adsabs/SciX_UAT_keywords huggingface dataset:
Reconciling Power-law Slopes in Solar Flare and Nanoflare Size Distributions

We unify the power laws of size distributions of solar flare and nanoflare energies. We present three models that predict the power-law slopes α E of flare energies defined in terms of the 2D and 3D fractal dimensions (D A , D V ): (i) the spatiotemporal standard self-organized criticality model, defined by the power-law slope α E1 =1 + 2/(D V + 2) = (13/9) ≈ 1.44; (ii) the 2D thermal energy model, α E2 = 1 + 2/D A = (7/3) ≈ 2.33; and (iii) the 3D thermal energy model, α E3 = 1 + 2/D V = (9/5) ≈ 1.80. The theoretical predictions of energies are consistent with the observational values of these three groups, i.e., α E1 = 1.47 ± 0.07, α E2 = 2.38 ± 0.09, and α E3 = 1.80 ± 0.18. These results corroborate that the energy of nanoflares does not diverge at small energies, since (α E1 < 2) and (α E3 < 2), except for the 2D model (α E2 > 2). Thus, while this conclusion does not support nanoflare scenarios of coronal heating from a dimensionality point of view, magnetic reconnection processes with quasi-1D or quasi-2D current sheets cannot be ruled out.

Keywords: active solar corona; solar flares; solar x-ray flares
A Precursor Balloon Mission for Venusian Astrobiology

The potential detection of phosphine in the atmosphere of Venus has reignited interest in the possibility of life aloft in this environment. If the cloud decks of Venus are indeed an abode of life, it should reside in the "habitable zone" between ∼50-60 km altitude, roughly coincident with the middle cloud deck, where the temperature and pressure (but not the atmospheric composition) are similar to conditions at the Earth's surface. We map out a precursor astrobiological mission to search for such putative lifeforms in situ with instrument balloons, which could be delivered to Venus via launch opportunities in 2022-2023. This mission would collect aerosol and dust samples by means of small balloons floating in the Venusian cloud deck and directly scrutinize whether they include any apparent biological materials and, if so, their shapes, sizes, and motility. Our balloon mission would also be equipped with a miniature mass spectrometer that should permit the detection of complex organic molecules. The mission is augmented by contextual cameras to search for macroscopic signatures of life in the Venusian atmospheric habitable zone. Finally, mass and power constraints permitting, radio interferometric determinations of the motion of the balloons in Venusian winds, together with in situ temperature and pressure measurements, will provide valuable insights into the poorly understood meteorology of the middle cloud region.

Keywords: venus; astrobiology; biosignatures; space vehicles; space vehicle instruments
Molecules with ALMA at Planet-forming Scales (MAPS). VII. Substellar O/H and C/H and Superstellar C/O in Planet-feeding Gas

The elemental composition of the gas and dust in a protoplanetary disk influences the compositions of the planets that form in it. We use the Molecules with ALMA at Planet-forming Scales (MAPS) data to constrain the elemental composition of the gas at the locations of potentially forming planets. The elemental abundances are inferred by comparing source-specific gas-grain thermochemical models with variable C/O ratios and small-grain abundances from the DALI code with CO and C2H column densities derived from the high-resolution observations of the disks of AS 209, HD 163296, and MWC 480. Elevated C/O ratios (~2.0), even within the CO ice line, are necessary to match the inferred C2H column densities over most of the pebble disk. Combined with constraints on the CO abundances in these systems, this implies that both the O/H and C/H ratios in the gas are substellar by a factor of 4-10, with the O/H depleted by a factor of 20-50, resulting in the high C/O ratios. This necessitates that even within the CO ice line, most of the volatile carbon and oxygen is still trapped on grains in the midplane. Planets accreting gas in the gaps of the AS 209, HD 163296, and MWC 480 disks will thus acquire very little carbon and oxygen after reaching the pebble isolation mass. In the absence of atmosphere-enriching events, these planets would thus have a strongly substellar O/H and C/H and superstellar C/O atmospheric composition. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Keywords: abundance ratios; astrochemistry; protoplanetary disks
Probing the Influence of a Tachocline in Simulated M-dwarf Dynamos

