adsabs/SciX_UAT_keywords huggingface dataset:
Methanol-A Poor Biosignature Gas in Exoplanet Atmospheres

Biosignature gas research has been growing in recent years thanks to next-generation space- and ground-based telescopes. Methanol (CH3OH) has many advantages as a biosignature gas candidate. First, CH3OH's hydroxyl group (OH) has a unique spectral feature not present in other anticipated gases in the atmospheres of rocky exoplanets. Second, there are no significant known abiotic CH3OH sources on terrestrial planets in the solar system. Third, life on Earth produces CH3OH in large quantities. However, despite CH3OH's advantages, we consider it a poor biosignature gas in the atmospheres of terrestrial exoplanets due to the enormous production flux required to reach its detection limit. CH3OH's high water solubility makes it very difficult to accumulate in the atmosphere. For the highly favorable planetary scenario of an exoplanet with an H2-dominated atmosphere orbiting an M5V dwarf star, we find that only when the column-averaged mixing ratio of CH3OH reaches at least 10 ppm can we detect it with the James Webb Space Telescope (JWST). The CH3OH bioproduction flux required to reach the JWST detection threshold of 10 ppm must be of the order of 1014 molecules cm-2 s-1, which is roughly three times the annual O2 production on Earth. Considering that such an enormous flux of CH3OH is essentially a massive waste of organic carbon-a major building block of life, we think this flux, while mathematically possible, is likely biologically unattainable. Although CH3OH can theoretically accumulate on exoplanets with CO2- or N2-dominated atmospheres, such planets' small atmospheric scale heights and weak atmospheric signals put them out of reach for near-term observations.

Keywords: exoplanet atmospheres; biosignatures; astrobiology
Nuclear and Orbital Characterization of the Transition Object (4015) 107P/Wilson-Harrington

Comet 107P/Wilson-Harrington, cross-listed as asteroid 4015, is one of the original transition objects whose properties do not neatly fit into a cometary or asteroidal origin. Discovered in a period of apparently gas-dominated activity in 1949, it was subsequently lost and recovered as the inactive asteroid 1979 VA. We obtained new and reanalyzed archival observations of the object, compared to meteorites, and conducted new orbital integrations in order to understand the nature of this object and to understand where it falls on the asteroid-comet continuum. Wilson-Harrington's reflectance spectrum is approximately neutral from visible to near-infrared wavelengths, but has a reflectance maximum near 0.8-0.9 μm. The object's spectrum is well matched by laboratory spectra of carbonaceous chondrite meteorites like the CM Murchison or the C I Ivuna. The object's phase curve slope is compatible with either an asteroidal or cometary origin, and its recent orbital history has no periods with high enough temperatures to have altered its surface. While it is possible that some unknown process has acted to change the surface from an originally cometary one, we instead prefer a fundamentally asteroidal origin for Wilson-Harrington, which can explain its surface and orbital properties. However, this would require a way to maintain significant (hyper)volatile supplies on the near-Earth objects beyond what is currently expected. Wilson-Harrington's similar meteorite affinity and possible orbital link to sample return targets (162173) Ryugu and (101955) Bennu suggest that the returned samples from the Hayabusa-2 and OSIRIS-REx missions might hold the key to understanding this object.

Keywords: comets; asteroids; surface composition; orbital evolution
Pluto's Ultraviolet Spectrum, Surface Reflectance, and Airglow Emissions

During the New Horizons spacecraft's encounter with Pluto, the Alice ultraviolet spectrograph conducted a series of observations that detected emissions from both the interplanetary medium (IPM) and Pluto. In the direction of Pluto, the IPM was found to be 133.4 ± 0.6 R at Lyα, 0.24 ± 0.02 R at Lyβ, and <0.10 R at He I 584 Å. We analyzed 3900 s of data obtained shortly before closest approach to Pluto and detect airglow emissions from H I, N I, N II, N2, and CO above the disk of Pluto. We find Pluto's brightness at Lyα to be 29.3 ± 1.9 R, in good agreement with preencounter estimates. The detection of the N II multiplet at 1085 Å marks the first direct detection of ions in Pluto's atmosphere. We do not detect any emissions from noble gases and place a 3σ upper limit of 0.14 R on the brightness of the Ar I 1048 Å line. We compare preencounter model predictions and predictions from our own airglow model, based on atmospheric profiles derived from the solar occultation observed by New Horizons, to the observed brightness of Pluto's airglow. Although completely opaque at Lyα, Pluto's atmosphere is optically thin at wavelengths longer than 1425 Å. Consequently, a significant amount of solar far-UV light reaches the surface, where it can participate in space weathering processes. From the brightness of sunlight reflected from Pluto, we find the surface has a reflectance factor (I/F) of 17% between 1400 and 1850 Å. We also report the first detection of a C3 hydrocarbon molecule, methylacetylene, in absorption, at a column density of ∼5 × 1015 cm-2, corresponding to a column-integrated mixing ratio of 1.6 × 10-6.

