adsabs/SciX_UAT_keywords huggingface dataset:
Solar Contamination in Extreme-precision Radial-velocity Measurements: Deleterious Effects and Prospects for Mitigation

Solar contamination, due to moonlight and atmospheric scattering of sunlight, can cause systematic errors in stellar radial velocity (RV) measurements that significantly detract from the ∼10 cm s-1 sensitivity required for the detection and characterization of terrestrial exoplanets in or near habitable zones of Sun-like stars. The addition of low-level spectral contamination at variable effective velocity offsets introduces systematic noise when measuring velocities using classical mask-based or template-based cross-correlation techniques. Here we present simulations estimating the range of RV measurement error induced by uncorrected scattered sunlight contamination. We explore potential correction techniques, using both simultaneous spectrometer sky fibers and broadband imaging via coherent fiber imaging bundles, that could reliably reduce this source of error to below the photon-noise limit of typical stellar observations. We discuss the limitations of these simulations, the underlying assumptions, and mitigation mechanisms. We also present and discuss the components designed and built into the NEID (NN-EXPLORE Exoplanet Investigations with Doppler spectroscopy) precision RV instrument for the WIYN 3.5 m telescope, to serve as an ongoing resource for the community to explore and evaluate correction techniques. We emphasize that while "bright time" has been traditionally adequate for RV science, the goal of 10 cm s-1 precision on the most interesting exoplanetary systems may necessitate access to darker skies for these next-generation instruments.

Keywords: exoplanet astronomy; radial velocity; high resolution spectroscopy; sky brightness; astronomy data analysis
Seasonal Variations of Chemical Species and Haze in Titan's Upper Atmosphere

Seasonal variation is significant in Titan's atmosphere owing to the large change of solar insolation resulting from Titan's 26.7° axial tilt relative to the plane of Saturn's orbit. Here we present an investigation of hydrocarbon and nitrile species in Titan's upper atmosphere at 400-1200 km, which includes the mesosphere and the lower thermosphere, over more than one-fourth of Titan's year (2006-2014, L S = 318°-60°), using 18 stellar occultation observations obtained by Cassini/Ultraviolet Imaging Spectrograph. Vertical profiles of eight chemical species (CH4, C2H2, C2H4, C2H6, C4H2, C6H6, HCN, HC3N) and haze particles are retrieved from these observations using an instrument forward model, which considers the technical issue of pointing motion. The Markov Chain Monte Carlo algorithm is used to obtain the posterior probability distributions of parameters in the retrieval, which inherently tests the extent to which species profiles can be constrained. The results show that no change of the species profiles is noticeable before the equinox, while the decrease of atmospheric temperature and significant upwelling in the summer hemisphere are found five terrestrial years afterward. Altitude of the detached haze layer decreases toward the vernal equinox and then disappears, and no reappearance is identified within the time range of our data, which is consistent with observations from Cassini/Imaging Science Subsystem. This study provides observational constraints on the seasonal change of Titan's upper atmosphere and suggests further investigations of the atmospheric chemistry and dynamics therein.

Keywords: planetary atmospheres; atmospheric composition; atmospheric variability; upper atmosphere; titan
Characterizing and Mitigating Telluric Absorption in Precise Radial Velocities. II. A Study of an M2-type Star

Telluric absorption lines impact the measuring of precise radial velocities (RVs) from high-resolution ground-based spectrographs. In this paper, we simulate the dependence of this impact on stellar spectral type and extend the work of the first paper in this series, which studied a G-type star, to a synthetic M-dwarf star. We quantify the bias in precise RV measurements in the visible and near-infrared (NIR) from the presence of tellurics in a simulated set of observations. We find that M-dwarf RVs are more impacted by tellurics compared to G-type stars. Specifically, for an M-dwarf star, tellurics can induce RV errors of up to 16 cm s-1 in the red optical and in excess of 220 cm s-1 in the NIR. For a G dwarf, the comparable RV systematics are 3 cm s-1 in the red optical and 240 cm s-1 in the NIR. We attribute this relative increase for M-dwarf stars to the increased concordance in wavelength between telluric lines and stellar Doppler information content. We compare the results of our simulation to data collected for Barnard's star from the iSHELL spectrograph at the NASA Infrared Telescope Facility. This study was conducted as a follow-up to the NASA probe mission concept study EarthFinder.