M-type stars are among the best candidates in searches for habitable Earth-like exoplanets, and yet many M-dwarfs exhibit extraordinary flaring that would bombard otherwise habitable planets with ionizing radiation. Observers have found that the fraction of M-stars demonstrating significant activity transitions from roughly 10% for main-sequence stars more massive than 0.35 M⊙ to nearly 90% for less massive stars. The latter are typically rotating quite rapidly, suggesting differing spin-down histories. It is also below 0.35 M⊙ when main-sequence stars become fully convective and may no longer contain a tachocline. We turn here to the more massive M-stars to study the impact such a layer may have on their internal dynamics. Using the global MHD code Rayleigh, we compare the properties of convective dynamos generated within rapidly rotating 0.4 M⊙ stars, with the computational domain either terminating at the base of the convection zone or permitting overshoot into the underlying stable region. We find that a tachocline is not necessary for the organization of strong toroidal wreaths of magnetism in these stars, though it can increase the coupling of mean field amplitudes to the stellar rotation rate. Additionally, we note that the presence of a tachocline tends to make magnetic cycles more regular than they would otherwise have been, and can permit alternative field configurations with much longer cycles. Finally, we find that the tachocline helps enhance the emergent fields and organize them into larger spatial scales, providing favorable conditions for more rapid spin-down via the stellar wind.

Keywords: m dwarf stars; magnetic fields; magnetohydrodynamical simulations; stellar convection envelopes; stellar evolution; low mass stars
Resolving the Dust-to-Metals Ratio and CO-to-H2 Conversion Factor in the Nearby Universe

We investigate the relationship between the dust-to-metals ratio (D/M) and the local interstellar medium environment at ∼2 kpc resolution in five nearby galaxies: IC 342, M31, M33, M101, and NGC 628. A modified blackbody model with a broken power-law emissivity is used to model the dust emission from 100 to 500 μm observed by Herschel. We utilize the metallicity gradient derived from auroral line measurements in H I regions whenever possible. Both archival and new CO rotational line and H I 21 cm maps are adopted to calculate gas surface density, including new wide-field CO and H I maps for IC 342 from IRAM and the VLA, respectively. We experiment with several prescriptions of the CO-to-H2 conversion factor and compare the resulting D/M-metallicity and D/M-density correlations, both of which are expected to be nonnegative from depletion studies. The D/M is sensitive to the choice of the conversion factor. The conversion factor prescriptions based on metallicity only yield too much molecular gas in the center of IC 342 to obtain the expected correlations. Among the prescriptions tested, the one that yields the expected correlations depends on both metallicity and surface density. The 1σ range of the derived D/M spans 0.40-0.58. Compared to chemical evolution models, our measurements suggest that the dust growth timescale is much shorter than the dust destruction timescale. The measured D/M is consistent with the D/M in galaxy-integrated studies derived from infrared dust emission. Meanwhile, the measured D/M is systematically higher than the D/M derived from absorption, which likely indicates a systematic offset between the two methods.

Keywords: interstellar dust; metallicity; molecular gas; interstellar dust processes; dust continuum emission
Molecules with ALMA at Planet-forming Scales (MAPS). XII. Inferring the C/O and S/H Ratios in Protoplanetary Disks with Sulfur Molecules

Sulfur-bearing molecules play an important role in prebiotic chemistry and planet habitability. They are also proposed probes of chemical ages, elemental C/O ratio, and grain chemistry processing. Commonly detected in diverse astrophysical objects, including the solar system, their distribution and chemistry remain, however, largely unknown in planet-forming disks. We present CS (2 - 1) observations at ~0"3 resolution performed within the ALMA MAPS Large Program toward the five disks around IM Lup, GM Aur, AS 209, HD 163296, and MWC 480. CS is detected in all five disks, displaying a variety of radial intensity profiles and spatial distributions across the sample, including intriguing apparent azimuthal asymmetries. Transitions of C2S and SO were also serendipitously covered, but only upper limits are found. For MWC 480, we present complementary ALMA observations at ~ 0"5 of CS, 13CS, C34S, H2CS, OCS, and SO2. We find a column density ratio N(H2CS)/N(CS) ~ 2/3, suggesting that a substantial part of the sulfur reservoir in disks is in organic form (i.e., CxHySz). Using astrochemical disk modeling tuned to MWC 480, we demonstrate that N(CS)/N(SO) is a promising probe for the elemental C/O ratio. The comparison with the observations provides a supersolar C/O. We also find a depleted gas-phase S/H ratio, suggesting either that part of the sulfur reservoir is locked in solid phase or that it remains in an unidentified gas-phase reservoir. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.

