adsabs/SciX_UAT_keywords huggingface dataset:
Connecting Low- and High-redshift Weak Emission-line Quasars via Hubble Space Telescope Spectroscopy of Lyα Emission

We present ultraviolet spectroscopy covering the Lyα + N V complex of six candidate low-redshift (0.9 < z < 1.5) weak emission-line quasars (WLQs) based on observations with the Hubble Space Telescope. The original systematic searches for these puzzling Type 1 quasars with intrinsically weak broad emission lines revealed an N ≈ 100 WLQ population from optical spectroscopy of high-redshift (z > 3) quasars, defined by a Lyα + N V rest-frame equivalent width (EW) threshold <15.4 Å. Identification of lower-redshift (z < 3) WLQ candidates, however, has relied primarily on optical spectroscopy of weak broad emission lines at longer rest-frame wavelengths. With these new observations expanding existing optical coverage into the ultraviolet, we explore unifying the low- and high-z WLQ populations via EW[Lyα+N V]. Two objects in the sample unify with high-z WLQs, three others appear consistent with the intermediate portion of the population connecting WLQs and normal quasars, and the final object is consistent with typical quasars. The expanded wavelength coverage improves the number of available line diagnostics for our individual targets, allowing a better understanding of the shapes of their ionizing continua. The ratio of EW[Lyα+N V] to EW[Mg II] in our sample is generally small but varied, favoring a soft ionizing continuum scenario for WLQs, and we find a lack of correlation between EW[Lyα+N V] and the X-ray properties of our targets, consistent with a "slim-disk" shielding gas model. We also find indications that weak absorption may be a more significant contaminant in low-z WLQ populations than previously thought.

Keywords: quasars; radio quiet quasars; supermassive black holes; active galactic nuclei; high energy astrophysics
Probing the Relationship Between Early Star Formation and CO in the Dwarf Irregular Galaxy WLM with JWST

Wolf–Lundmark–Melotte (WLM) is a Local Group dwarf irregular (dIrr) galaxy with a metallicity 13% of solar. At 1 Mpc, the relative isolation of WLM provides a unique opportunity to investigate the internal mechanisms of star formation at low metallicities. The earliest stages of star formation in larger spirals occur in embedded clusters within molecular clouds, but dIrrs lack the dust, heavy metals, and organized structure of spirals believed necessary to collapse the molecular clouds into stars. Despite actively forming stars, the early stages of star formation in dIrrs is not well understood. We examine the relationship between early star formation and molecular clouds at low metallicities. We utilize ALMA-detected CO cores, JWST near-infrared (NIR) images (F090W, F150W, F250M, and F430M), and GALEX far-ultraviolet (FUV) images of WLM to trace molecular clouds, early star formation, and longer star formation timescales respectively. We compare clumps of NIR-bright sources (referred to as objects) categorized into three types based on their proximity to FUV sources and CO cores. We find objects, independent of their location, have similar colors and magnitudes and no discernible difference in temperature. However, we find that objects near CO have higher masses than objects away from CO, independent of proximity to FUV. Additionally, objects near CO are coincident with Spitzer 8 μm sources at a higher frequency than objects elsewhere in WLM. This suggests objects near CO may be embedded star clusters at an earlier stage of star formation, but accurate age estimates for all objects are required for confirmation.

Keywords: local group; dwarf irregular galaxies; star formation; star forming regions; molecular clouds
Exact Calculation of Nonideal Fields Demonstrates Their Dominance of Injection in Relativistic Reconnection

Magnetic reconnection is an important source of energetic particles in systems ranging from astrophysics to the laboratory. The large separation of spatiotemporal scales involved makes it critical to determine the minimum physical model containing the necessary physics for modeling particle acceleration. By resolving the energy gain from ideal and nonideal magnetohydrodynamic electric fields self-consistently in kinetic particle-in-cell simulations of reconnection, we conclusively show the dominant role of the nonideal field for the early stage of energization known as injection. The importance of the nonideal field increases with magnetization, guide field, and in three dimensions, indicating its general importance for reconnection in natural astrophysical systems. We obtain the statistical properties of the injection process from the simulations, paving the way for the development of extended MHD models capable of accurately modeling particle acceleration in large-scale systems. The novel analysis method developed in this study can be applied broadly to give new insight into a wide range of processes in plasma physics.

