adsabs/SciX_UAT_keywords huggingface dataset:
A Framework for Relative Biosignature Yields from Future Direct Imaging Missions

Future exoplanet direct imaging missions, such as HabEx and LUVOIR, will select target stars to maximize the number of Earth-like exoplanets that can have their atmospheric compositions characterized. Because one of these missions' aims is to detect biosignatures, they should also consider the expected biosignature yield of planets around these stars. In this work, we develop a method of computing relative biosignature yields among potential target stars, given a model of habitability and biosignature genesis, and using a star's habitability history. As an illustration and first application of this method, we use MESA stellar models to calculate the time evolution of the habitable zone and examine three simple models for biosignature genesis to calculate the relative biosignature yield for different stars. We find that the relative merits of K stars versus F stars depend sensitively on model choice. In particular, use of the present-day habitable zone as a proxy for biosignature detectability favors young, luminous stars lacking the potential for long-term habitability. Biosignature yields are also sensitive to whether life can arise on Cold Start exoplanets that enter the habitable zone after formation, an open question deserving of more attention. Using the case study of biosignature yields calculated for θ Cygni and 55 Cancri, we find that robust mission design and target selection for HabEx and LUVOIR depend on choosing a specific model of biosignature appearance with time, the terrestrial planet occurrence rate as a function of orbital separation, precise knowledge of stellar properties, and accurate stellar evolutionary histories.

Keywords: exoplanets; exoplanet evolution; astrobiology; biosignatures; direct imaging; planet hosting stars; habitable zone; stellar properties
Spectral Retrieval with JWST Photometric data: a Case Study for HIP 65426 b

Half of the JWST high-contrast imaging objects will only have photometric data as of Cycle 2. However, to better understand their atmospheric chemistry that informs formation origin, spectroscopic data are preferred. Using HIP 65426 b, we investigate to what extent planet properties and atmospheric chemical abundance can be retrieved with only JWST photometric data points (2.5-15.5 μm) in conjunction with ground-based archival low-resolution spectral data (1.0-2.3 μm). We find that the data is consistent with an atmosphere with solar metallicity and C/O ratios at 0.40 and 0.55. We rule out 10× solar metallicity and an atmosphere with C/O = 1.0. We also find strong evidence of silicate clouds but no sign of an enshrouding featureless dust extinction. This work offers guidance and cautionary tales on analyzing data in the absence of medium-to-high-resolution spectral data.

Keywords: exoplanet atmospheres; exoplanet atmospheric composition
Simulations of Stochastic Long-term Variability in Leptonic Models for External-Compton and Synchrotron Self-Compton Dominated Blazars

In this work we investigate the nature of multiwavelength variability of blazars from a purely numerical approach. We use a time-dependent one-zone leptonic blazar emission model to simulate multiwavelength variability by introducing stochastic parameter variations in the emission region. These stochastic parameter variations are generated by Monte Carlo methods and have a characteristic power-law index of α = -2 in their power spectral densities. We include representative blazar test cases for a flat spectrum radio quasar and a high-synchrotron peaked BL Lacertae object for which the high-energy component of the spectral energy distribution is dominated by external-Compton and synchrotron self-Compton emission, respectively. The simulated variability is analyzed in order to characterize the distinctions between the two blazar cases and the physical parameters driving the variability. We show that the variability's power spectrum is closely related to underlying stochastic parameter variations for both cases. Distinct differences between the different progenitor variations are present in the multiwavelength cross-correlation functions.

Keywords: blazars; jets; radiative processes
Magnetic Fields, Star Formation Rates, and Gas Densities at Sub-kiloparsec Scales in a Pilot Sample of Nearby Galaxies

We have estimated the magnetic field strengths of a sample of seven galaxies using their nonthermal synchrotron radio emission at meter wavelengths, and assuming energy equipartition between magnetic fields and cosmic-ray particles. We tested for deviation of magnetic fields from energy equipartition with cosmic-ray particles, and found that deviations of ~25% are typical for the sample galaxies. Spatially resolved star formation rates (SFRs) were estimated for the seven galaxies along with five galaxies studied previously. For the combined sample of 12 galaxies, the equipartition magnetic fields (B eq) are correlated with the SFR surface densities (ΣSFR) at sub-kiloparsec scales with B eq ∝ ${{\rm{\Sigma }}}_{\mathrm{SFR}}^{0.31\pm 0.06}$ , consistent with model predictions. We estimated gas densities (ρ gas) for a subsample of seven galaxies using archival observations of the CO rotational transitions and the atomic hydrogen (H I) 21 cm line and studied the spatially resolved correlation between the magnetic fields and ρ gas. Magnetic fields and gas densities are found to be correlated at sub-kiloparsec scale as B eq ∝ ${\rho }_{\mathrm{gas}}^{0.40\pm 0.09}$ . This is broadly consistent with models, which typically predict B ∝ ${\rho }_{\mathrm{gas}}^{0.5}$ .