Keywords: protoplanetary disks; planet formation; interstellar molecules; radio astronomy; interferometry; astronomical models; chemical abundances; interstellar abundances; astrochemistry
π Earth: A 3.14 day Earth-sized Planet from K2's Kitchen Served Warm by the SPECULOOS Team

We report on the discovery of a transiting Earth-sized (0.95R⊕) planet around an M3.5 dwarf star at 57 pc, EPIC 249631677. The planet has a period of ∼3.14 days, i.e., ∼π, with an installation of 7.45 S⊕. The detection was made using publicly available data from K2's Campaign 15. We observed three additional transits with SPECULOOS Southern and Northern Observatories, and a stellar spectrum from Keck/HIRES, which allowed us to validate the planetary nature of the signal. The confirmed planet is well suited for comparative terrestrial exoplanetology. While exoplanets transiting ultracool dwarfs present the best opportunity for atmospheric studies of terrestrial exoplanets with the James Webb Space Telescope, those orbiting mid-M dwarfs within 100 pc such as EPIC 249631677b will become increasingly accessible with the next generation of observatories.

Keywords: exoplanet detection methods; transit photometry
Probing the Capability of Future Direct-imaging Missions to Spectrally Constrain the Frequency of Earth-like Planets

A critical question in astrobiology is whether exo-Earth candidates (EECs) are Earth-like, in that they originate life that progressively oxygenates their atmospheres similarly to Earth. We propose answering this question statistically by searching for O2 and O3 on EECs with missions such as HabEx or LUVOIR. We explore the ability of these missions to constrain the fraction, fE, of EECs that are Earth-like in the event of a null detection of O2 or O3 on all observed EECs. We use the Planetary Spectrum Generator to simulate observations of EECs with O2 and O3 levels based on Earth's history. We consider four instrument designs—LUVOIR-A (15 m), LUVOIR-B (8 m), HabEx with a starshade (4 m, "HabEx/SS"), and HabEx without a starshade (4 m, "HabEx/no-SS")—as well as three estimates of the occurrence rate of EECs (ηearth): 24%, 5%, and 0.5%. In the case of a null detection, we find that for ηearth = 24%, LUVOIR-A, LUVOIR-B, and HabEx/SS would constrain fE to ≤0.094, ≤0.18, and ≤0.56, respectively. This also indicates that if fE is greater than these upper limits, we are likely to detect O3 on at least one EEC. Conversely, we find that HabEx/no-SS cannot constrain fE, due to the lack of a coronagraph ultraviolet channel. For ηearth = 5%, only LUVOIR-A and LUVOIR-B would be able to constrain fE, to ≤0.45 and ≤0.85, respectively. For ηearth = 0.5%, none of the missions would allow us to constrain fE, due to the low number of detectable EECs. We conclude that the ability to constrain fE is more robust to uncertainties in ηearth for missions with larger aperture mirrors. However, all missions are susceptible to an inconclusive null detection if ηearth is sufficiently low.

Keywords: astrobiology; exoplanet atmospheres; direct imaging; biosignatures
Stellar Spins in the Open Cluster NGC 2516

Measuring the distribution of stellar spin axis orientations in a coeval group of stars probes the physical processes underlying the stars' formation. In this paper, we use spectrophotometric observations of the open cluster NGC 2516 to determine the degree of spin alignment among its stars. We combine TESS light curves, ground-based spectroscopy from the Gaia-ESO and GALAH surveys, broadband stellar magnitudes from several surveys, and Gaia astrometry to measure 33 stellar inclinations and quantify overall cluster rotation. Our measurements suggest that stellar spins in this cluster are isotropically oriented, while allowing for the possibility that they are moderately aligned. An isotropic distribution of NGC 2516 spins would imply a star-forming environment in which turbulence dominated ordered motion, while a moderately aligned distribution would suggest a more substantial contribution from rotation. We also perform a three-dimensional analysis of the cluster's internal kinematics, finding no significant signatures of overall rotation. Stemming from this analysis, we identify evidence of cluster contraction, suggesting possible ongoing mass segregation in NGC 2516.