Keywords: exoplanets; exoplanet astronomy
Hydrogen Burning of 29Si and Its Impact on Presolar Stardust Grains from Classical Novae

Presolar stardust grains found in primitive meteorites are believed to retain the isotopic composition of stellar outflows at the time of grain condensation. Therefore, laboratory measurements of their isotopic ratios represent sensitive probes for investigating open questions related to stellar evolution, stellar explosions, nucleosynthesis, mixing mechanisms, dust formation, and galactic chemical evolution. For a few selected presolar grains, classical novae have been discussed as a potential source. For SiC, silicate, and graphite presolar grains, the association is based on the observation of small N(12C)/N(13C) and N(14N)/N(15N) number abundance ratios compared to solar values, and abundance excesses in 30Si relative to 29Si, as previously predicted by models of classical novae. We report on a direct measurement of the 29Si(p,γ)30P reaction, which strongly impacts simulated δ 29Si values from classical novae. Our new experimental 29Si(p,γ)30P thermonuclear reaction rate differs from previous results by up to 50% in the classical nova temperature range (T = 100-400 MK), while the rate uncertainty is reduced by up to a factor of 3. Using our new reaction rate in Monte Carlo reaction network and hydrodynamic simulations of classical novae, we estimate δ 29Si values with much reduced uncertainties. Our results establish δ 29Si values measured in presolar grains as a sensitive probe for assessing their classical nova paternity. We also demonstrate that δ 30Si values from nova simulations are currently not a useful diagnostic tool unless the large uncertainty of the 30P(p,γ)31S reaction rate can be significantly reduced.

Keywords: classical novae; explosive nucleosynthesis; nuclear reaction cross sections; meteorites
A Census of 163 Large-scale (≥10 pc), Velocity-coherent Filaments in the Inner Galactic Plane: Physical Properties, Dense-gas Fraction, and Association with Spiral Arms

The interstellar medium has a highly filamentary and hierarchical structure that may play a significant role in star formation. A systematical study of the large-scale filaments toward their physical parameters, distribution, structures, and kinematics will inform us about which types of filaments have the potential to form stars, how the material feeds protostars through filaments, and the connection between star formation and Galactic spiral arms. Unlike the traditional by-eye searches, we use a customized minimum spanning tree algorithm to identify filaments by linking Galactic clumps from the APEX Telescope Large Area Survey of the Galaxy catalog. In the inner Galactic plane (∣l∣ < 60°), we identify 163 large-scale filaments with physical properties derived, including the dense-gas mass fraction, and we compare them with an updated spiral arm model in position-position-velocity space. The dense-gas mass fraction is found not to differ significantly in various Galactic positions or in different spiral arms. We also find that most filaments are interarm filaments after adding a distance constraint, and filaments in arms differ a little with those not in arms. One surprising result is that clumps on and off filaments have no significant distinction in their mass at the same size.

Keywords: catalogs; interstellar filaments; galaxy structure; star formation; interstellar clouds
Systematics in the Cepheid and TRGB Distance Scales: Metallicity Sensitivity of the Wesenheit Leavitt Law

Using an updated and significantly augmented sample of Cepheid and tip of the red giant branch (TRGB) distances to 28 nearby spiral and irregular galaxies, covering a wide range of metallicities, we have searched for evidence of a correlation of the zero-point of the Cepheid period–luminosity relation with H II region (gas-phase) metallicities. Our analysis, for the 21 galaxies closer than 12.5 Mpc, results in the following conclusions: (1) The zero-points of the Cepheid and TRGB distance scales are in remarkably good agreement, with the mean offset in the zero-points of the most nearby distance-selected sample being close to zero, Δμ o (Cepheid—TRGB) = ‑0.026 ± 0.015 mag (for an I-band TRGB zero-point of M I = ‑4.05 mag); however, for the more distant sample, there is a larger offset between the two distance scales, amounting to ‑0.073 ± 0.057 mag <Δμ o > (Cepheids—TRGB) = ‑0.026 ± 0.015 mag, for an I-band TRGB zero-point of M I = ‑4.05 mag. (2) The individual differences, about that mean, have a measured scatter of ±0.068 mag. (3) We find no statistically significant evidence for a metallicity dependence in the Cepheid distance scale using the reddening-free W(V, VI) period–luminosity relation: Δμ o (Cepheid ‑ TRGB) = ‑ 0.022( ± 0.015) × ([O/H] ‑ 8.50) ‑ 0.003(±0.007).