Keywords: magnetic fields; star formation; interstellar medium; radio continuum emission
Breakthrough Listen Search for Technosignatures toward the Kepler-160 System

We have conducted a search for artificial radio emission associated with the Kepler-160 system following the report of the discovery of the Earth-like planet candidate KOI-456.04 on 2020 June 4 (Heller et al. 2020). Our search targeted both narrowband (2.97 Hz) drifting (±4 Hz s-1) and wideband pulsed (5 ms at all bandwidths) artificially dispersed technosignatures using the turboSETI (Enriquez et al. 2017) and SPANDAK (V. Gajjar et al. 2020, in preparation) pipelines, respectively, from 1 to 8 GHz. No candidates were identified above an upper limit Equivalent Isotropic Radiated Power of 5.9 × 1014 W for narrowband emission and 7.3 × 1012 W for wideband emission. Here we briefly describe our observations and data reduction procedure.

Keywords: search for extraterrestrial intelligence; technosignatures
Optical Study of PKS B1322-110, the Intra-hour Variable Radio Source

Observations with the Australia Telescope Compact Array revealed intrahour variations in the radio source PKS B1322-110. As part of an optical follow-up, we obtained Gemini Hα and Hα continuum (HαC) images of the PKS B1322-110 field. A robust 19σ detection of PKS B1322-110 in the Hα-HαC image prompted us to obtain the first optical spectrum of PKS B1322-110. With the Gemini spectrum we determine that PKS B1322-110 is a flat-spectrum radio quasar at a redshift of z = 3.007 ± 0.002. The apparent flux detected in the Hα filter is likely to originate from He II emission redshifted precisely on the Galactic Hα narrowband filter. We set upper limits on the emission measure of the Galactic plasma, for various possible cloud geometries.

Keywords: interstellar medium
Bringing 2D Eclipse Mapping out of the Shadows with Leave-one-out Cross Validation

Eclipse mapping is a technique for inferring 2D brightness maps of transiting exoplanets from the shape of an eclipse light curve. With JWST's unmatched precision, eclipse mapping is now possible for a large number of exoplanets. However, eclipse mapping has only been applied to two planets, and the nuances of fitting eclipse maps are not yet fully understood. Here, we use Leave-one-out Cross Validation (LOO-CV) to investigate eclipse mapping, with application to a JWST NIRISS/SOSS observation of the ultrahot Jupiter WASP-18b. LOO-CV is a technique that provides insight into the out-of-sample predictive power of models on a data-point-by-data-point basis. We show that constraints on planetary brightness patterns behave as expected, with large-scale variations driven by the phase-curve variation in the light curve and smaller-scale structures constrained by the eclipse ingress and egress. For WASP-18b we show that the need for higher model complexity (smaller-scale features) is driven exclusively by the shape of the eclipse ingress and egress. We use LOO-CV to investigate the relationship between planetary brightness map components when mapping under a positive-flux constraint to better understand the need for complex models. Finally, we use LOO-CV to understand the degeneracy between the competing "hot spot" and "plateau" brightness map models of WASP-18b, showing that the plateau model is driven by the ingress shape and the hot spot model is driven by the egress shape, but preference for neither model is due to outliers or unmodeled signals. Based on this analysis, we make recommendations for the use of LOO-CV in future eclipse-mapping studies.