Keywords: open star clusters; inclination; stellar rotation; star formation
Uncovering an Excess of X-Ray Point Sources in the Halos of Virgo Late-type Galaxies

We present a systematic search for extraplanar X-ray point sources around 19 late-type, highly inclined disk galaxies residing in the Virgo cluster, based on archival Chandra observations reaching a source detection sensitivity of L(0.5–8 keV) ∼ 1038 erg s‑1. Based on the cumulative source surface density distribution as a function of projected vertical distance from the disk midplane, we identify a statistically significant (∼3.3σ) excess of ∼20 X-ray sources within a projected vertical off-disk distance of (∼4.4–12 kpc), the presence of which cannot be explained by the bulk stellar content of the individual galaxies, nor by the cosmic X-ray background. On the other hand, there is no significant evidence for an excess of extraplanar X-ray sources in a comparison sample of field late-type edge-on galaxies, for which Chandra observations reaching a similar source detection sensitivity are available. We discuss the possible origins for the observed excess, including low-mass X-ray binaries (LMXBs) associated with globular clusters, supernova-kicked LMXBs, and high-mass X-ray binaries born in recent star formation induced by ram pressure stripping of the disk gas, as well as a class of intracluster X-ray sources previously identified around early-type member galaxies of Virgo. We find that none of these X-ray populations can naturally dominate the observed extraplanar excess, although supernova-kicked LMXBs and the effect of ram pressure are most likely relevant.

Keywords: virgo cluster; x-ray astronomy; late-type galaxies; disk galaxies; x-ray binary stars
Stellar Properties of z ≳ 8 Galaxies in the Reionization Lensing Cluster Survey

Measurements of stellar properties of galaxies when the universe was less than one billion years old yield some of the only observational constraints on the onset of star formation. We present here the inclusion of Spitzer/IRAC imaging in the fitting of the spectral energy distribution of the seven highest-redshift galaxy candidates selected from the Hubble Space Telescope (HST) imaging of the Reionization Lensing Cluster Survey. We find that for six out of eight HST-selected z ∼ 8 sources, the z ∼ 8 solutions are still strongly preferred over z ∼ 1-2 solutions after the inclusion of Spitzer fluxes, and two prefer a z ∼ 7 solution, which we defer to a later analysis. We find a wide range of intrinsic stellar masses (5 × 106-4 × 109 M⊙), star formation rates (0.2-14 M⊙ yr-1), and ages (30-600 Myr) among our sample. Of particular interest is A1763-1434, which shows evidence of an evolved stellar population (∼500 Myr) at z ∼ 8, implying that its first generation of star formation occurred <100 Myr after the Big Bang. SPT0615-JD, a spatially resolved z ∼ 10 candidate, remains at its high redshift, supported by deep Spitzer/IRAC data, and also shows some evidence for an evolved stellar population. Even with the lensed, bright apparent magnitudes of these z ≳ 8 candidates (H = 26.1-27.8 AB mag), only the James Webb Space Telescope will be able to exclude the possibility of abnormally strong nebular emission, large dust content, or some combination thereof, and confirm the presence of evolved stellar populations early in the universe.

Keywords: high-redshift galaxies; broad band photometry
Galactic Cosmic Rays at Mars and Venus: Temporal Variations from Hours to Decades Measured as the Background Signal of Onboard Microchannel Plates

A microchannel plate (MCP) is a component widely used for counting particles in space. Using the background counts from MCPs on the Mars Express and Venus Express orbiters-operating over 17 yr and 8 yr, respectively-we investigated the galactic cosmic ray (GCR) characteristics of the inner solar system. The MCP background counts at Mars and Venus, on a solar cycle timescale, exhibited clear anticorrelation with the sunspot number. We concluded that the measured MCP background counts contained GCR information. The GCR characteristics measured using the MCP background counts at Mars showed features consistent with measurements on Earth in Solar Cycle 24. The time lag between the sunspot number and the MCP background counts was found to be ~9 months at Mars. The shorter-term background data recorded along the orbits (with a timescale of several hours) also showed evident depletion of the background counts, due to absorption of the GCR particles by the planets. Thanks to the visible planetary size change along an orbit, we developed a model to separate the GCR contribution to the MCP background counts from the internal contribution caused by the β-decay of radioactive elements in the MCP glass. Our statistical analysis of the GCR absorption signatures at Mars implies that the effective absorption radius of Mars for the GCR particles is >100 km larger than the radius of the planet. However, the cause remains an open question.