Keywords: pluto; spectrometers; ultraviolet astronomy; planetary atmospheres; planetary surfaces
Redeveloping a CLEAN Deconvolution Algorithm for Scatter-broadened Radio Pulsar Signals

Broadband radio waves emitted from pulsars are distorted and delayed as they propagate toward the Earth due to interactions with the free electrons that compose the interstellar medium (ISM), with lower radio frequencies being more impacted than higher frequencies. Multipath propagation in the ISM results in both later times of arrival for the lower frequencies and causes the observed pulse to arrive with a broadened tail described via the pulse broadening function. We employ the CLEAN deconvolution technique to recover the pulse broadening timescale and by proxy the intrinsic pulse shape. This work expands upon previous descriptions of CLEAN deconvolution used in pulse broadening analyses by parameterizing the efficacy on simulated data and developing a suite of tests to establish which of a set of figures of merit leads to an automatic and consistent determination of the scattering timescale and its uncertainty. We compare our algorithm to the cyclic spectroscopy method of estimating the scattering timescale, specifically to the simulations performed in Dolch et al. (2021). We test our improved algorithm on the highly scattered millisecond pulsar J1903+0327, showing the scattering timescale to change over years, consistent with estimates of the refractive timescale of the pulsar.

Keywords: radio pulsars; interstellar medium; deconvolution; interstellar scattering
Forecasting Pulsar Timing Array Sensitivity to Anisotropy in the Stochastic Gravitational Wave Background

Statistical anisotropy in the nanohertz-frequency gravitational wave background (GWB) is expected to be detected by pulsar timing arrays (PTAs) in the near future. By developing a frequentist statistical framework that intrinsically restricts the GWB power to be positive, we establish scaling relations for multipole-dependent anisotropy decision thresholds that are a function of the noise properties, timing baselines, and cadences of the pulsars in a PTA. We verify that (i) a larger number of pulsars, and (ii) factors that lead to lower uncertainty on spatial cross-correlation measurements between pulsars, lead to a higher overall GWB signal-to-noise ratio, and lower anisotropy decision thresholds with which to reject the null hypothesis of isotropy. Using conservative simulations of realistic NANOGrav data sets, we predict that an anisotropic GWB with angular power C l=1 > 0.3C l=0 may be sufficient to produce tension with isotropy at the p = 3 × 10-3 (~3σ) level in near-future NANOGrav data with a 20 yr baseline. We present ready-to-use scaling relationships that can map these thresholds to any number of pulsars, configuration of pulsar noise properties, or sky coverage. We discuss how PTAs can improve the detection prospects for anisotropy, as well as how our methods can be adapted for more versatile searches.

Keywords: millisecond pulsars; gravitational wave astronomy; gravitational waves; radio pulsars
Substructure of Visibility Functions from Scattered Radio Emission of Pulsars through Space VLBI

We report on the substructure of visibility functions in the delay domain of PSRs B0329+54, B0823+26, B0834+06, B1933+16, and B0833-45 (Vela) observed with Earth-Earth and RadioAstron space-Earth two-element interferometers at frequencies of 324 and 1668 MHz. All visibility functions display unresolved spikes distributed over a range of delays. They are due to band-limited scintillation noise and related to the scattering time. The envelopes for each but the Vela pulsar are well fit by a single Lorentzian, which we interpret as being indicative of isotropic scattering on the plane of the sky due to a thin scattering screen between the pulsar and us. In contrast, the envelope for the Vela pulsar needs to be mostly fit by at least two Lorentzians, a narrow and a broad one at the same zero delay. We interpret this characteristic as indicative of anisotropic scattering due to a more complex structure of scattering screens in the supernova remnant. The possibility of describing the delay visibility functions by Lorentzians is likely a general property of pulsars and offers a new way of describing the scattering parameters of the intervening interstellar medium. Furthermore, for all of our pulsars, the unresolved spikes in visibility functions of similar projected baselines were well correlated, indicating that the telescopes are located in the same diffraction spot. The correlation vanished for visibilities from largely different baselines, when some radio telescopes are not in the same spot.