Keywords: exoplanet astronomy; exoplanet atmospheres; exoplanet atmospheric structure; exoplanets; extrasolar gaseous planets; hot jupiters
WASP-117 b: An Eccentric Hot Saturn as a Future Complex Chemistry Laboratory

We present spectral analysis of the transiting Saturn-mass planet WASP-117 b, observed with the G141 grism of the Hubble Space Telescope's (HST) Wide Field Camera 3. We reduce and fit the extracted spectrum from the raw transmission data using the open-source software Iraclis before performing a fully Bayesian retrieval using the publicly available analysis suite TauREx 3.0. We detect water vapor alongside a layer of fully opaque cloud, retrieving a terminator temperature of ${T}_{\mathrm{term}}={833}_{-156}^{+260}$ K. In order to quantify the statistical significance of this detection, we employ the atmospheric detectability index (ADI), deriving a value of ADI = 2.30, which provides positive but not strong evidence against the flat-line model. Due to the eccentric orbit of WASP-117 b, it is likely that chemical and mixing timescales oscillate throughout orbit due to the changing temperature, possibly allowing warmer chemistry to remain visible as the planet begins transit, despite the proximity of its point of ingress to apastron. We present simulated spectra of the planet as would be observed by the future space missions such as the Atmospheric Remote-sensing Infrared Exoplanet Large-survey and the James Webb Space Telescope and show that, despite not being able to probe such chemistry with current HST data, these observatories should make it possible in the not too distant future.

Keywords: exoplanet atmospheres; astronomy data analysis; radiative transfer simulations; transmission spectroscopy
Kinetic Model of Large-amplitude Oscillations in Neutron Star Pair Cascades

Electron-positron pair cascades developed in the extreme electromagnetic fields of neutron star polar caps are considered a key source of magnetospheric plasma in these objects. We use a simplified model that maps the quantum electrodynamics processes governing pair cascades to analytically and numerically model these cascades, and show that large-amplitude oscillations of the electric field are inductively driven by the resulting plasma. A plasma instability arises in these oscillations, and particles accelerated in growing electric field perturbations can drive secondary pair bursts that damp the large-amplitude oscillations. An analytical model is proposed to describe this interplay between the pair production and kinetic collective plasma processes. All analytical results are shown to be in excellent agreement with particle-in-cell simulations.

Keywords: neutron stars; plasma physics; high energy astrophysics
Onset of Turbulent Fast Magnetic Reconnection Observed in the Solar Atmosphere

Fast magnetic reconnection powers explosive events throughout the universe, from gamma-ray bursts to solar flares. Despite its importance, the onset of astrophysical fast reconnection is the subject of intense debate and remains an open question in plasma physics. Here we report high-cadence observations of two reconnection-driven solar microflares obtained by the Interface Region Imaging Spectrograph that show persistent turbulent flows preceding flaring. The speeds of these flows are comparable to the local sound speed initially, suggesting the onset of fast reconnection in a highly turbulent plasma environment. Our results are in close quantitative agreement with the theory of turbulence-driven reconnection as well as with numerical simulations in which fast magnetic reconnection is induced by turbulence.

Keywords: solar magnetic reconnection; solar atmosphere; solar magnetic fields; spectroscopy; plasma physics; magnetohydrodynamics; solar transition region; solar chromosphere; solar flares; solar active regions; solar ultraviolet emission
Quasi-adiabatic and Stochastic Heating and Particle Acceleration at Quasi-perpendicular Shocks

Based on Magnetospheric Multiscale observations from the Earth&'s bow shock, we have identified two plasma heating processes that operate at quasi-perpendicular shocks. Ions are subject to stochastic heating in a process controlled by the heating function ${\chi }_{j}={m}_{j}{q}_{j}^{-1}{B}^{-2}\mathrm{div}({{\boldsymbol{E}}}_{\perp })$ for particles with mass mj and charge qj in the electric and magnetic fields ${\boldsymbol{E}}$ and ${\boldsymbol{B}}$ . Test-particle simulations are employed to identify the parameter ranges for bulk heating and stochastic acceleration of particles in the tail of the distribution function. The simulation results are used to show that ion heating and acceleration in the studied bow shock crossings is accomplished by waves at frequencies (2-10)fcp (proton gyrofrequency) for the bulk heating, and $f\gt 10{f}_{{cp}}$ for the tail acceleration. When electrons are not in the stochastic heating regime, $| {\chi }_{e}| \lt 1$ , they undergo a quasi-adiabatic heating process characterized by the isotropic temperature relation $T/B={({T}_{0}/{B}_{0})({B}_{0}/B)}^{1/3}$ . This is obtained when the energy gain from the conservation of the magnetic moment is redistributed to the parallel energy component through the scattering by waves. The results reported in this paper may be applicable also to particle heating and acceleration at astrophysical shocks.