Keywords: galactic cosmic rays; mars; venus; planetary atmospheres; cosmic ray detectors
The Fermi Paradox Revisited: Technosignatures and the Contact Era

A new solution to the Fermi Paradox is presented: probes or visits from putative alien civilizations have a very low probability until a civilization reaches a certain age (called the "Contact Era") after the onset of radio communications. If biotic planets are common, putative advanced civilizations may send probes not to any planet showing biosignatures, but rather to planets with technosignatures, such as radio broadcasts. The contact probability is defined as the chance to find a nearby civilization located close enough so that it could have detected the earliest radio emissions (the "radiosphere") and sent a probe that would reach the solar system at present. It is found that the current contact probability for Earth is very low unless civilizations are extremely abundant. Since the radiosphere expands with time, so does the contact probability. The Contact Era is defined as the time (since the onset of radio transmissions) at which the contact probability becomes of order unity. At that time alien probes (or messages) become more likely. Unless civilizations are highly abundant, the Contact Era is shown to be of the order of a few hundred to a few thousand years and may be applied not only to physical probes but also to transmissions (i.e., search for extraterrestrial intelligence). Consequently, it is shown that civilizations are unlikely to be able to intercommunicate unless their communicative lifetime is at least a few thousand years.

Keywords: astrobiology; exoplanet astronomy; habitable planets
Photometric Metallicity and Distance Estimates for 136,000 RR Lyrae Stars from Gaia Data Release 3

We present a sample of 135,873 RR Lyrae stars (RRLs) with precise photometric metallicity and distance estimates from our newly calibrated P-ϕ 31-R 21-[Fe/H]/P-R 21-[Fe/H] and G-band absolute magnitude-metallicity relations. The P-ϕ 31-R 21-[Fe/H] and P-R 21-[Fe/H] relations for type RRab and type RRc stars are obtained from nearly 2700 Gaia-identified RRLs, with precise ϕ 31 and R 21 measurements from light curves and metallicity estimates from spectroscopy. Using a few hundreds of nearby RRLs, with accurate distances estimated from the parallax measurements in Gaia Early Data Release 3, new G-band absolute magnitude-metallicity relations and near-IR period-absolute magnitude-metallicity relations are constructed. External checks, using other high-resolution spectroscopic samples of field RRLs and RRL members of globular clusters, show that the typical uncertainties in our photometric metallicity estimates are about 0.24 and 0.16 dex for type RRab and type RRc stars, respectively, without significant systematic bias with respect to the high-resolution spectroscopic metallicity measurements. The accuracies of these metallicity estimates are much improved, especially for type RRab stars, when compared to those provided by Gaia Data Release 3. Validations of our distance estimates, again using members of globular clusters, show that the typical distance errors are only 3%-4%. The distance moduli μ 0 = 18.503 ± 0.001 (stat) ± 0.040 (syst) mag for the Large Magellanic Cloud (LMC) and μ 0 = 19.030 ± 0.003 (stat) ± 0.043 (syst) mag for the Small Magellanic Cloud (SMC) are estimated from our type RRab star sample and are in excellent agreement with previous measurements. The mean metallicities of the LMC and SMC derived in this work are also consistent with previous determinations. Using our sample, a steep metallicity gradient of -0.024 ± 0.001 dex kpc-1 is found for the LMC, while a negligible metallicity gradient is obtained for the SMC.

Keywords: rr lyrae variable stars; distance indicators; metallicity; magellanic clouds
Bias in Polarization CLEAN Imaging

Various deconvolution algorithms, such as Hogbom CLEAN, have been developed to construct radio images of the sky. The algorithms used for synthesis imaging in CASA are sub-optimal for polarimetric imaging of sources with nonzero rotation measures. This method focuses on linear polarization in Stokes Q and U and the bias introduced in cleaning linearly polarized sources. We propose a hybrid method of cleaning based on QU fitting to reduce this bias. This method acquires a fit for a source's spectral index, polarization angle, Stokes I amplitude, polarized fraction, and rotation measure from a dirty image and uses these parameters to form a model that is fed into CASA for cleaning. Preliminary results show that this method cleans faster than the existing CASA task but introduces some degeneracies that need to be resolved prior to polarimetric imaging.

Keywords: radio astronomy; algorithms; astronomy image processing