Keywords: interstellar scintillation; radio pulsars; very long baseline interferometry; interstellar scattering
On the XUV Luminosity Evolution of TRAPPIST-1

We model the long-term X-ray and ultraviolet (XUV) luminosity of TRAPPIST-1 to constrain the evolving high-energy radiation environment experienced by its planetary system. Using a Markov Chain Monte Carlo (MCMC) method, we derive probabilistic constraints for TRAPPIST-1's stellar and XUV evolution that account for observational uncertainties, degeneracies between model parameters, and empirical data of low-mass stars. We constrain TRAPPIST-1's mass to m⋆ = 0.089 ± 0.001 M⊙ and find that its early XUV luminosity likely saturated at log10({L}XUV/{L}bol)=-{3.03}-0.12+0.23 . From the posterior distribution, we infer that there is a ∼40% chance that TRAPPIST-1 is still in the saturated phase today, suggesting that TRAPPIST-1 has maintained high activity and LXUV/Lbol ≈ 10-3 for several gigayears. TRAPPIST-1's planetary system therefore likely experienced a persistent and extreme XUV flux environment, potentially driving significant atmospheric erosion and volatile loss. The inner planets likely received XUV fluxes ∼103-104 times that of the modern Earth during TRAPPIST-1's ∼1 Gyr long pre-main-sequence phase. Deriving these constraints via MCMC is computationally nontrivial, so scaling our methods to constrain the XUV evolution of a larger number of M dwarfs that harbor terrestrial exoplanets would incur significant computational expenses. We demonstrate that approxposterior, an open source Python machine learning package for approximate Bayesian inference using Gaussian processes, accurately and efficiently replicates our analysis using 980 times less computational time and 1330 times fewer simulations than MCMC sampling using emcee. We find that approxposterior derives constraints with mean errors on the best-fit values and 1σ uncertainties of 0.61% and 5.5%, respectively, relative to emcee.

Keywords: exoplanet evolution; stellar evolution; stellar luminosities; stellar activity; stellar properties
Magnetic Imprisonment of Dusty Pinballs by a Supernova Remnant

Motivated by recent measurements of deposits of 60Fe on the ocean floor and the lunar surface, we model the transport of dust grains containing 60Fe from a near-Earth (I.e., within 100 pc) supernova (SN). We inject dust grains into the environment of an SN remnant (SNR) and trace their trajectories by applying a 1D hydrodynamic description assuming spherical symmetry to describe the plasma dynamics, and we include a rudimentary, 3D magnetic field description to examine its influence on charged dust grains. We assume the interstellar medium (ISM) magnetic fields are turbulent and are amplified by the SNR shock, while the SN wind and ejecta fields are negligible. We examine the various influences on the dust grains within the SNR to determine when/if the dust decouples from the plasma, how much it is sputtered, and where within the SNR the dust grains are located. We find that Rayleigh-Taylor instabilities are important for dust survival, as they influence the location of the SN's reverse shock. We find that the presence of a magnetic field within the shocked ISM material limits the passage of SN dust grains, with the field either reflecting or trapping the grains within the heart of the SNR. These results have important implications for in situ 60Fe measurements and for dust evolution in SNRs generally.