Keywords: shocks; interplanetary physics
Precise Measurements of the Speed of Light with High-redshift Quasars: Ultra-compact Radio Structure and Strong Gravitational Lensing

Although the speed of light has been measured with very high precision, most of these measurements were carried out on Earth or in our close cosmic surroundings. In this Letter, we propose an original idea to combine the observations of ultra-compact structure in radio quasars and strong gravitational lensing with quasars acting as background sources to estimate the speed of light. The method will provide precise measurements of the speed of light using extragalactic objects at different redshifts. We evaluate if current or future missions such as the Large Synoptic Survey Telescope (LSST) and Dark Energy Survey (DES) can be sensitive enough to detect any variation of c. Our results show that strongly lensed quasars observed by LSST would produce robust constraints on Δc/c at the level of 10-4, if the compact radio structure measurements are available.

Keywords: quasars; strong gravitational lensing; cosmological parameters; radio sources
Constraining the Jet Launching Time of GRB 170817A by Utilizing the Baryon Loading

The observed delay of GRB 170817A relative to GW170817 provides significant information about gamma-ray burst (GRB) physics, and is subject to intense debate. In this Letter, we present an approach for discussing the major source of this time delay. First, we use the structured jet model to fit the X-ray/optical/radio afterglows of GRB 170817A together with superluminal motion measured using Very Long Baseline Interferometry. Our structured jet is modeled with angle-dependent energy and baryon loading. It is found that our model well fits the afterglows of GRB 170817A. Then, the baryon loading in the jet is inferred based on our fitting results. By comparing the baryon loading to the mass outflow in different stages, we infer that the time lag of the jet launch relative to the merger is less than hundreds or tens of milliseconds. It suggests that the time delay of GRB 170817A relative to GW170817 is defined mostly by the spreading time of the jet propagating to its dissipation radius.

Keywords: gamma-ray bursts; gravitational waves; relativistic jets; high energy astrophysics
The ALMA Survey of 70 μm Dark High-mass Clumps in Early Stages (ASHES). X. Hot Gas Reveals Deeply Embedded Star Formation

Massive infrared dark clouds (IRDCs) are considered to host the earliest stages of high-mass star formation. In particular, 70 μm dark IRDCs are the colder and more quiescent clouds. At a scale of about 5000 au using formaldehyde (H2CO) emission, we investigate the kinetic temperature of dense cores in 12 IRDCs obtained from the pilot Atacama Large Millimeter/submillimeter Array Survey of 70 μm dark High-mass clumps in Early Stages (ASHES). Compared to the 1.3 mm dust continuum and other molecular lines, such as C18O and deuterated species, we find that H2CO is mainly sensitive to low-velocity outflow components rather than to quiescent gas expected in the early phases of star formation. The kinetic temperatures of these components range from 26 to 300 K. The Mach number reaches about 15 with an average value of about 4, suggesting that the velocity distribution of gas traced by H2CO is significantly influenced by a supersonic nonthermal component. In addition, we detect warm line emission from HC3N and OCS in 14 protostellar cores, which requires high excitation temperatures (E u /k ∼ 100 K). These results show that some of the embedded cores in the ASHES fields are in an advanced evolutionary stage, previously unexpected for 70 μm dark IRDCs.

Keywords: infrared dark clouds; star formation; star forming regions; massive stars; protoclusters; protostars; interstellar medium
Fallback Supernova Assembly of Heavy Binary Neutron Stars and Light Black Hole-Neutron Star Pairs and the Common Stellar Ancestry of GW190425 and GW200115

The detection of the unusually heavy binary neutron star merger GW190425 marked a stark contrast to the mass distribution from known Galactic pulsars in double neutron star binaries and gravitational-wave source GW170817. We suggest here a formation channel for heavy binary neutron stars and light black hole-neutron star binaries in which massive helium stars, which had their hydrogen envelope removed during a common-envelope phase, remain compact and avoid mass transfer onto the neutron star companion, possibly avoiding pulsar recycling. We present three-dimensional simulations of the supernova explosion of the massive stripped helium star and follow the mass fallback evolution and the subsequent accretion onto the neutron star companion. We find that fallback leads to significant mass growth in the newly formed neutron star. This can explain the formation of heavy binary neutron star systems such as GW190425, as well as predict the assembly of light black hole-neutron star systems such as GW200115. This formation avenue is consistent with the observed mass-eccentricity correlation of binary neutron stars in the Milky Way. Finally, avoiding mass transfer suggests an unusually long spin-period population of pulsar binaries in our Galaxy.

