger than the rotational forces, and rapid rotation alone cannot explain the lack of surface debris. It is possible that surface dust was removed previously, perhaps via outgassing activity that increased the rotation rates to their present-day value. We calculate dust production rates of order ~10-4 g s-1 in each object, assuming that the nuclei are bare, within the upper limits of dust production from a sample stacked image of 1998 KY26 of ${\dot{M}}_{\mathrm{Dust}}\lt 0.2$ g s-1. This production corresponds to brightness variations of order ~0.0025%, which are undetectable in extant photometric data. We assess the future observability of each of these targets and find that the orbit of 1998 KY26-which is also the target of the extended Hayabusa2 mission-exhibits favorable viewing geometry before 2025.

Keywords: comae; asteroids
Disk Tearing: Numerical Investigation of Warped Disk Instability

We present numerical simulations of misaligned disks around a spinning black hole covering a range of parameters. Previous simulations have shown that disks that are strongly warped by a forced precession—in this case, the Lense-Thirring effect from the spinning black hole—can break apart into discrete disks or rings that can behave quasi-independently for short timescales. With the simulations we present here, we confirm that thin and highly inclined disks are more susceptible to disk tearing than thicker disks or those with lower inclination, and we show that lower values of the disk viscosity parameter lead to instability at lower warp amplitudes. This is consistent with detailed stability analysis of the warped disk equations. We find that the growth rates of the instability seen in the numerical simulations are similar across a broad range of parameters, and are of the same order as the predicted growth rates. However, we did not find the expected trend of growth rates with viscosity parameter. This may indicate that the growth rates are affected by numerical resolution, or that the wavelength of the fastest-growing mode is a function of local disk parameters. Finally, we also find that disk tearing can occur for disks with a viscosity parameter that is higher than predicted by a local stability analysis of the warped disk equations. In this case, the instability manifests differently, producing large changes in the disk tilt locally in the disk, rather than the large changes in disk twist that typically occur in lower-viscosity disks.

Keywords: accretion; relativistic disks; circumstellar disks; stellar accretion disks; hydrodynamics; hydrodynamical simulations; black hole physics
Gemini Speckle Imaging of Dual Quasar Candidates

We observed the candidate dual quasar J2122-0026 with speckle interferometric imaging at the Gemini-North Observatory. Results of our one-hour high resolution 'Alopeke imaging observation agree with and confirm Hubble Space Telescope imaging of the pair separated at ~0"5. These observations show that sub-arcsec dual quasar candidates as faint as r ~ 19 can be observed and detected using speckle imaging at Gemini.

Keywords: quasars; speckle interferometry