Keywords: supernova dynamics; supernovae; astrophysical dust processes; interstellar dust processes; supernova remnants
Solar Vortex Tubes. II. On the Origin of Magnetic Vortices

The solar atmosphere presents a wealth of dynamics due to a constant interplay between the plasma flows and magnetic fields. Twisted flux tubes are an essential magnetic structure, believed to be driven by the rotational surface's motions and linked to plasma heating, jets, and eruptive phenomena. Despite extensive investigations, twisted magnetic flux tubes lack a proper mathematical definition, precluding their automatic detection. This work addresses this issue by defining them as magnetic vortices and introduces a formal definition that is based on a recently developed magnetic vortex detection technique, the integrated averaged current deviation method. We applied this method and a kinetic vortex identification technique to realistic magnetoconvection simulations obtained from the MURaM code. The preferential site for these two types of vortices is the intergranular downflow, but while the magnetic vortices are found mostly in the small areas where plasma-β > 1, the rotational flow structures (the kinetic vortices), were detected in locations where plasma-β < 1. The magnetic vortices locally concentrate the magnetic field's vertical components and current, lasting, on average, around a minute. Two types of magnetic vortices are introduced based on their magnetic-to-kinetic energy ratio. For the first type, the magnetic energy prevails, and the magnetic vortices are mostly vertical. The second type of magnetic vortex presents distinct shapes and a lower magnetic-to-kinetic energy ratio. We have found that magnetic vortices may appear if two conditions are simultaneously present: (i) shear flow and (ii) plasma-β > 1. The presence of rotational motion is not necessary.

Keywords: solar magnetic fields; solar atmospheric motions; solar photosphere; magnetohydrodynamics
Coupling between the Magnetospheric Dipolarization Front and the Earth's Ionosphere by Ultralow-frequency Waves

Magnetosphere-ionosphere coupling is representative of a class of astrophysical problems involving the interaction between very different plasma regimes that are connected by magnetic fields. It is a major issue of the planet's (e.g., Earth, Jupiter, Saturn) system how the magnetospheric plasma is coupled to the planet's ionosphere/atmosphere. Due to their roles on energy transport and conversion in the terrestrial magnetosphere, dipolarization fronts (DFs) in the magnetotail have been intensively studied. While the energy may also be transported toward the ionosphere during the magnetosphere-ionosphere coupling, the coupling processes during DF events remain unclear. Using high-quality MMS and Swarm measurements, we present new conjugated observations of ultralow-frequency waves associated with the DF in the magnetotail and multiscale field-aligned currents (FACs) in the ionosphere. The tail large-amplitude low-frequency magnetic disturbances and associated FACs were found connected with a pair of ionospheric FACs near the magnetic footprints of MMS. The earthward field-aligned Poynting flux of low-frequency waves reveals the energy transport from the DF toward the ionosphere. Meanwhile, small-scale and highly temporal ionospheric FACs suggest that the magnetotail shear Alfvén waves may evolve into kinetic Alfvén waves during the coupling processes. This finding implies that a very localized dynamic process (e.g., DFs in the terrestrial magnetotail) in the magnetosphere could couple the ionosphere with Alfvén waves.

Keywords: earth (planet); alfven waves; geomagnetic fields
BayeSED-GALAXIES. I. Performance Test for Simultaneous Photometric Redshift and Stellar Population Parameter Estimation of Galaxies in the CSST Wide-field Multiband Imaging Survey

The forthcoming Chinese Space Station Telescope (CSST) wide-field multiband imaging survey will produce seven-band photometric spectral energy distributions (SEDs) for billions of galaxies. The effective extraction of astronomical information from these massive data sets of SEDs relies on the techniques of SED synthesis (or modeling) and SED analysis (or fitting). We evaluate the performance of the latest version of the BayeSED code combined with SED models with increasing complexity for simultaneously determining the photometric redshifts and stellar population parameters of galaxies in this survey. By using an empirical statistics-based mock galaxy sample without SED modeling errors, we show that the random observational errors in photometries are more important sources of errors than the parameter degeneracies and Bayesian analysis method and tool. By using a Horizon-AGN hydrodynamical simulation-based mock galaxy sample with SED modeling errors about the star formation histories (SFHs) and dust attenuation laws (DALs), the simple typical assumptions lead to significantly worse parameter estimation with CSST photometries only. SED models with more flexible (or complicated) forms of SFH/DAL do not necessarily lead to better estimation of redshift and stellar population parameters. We discuss the selection of the best SED model by means of Bayesian model comparison in different surveys. Our results reveal that Bayesian model comparison with Bayesian evidence may favor SED models with different complexities when using photometries from different surveys. Meanwhile, the SED model with the largest Bayesian evidence tends to give the best performance of parameter estimation, which is clearer for photometries with higher discriminative power.