Keywords: neutron stars; black holes; stellar remnants; compact objects; massive stars; supernovae; gravitational wave sources; gravitational waves; gravitational wave astronomy
Stratospheric Transparency and Color of the Total Lunar Eclipse of 1794 February 14 Observed by Jovellanos from Gijón (Spain)

Gaspar Melchor de Jovellanos is the major figure of the Age of Enlightenment in Spain. He observed the total lunar eclipse of 1794 February 14 from Gijón (north of Spain). Jovellanos described the moon color as "glowing iron." It indicates that the stratosphere around 1794 seems to have been undisturbed volcanically as previous estimates indicate.

Keywords: lunar eclipses; atmospheric effects; history of astronomy
AT2018fyk: Candidate Tidal Disruption Event by a (Super)Massive Black Hole Binary

The tidal disruption event (TDE) AT2018fyk has unusual X-ray, UV, and optical light curves that decay over the first ∼600 days, rebrighten, and decay again around 1200 days. We explain this behavior as a one-off TDE associated with a massive black hole (BH) binary. The sharp drop-offs from t ‑5/3 power laws at around 600 days naturally arise when one BH interrupts the debris fallback onto the other BH. The BH mass M • derived from fitting X-ray spectra with a slim disk accretion model and, independently, from fitting the early UV/optical light curves, is smaller by 2 orders of magnitude than predicted from the M •–σ * host galaxy relation, suggesting that the debris is accreted onto the secondary, with the fallback cut off by the primary. Furthermore, if the rebrightening were associated with the primary, it should occur around 5000 days, not the observed 1200 days. The secondary's mass and dimensionless spin is and a •,s > 0.3 (X-ray spectral fitting), while the primary's mass is M •,p = 107.7±0.4 M ⊙ (M •–σ * relation). An intermediate mass BH secondary is consistent with the observed UV/optical light-curve decay, i.e., the secondary's outer accretion disk is too faint to produce a detectable emission floor. The time of the first accretion cutoff constrains the binary separation to be (6.7 ± 1.2) × 10‑3 pc. X-ray spectral fitting and timing analyses indicate that the hard X-rays arise from a corona above the secondary's disk. The early UV/optical emission, suggesting a super-Eddington phase for the secondary, possibly originates from shocks arising from debris circularization.

Keywords: tidal disruption; accretion; black hole physics; x-ray transient sources
The Nature of the Milky Way's Stellar Halo Revealed by the Three Integrals of Motion

We developed a new selection method of halo stars in the phase-space distribution defined by the three integrals of motion in an axisymmetric Galactic potential (E, Lz, I3), where I3 is the third integral of motion. The method is used to explore the general chemodynamical structure of the halo based on stellar samples from SDSS-SEGUE DR7 and APOGEE DR16, matched with Gaia DR2. We found the following. (a) Halo stars can be separated from disk stars by selecting over (1) 0 < Lz < 1500 kpc km s-1, ${(2{I}_{3})}^{1/2}\gt 1000$ kpc km s-1 (orbital angle θorb > 15-20 deg), and E < -1.5 × 105 km2 s-2, and (2) Lz < 0 kpc km s-1. These selection criteria are free from kinematical biases introduced by the simple high-velocity cuts adopted in recent literature. (b) The averaged, or coarse-grained, halo phase-space distribution shows a monotonic, exponential decrease with increasing E and I3 like the Michie-Bodenheimer models. (c) The inner stellar halo described in Carollo et al. is found to comprise a combination of Gaia Enceladus debris (GE), lowest-E stars (likely in situ stars), and metal-poor prograde stars missed by the high-velocity cut selection. (d) The very metal-poor outer halo, ([Fe/H] < -2.2), exhibits both retrograde and prograde rotation, with an asymmetric Lz distribution toward high retrograde motions and larger θorb than those possessed by the GE-dominated inner halo. (e) The Sgr dSph galaxy could induce a long-range dynamical effect on local halo stars. Implications for the formation of the stellar halo are also discussed.

Keywords: milky way stellar halo; milky way dynamics; milky way formation; stellar abundances