Keywords: cepheid variable stars; red giant branch; metallicity
The Chiral Puzzle of Life

Biological molecules chose one of two structurally chiral systems which are related by reflection in a mirror. It is proposed that this choice was made, causally, by cosmic rays, which are known to play a major role in mutagenesis. It is shown that magnetically polarized cosmic rays that dominate at ground level today can impose a small, but persistent, chiral bias in the rate at which they induce structural changes in simple, chiral monomers that are the building blocks of biopolymers. A much larger effect should be present with helical biopolymers, in particular, those that may have been the progenitors of ribonucleic acid and deoxyribonucleic acid. It is shown that the interaction can be both electrostatic, just involving the molecular electric field, and electromagnetic, also involving a magnetic field. It is argued that this bias can lead to the emergence of a single, chiral life form over an evolutionary timescale. If this mechanism dominates, then the handedness of living systems should be universal. Experiments are proposed to assess the efficacy of this process.

Keywords: astrobiology; cosmic rays
Comprehensive Radiative MHD Simulations of Eruptive Flares above Collisional Polarity Inversion Lines

We present a new simulation setup using the MURaM radiative MHD code that allows the study of the formation of collisional polarity inversion lines (cPILs) in the photosphere and the coronal response including flares. In this scheme, we start with a bipolar sunspot configuration and set the spots on collision course by imposing the appropriate velocity field at the footpoints in the subphotospheric boundary. We produce different setups with the same initial spot separation by varying physical parameters such as the collision speed and minimum collision distance. While all setups lead to the formation of an EUV and X-ray sigmoid structure, only the cases with a close passing of the spots cause flares and mass eruptions. The energy release is in the 1-2 × 1031 erg range, putting the simulated flares into the upper C-class to lower M-class range of GOES X-ray 1-8 Å flux. While the setup with the more distant passing of the spots does not lead to a flare, the corona is nonetheless substantially heated, suggesting noneruptive energy-release mechanisms. We focus our discussion on two particular setups that differ in spot coherence and resulting cPIL length persistence. We find different timings in the transition from a sheared magnetic arcade to magnetic flux rope (MFR); the setup with a large length but shorter duration cPIL produces a MFR during the eruption, while the MFR is preexisting in the setup with a large length and longer duration cPIL. While both result in flares of comparable strength and the eruption of a coronal mass ejection, the setup with preexisting MFR (and embedded filament) leads to an MFR eruption with a larger mass content.

Keywords: active solar corona; solar flares; solar coronal mass ejections; radiative magnetohydrodynamics; solar photosphere; solar active regions; solar active region magnetic fields
Redshift-evolutionary X-Ray and UV Luminosity Relation of Quasars from Gaussian Copula

We construct a three-dimensional and redshift-evolutionary X-ray and ultraviolet (L X -L UV ) luminosity relation for quasars from the powerful statistic tool called copula, and find that the constructed L X -L UV relation from copula is more viable than the standard one and the observations favor the redshift-evolutionary relation more than 3σ. The Akaike and Bayes information criterions indicate that the quasar data support strongly the three-dimensional L X -L UV relation. Our results show that the quasars can be regarded as a reliable indicator of the cosmic distance if the L X -L UV relation from copula is used to calibrate quasar data.

Keywords: quasars; cosmology
Chemical Fractionation Modeling of Plumes Indicates a Gas-rich, Moderately Alkaline Enceladus Ocean