Keywords: spectral energy distribution; bayesian statistics; extragalactic astronomy; redshift surveys; stellar populations; galaxy stellar content; galaxy properties; galaxy photometry; galaxy formation; galaxy evolution
Energy and Waiting Time Distributions of FRB 121102 Observed by FAST

The energy and waiting time distributions are important properties for understanding the physical mechanism of repeating fast radio bursts (FRBs). Recently, the Five-hundred-meter Aperture Spherical radio Telescope (FAST; Nan et al. 2011; Li et al. 2018) detected the largest burst sample of FRB 121102, containing 1652 bursts Li et al. (2021a) We use this sample to investigate the energy and waiting time distributions. The energy count distribution dN/dE at the high-energy range (>1038 erg) can be fitted with a single power-law function with an index of -{1.86}_{-0.02}^{+0.02}$ , while the distribution at the low-energy range deviates from the power-law function. An interesting result of Li et al. (2021a) is that there is an apparent temporal gap between early bursts (occurring before MJD 58740) and late bursts (occurring after MJD 58740). We find that the energy distributions of high-energy bursts at different epochs are inconsistent. The power-law index is -{1.70}_{-0.03}^{+0.03}$ for early bursts and -{2.60}_{-0.14}^{+0.15}$ for late bursts. For bursts observed in a single day, a linear repetition pattern is found. We use the Weibull function to fit the distribution of waiting time of consecutive bursts. The shape parameter k={0.72}_{-0.01}^{+0.01}$ and the event rate r={736.43}_{-28.90}^{+26.55}$ day-1 are derived. If the waiting times with δt < 28 s are excluded, the burst behavior can be described by a Poisson process. The best-fitting values of k are slightly different for low-energy (E < 1.58 × 1038 erg) and high-energy (E > 1.58 × 1038 erg) bursts.

Keywords: radio transient sources; radio bursts
Stable Hydrogen-burning Limits in Rapidly Rotating Very Low Mass Objects

We present novel effects of uniform rapid stellar rotation on the minimum mass of stable hydrogen burning in very low mass stars, using an analytic model and relaxing the assumption of spherical symmetry. We obtain an analytic formula for the minimum mass of hydrogen burning as a function of the angular speed of stellar rotation. Further, we show the existence of a maximum mass of stable hydrogen burning in such stars, which is purely an artifact of rapid rotation. The existence of this extremum in mass results in a minimum admissible value of the stellar rotation period of ~22 minutes, below which a very low mass object does not reach the main sequence, within the ambit of our model. For a given angular speed, we predict a mass range beyond which such an object will not evolve into a main-sequence star.

Keywords: stellar physics; brown dwarfs; stellar rotation
Mass Upper Bounds for Over 50 Kepler Planets Using Low-S/N Transit Timing Variations

Prospects for expanding the available mass measurements of the Kepler sample are limited. Planet masses have typically been inferred via radial velocity (RV) measurements of the host star or time-series modeling of transit timing variations (TTVs) in multiplanet systems; however, the majority of Kepler hosts are too dim for RV follow-up, and only a select number of systems have strong enough TTVs for time-series modeling. Here, we develop a method of constraining planet mass in multiplanet systems using low signal-to-noise ratio (S/N) TTVs. For a sample of 175 planets in 79 multiplanet systems from the California-Kepler Survey, we infer posteriors on planet mass using publicly available TTV time series from Kepler. For 53 planets (>30% of our sample), low-S/N TTVs yield informative upper bounds on planet mass, i.e., the mass constraint strongly deviates from the prior on mass and yields a physically reasonable bulk composition. For 25 small planets, low-S/N TTVs favor volatile-rich compositions. Where available, low-S/N TTV-based mass constraints are consistent with RV-derived masses. TTV time series are publicly available for each Kepler planet, and the compactness of Kepler systems makes TTV-based constraints informative for a substantial fraction of multiplanet systems. Leveraging low-S/N TTVs offers a valuable path toward increasing the available mass constraints of the Kepler sample.

Keywords: transit timing variation method; exoplanet detection methods