Enceladus harbors an ocean beneath its ice crust that erupts spectacular plumes from fissures at the south pole. The plume composition was measured by the Cassini spacecraft, and provides evidence for the ocean's gas content, salinity, pH, and potential for life. Understanding the ocean's composition is complicated by physicochemical processes that alter the plume composition during eruption, such as water vapor condensation in the icy fissures and gas exsolution from the ocean surface. We developed a model that includes key fractionation processes, in particular fractionation during gas exsolution, which has not been previously considered. Our model predicts a moderately alkaline (pH 7.95-9.05), gas-rich ocean (~10-5-10-3 molal) with high concentrations of ammonium ions (10-2-10-1 molal). Our derived dissolved gas concentrations are higher than in recent studies because we account for gas exsolution, which depletes gases in the plume compared to the ocean, and because our model conserves mass flow rates between gas exsolution from the ocean and eruption from the tiger stripe fissures. We find carbon dioxide and hydrogen concentrations that could provide sufficient chemical energy for oceanic life in the form of hydrogenotrophic methanogens. Carbon dioxide concentrations of 10-5-10-3 molal indicate a more Earth-like pH than the pH ~8.5-13.5 in previous studies. The inferred bulk ammonium and total inorganic carbon concentrations are consistent with cometary levels. This corroborates evidence from cometary deuterium-hydrogen (D/H) ratios that Enceladus formed from comet-like planetesimals. Our results suggest a gas-rich ocean that inherited its high volatile concentrations from comet-like building blocks.

Keywords: astrobiology; saturnian satellites; chemical abundances; water vapor; comet volatiles; methane; carbon dioxide
Compact Sources in the A401-A399 Galaxy Cluster System Observed at 90 GHz with the MUSTANG-2 Camera

We report the fluxes of 10 compact sources in 90 GHz maps of the galaxy cluster A401 and the intercluster region between A401 and A399 from 66 hr of observations with the MUSTANG-2 receiver on the Green Bank Telescope. All sources have been previously detected in other bands (IR or radio). The flux and central coordinates of each source are fitted with 2D Gaussian profiles and fluxes are compared with an aperture photometry method. Future observations over a larger area would be a unique tool to compare theoretical predictions of source counts at 90 GHz with measured data, as well as refining their spectral index and improve extrapolations to higher frequencies.

Keywords: compact radiation sources; cosmology; galaxy clusters
SN 2017egm: A Helium-rich Superluminous Supernova with Multiple Bumps in the Light Curves

When discovered, SN 2017egm was the closest (redshift z = 0.03) hydrogen-poor superluminous supernova (SLSN-I) and a rare case that exploded in a massive and metal-rich galaxy. Thus, it has since been extensively observed and studied. We report spectroscopic data showing strong emission at around He I λ10830 and four He I absorption lines in the optical. Consequently, we classify SN 2017egm as a member of an emerging population of helium-rich SLSNe-I (i.e., SLSNe-Ib). We also present our late-time photometric observations. By combining them with archival data, we analyze high-cadence ultraviolet, optical, and near-infrared light curves spanning from early pre-peak (~-20 days) to late phases (~+300 days). We obtain its most complete bolometric light curve, in which multiple bumps are identified. None of the previously proposed models can satisfactorily explain all main light-curve features, while multiple interactions between the ejecta and circumstellar material (CSM) may explain the undulating features. The prominent infrared excess with a blackbody luminosity of 107-108 L ⊙ detected in SN 2017egm could originate from the emission of either an echo of a pre-existing dust shell or newly formed dust, offering an additional piece of evidence supporting the ejecta-CSM interaction model. Moreover, our analysis of deep Chandra observations yields the tightest-ever constraint on the X-ray emission of an SLSN-I, amounting to an X-ray-to-optical luminosity ratio ≲10-3 at late phases (~100-200 days), which could help explore its close environment and central engine.

Keywords: supernovae
The Electron Acoustic Wave and Its Role in Solar Flaring Loops Heating

From soft X-ray emission, the solar flare temperatures are from several MK to dozens of times MK, which are higher than the preflare coronal temperatures. A combination of several heating mechanisms may contribute to the heating problem in solar flare loops. In this paper, we propose an important mechanism of solar flaring loops heating, in which the excited electron acoustic wave (EAW) by flare-accelerated fast electron beams can lead to electron heating via collisionless Landau damping effect produced by wave-particle resonant interaction. Taking account of the return-current effect of fast electron beams, by use of numerical and analytic solutions, the plasma wave instability driven by fast electron beams is investigated in typical solar flare loop plasma parameters. The results show that the EAW is the strongest unstable wave mode rather than other wave modes. The dissipation of EAW via collisionless Landau damping and its application to solar flaring loops heating are discussed in detail.

Keywords: solar flares; plasma astrophysics; solar coronal waves
Monte Carlo Applications for Partially Polarized Inverse External-Compton Scattering (MAPPIES). I. Description of the Code and First Results

The radiation mechanisms responsible for the multiwavelength emission from relativistic jet sources are poorly understood. The modeling of the spectral energy distributions and light curves alone is not adequate to distinguish between existing models. Polarization in the X-ray and γ-ray regime of these sources may provide new and unique information about the jet physics and radiation mechanisms. Several upcoming projects will be able to deliver polarimetric measurements of the brightest X-ray sources, including active galactic nuclei jets and γ-ray bursts. This article describes the development of a new Monte Carlo code—MAPPIES (Monte Carlo Applications for Partially Polarized Inverse External-Compton Scattering)—for polarization-dependent Compton scattering in relativistic jet sources. Generic results for Compton polarization in the Thomson and Klein-Nishina regimes are presented.

Keywords: polarimetry; polarimeters; blazars; active galactic nuclei; active galaxies; relativistic jets; gamma-rays; x-ray active galactic nuclei; non-thermal radiation sources
The Extreme Scarcity of Dust-enshrouded Red Supergiants: Consequences for Producing Stripped Stars via Winds

Quiescent mass loss during the red supergiant (RSG) phase has been shown to be far lower than prescriptions typically employed in single-star evolutionary models. Importantly, RSG winds are too weak to drive the production of Wolf-Rayet (WR) stars and stripped-envelope supernovae (SE-SNe) at initial masses of roughly 20-40 M ⊙. If single stars are to make WR stars and SE-SNe, this shifts the burden of mass loss to rare dust-enshrouded RSGs (DE-RSGs), objects claimed to represent a short-lived, high-mass-loss phase. Here, we take a fresh look at the purported DE-RSGs. By modeling the mid-IR excesses of the full sample of RSGs in the Large Magellanic Cloud, we find that only one RSG has both a high mass-loss rate ( $\dot{M}$ ≥ 10-4 M ⊙ yr-1) and a high optical circumstellar dust extinction (7.92 mag). This RSG is WOH G64, and it is the only one of the 14 originally proposed DE-RSGs that is actually dust enshrouded. The rest appear to be either normal RSGs without strong IR excess, or lower-mass asymptotic giant branch stars. Only one additional object in the full catalog of RSGs (not previously identified as a DE-RSG) shows strong mid-IR excess. We conclude that if DE-RSGs do represent a pre-SN phase of enhanced $\dot{M}$ in single stars, it is extremely short-lived, only capable of removing ≤2 M ⊙ of material. This rules out the single-star post-RSG pathway for the production of WR stars, luminous blue variables, and SE-SNe. Single-star models should not employ $\dot{M}$ -prescriptions based on these extreme objects for any significant fraction of the RSG phase.

Keywords: massive stars; stellar evolution; core-collapse supernovae; stellar mass loss
The Field Substellar Mass Function Based on the Full-sky 20 pc Census of 525 L, T, and Y Dwarfs

We present final Spitzer trigonometric parallaxes for 361 L, T, and Y dwarfs. We combine these with prior studies to build a list of 525 known L, T, and Y dwarfs within 20 pc of the Sun, 38 of which are presented here for the first time. Using published photometry and spectroscopy as well as our own follow-up, we present an array of color-magnitude and color-color diagrams to further characterize census members, and we provide polynomial fits to the bulk trends. Using these characterizations, we assign each object a Teff value and judge sample completeness over bins of Teff and spectral type. Except for types ≥T8 and Teff < 600 K, our census is statistically complete to the 20 pc limit. We compare our measured space densities to simulated density distributions and find that the best fit is a power law ( ${dN}/{dM}\propto {M}^{-\alpha }$ <!-- --> ) with α = 0.6 ± 0.1. We find that the evolutionary models of Saumon & Marley correctly predict the observed magnitude of the space density spike seen at 1200 K < Teff < 1350 K, believed to be caused by an increase in the cooling timescale across the L/T transition. Defining the low-mass terminus using this sample requires a more statistically robust and complete sample of dwarfs ≥Y0.5 and with Teff < 400 K. We conclude that such frigid objects must exist in substantial numbers, despite the fact that few have so far been identified, and we discuss possible reasons why they have largely eluded detection.

Keywords: stellar mass functions; trigonometric parallax; brown dwarfs; solar neighborhood; absolute magnitude; stellar effective temperatures; l dwarfs; t dwarfs; y dwarfs; stellar classification; binary stars; two-color diagrams