Keywords: plasma astrophysics
Photons from Neutrinos: The Gamma-Ray Echo of a Supernova Neutrino Burst

When a star undergoes core collapse, a vast amount of energy is released in a ∼10 s long burst of neutrinos of all species. Inverse beta decay in the star's hydrogen envelope causes an electromagnetic cascade that ultimately results in a flare of gamma rays—an "echo" of the neutrino burst—at the characteristic energy of 0.511 MeV. We study the phenomenology and detectability of this flare. Its luminosity curve is characterized by a fast, seconds-long rise and an equally fast decline, with a minute- or hour-long plateau in between. For a near-Earth star (distance D ≲ 1 kpc) the echo will be observable at near future gamma-ray telescopes with an effective area of 103 cm2 or larger. Its observation will inform us on the envelope size and composition. In conjunction with the direct detection of the neutrino burst, it will also give information on the neutrino emission away from the line of sight and will enable tests of neutrino propagation effects between the stellar surface and Earth.

Keywords: supernova neutrinos; gamma-rays; core-collapse supernovae
Laboratory Optical Spectroscopy of Vibronic Transitions of the Thiophenoxy Radical

The thiophenoxy radical (C6H5S) is a species of possible astrophysical interest due to an electronic transition in a 5000 Å region. The ${B}^{2}{A}_{2}\leftarrow {X}^{2}{B}_{1}$ electronic transition of this radical in the discharge of thiophenol was measured using a cavity ring-down spectrometer. The optical absorption spectrum of this transition was obtained in the range covering from the origin band (0-0) to a frequency of 1750 cm-1. The vibronic bands in the 400-1700 cm-1 region are stronger than the origin band, suggesting structural difference between the ground and excited electronic states. The prominent progression was assigned to the 6a symmetric in-plane C-C-C bending mode starting from the $6{{\rm{b}}}_{0}^{1}$ forbidden band. Band origins of individual bands were determined by analysis of the rotational profiles. Although these vibronic bands were not found in optical spectra of diffuse clouds, the upper limits of the column densities for the thiophenoxy radical in the diffuse clouds toward HD 183143 and HD 204827 were evaluated to be ∼4 × 1013 cm-2.

Keywords: diffuse interstellar bands
Visibility Interpolation in Solar Hard X-Ray Imaging: Application to RHESSI and STIX

Space telescopes for solar hard X-ray imaging provide observations made of sampled Fourier components of the incoming photon flux. The aim of this study is to design an image reconstruction method relying on enhanced visibility interpolation in the Fourier domain. The interpolation-based method is applied to synthetic visibilities generated by means of the simulation software implemented within the framework of the Spectrometer/Telescope for Imaging X-rays (STIX) mission on board Solar Orbiter. An application to experimental visibilities observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is also considered. In order to interpolate these visibility data, we have utilized an approach based on Variably Scaled Kernels (VSKs), which are able to realize feature augmentation by exploiting prior information on the flaring source and which are used here, for the first time, in the context of inverse problems. When compared to an interpolation-based reconstruction algorithm previously introduced for RHESSI, VSKs offer significantly better performance, particularly in the case of STIX imaging, which is characterized by a notably sparse sampling of the Fourier domain. In the case of RHESSI data, this novel approach is particularly reliable when the flaring sources are either characterized by narrow, ribbon-like shapes or high-resolution detectors are utilized for observations. The use of VSKs for interpolating hard X-ray visibilities allows remarkable image reconstruction accuracy when the information on the flaring source is encoded by a small set of scattered Fourier data and when the visibility surface is affected by significant oscillations in the frequency domain.

Keywords: solar x-ray flares; x-ray telescopes; astronomy data analysis
The Exoplanet Radius Valley from Gas-driven Planet Migration and Breaking of Resonant Chains

The size frequency distribution of exoplanet radii between 1 and 4R ⊕ is bimodal with peaks at ~1.4 R ⊕ and ~2.4 R ⊕, and a valley at ~1.8 R ⊕. This radius valley separates two classes of planets-usually referred to as "super-Earths" and "mini-Neptunes"-and its origin remains debated. One model proposes that super-Earths are the outcome of photoevaporation or core-powered mass loss stripping the primordial atmospheres of the mini-Neptunes. A contrasting model interprets the radius valley as a dichotomy in the bulk compositions, where super-Earths are rocky planets and mini-Neptunes are water-ice-rich worlds. In this work, we test whether the migration model is consistent with the radius valley and how it distinguishes these views. In the migration model, planets migrate toward the disk's inner edge, forming a chain of planets locked in resonant configurations. After the gas disk dispersal, orbital instabilities "break the chains" and promote late collisions. This model broadly matches the period-ratio and planet-multiplicity distributions of Kepler planets and accounts for resonant chains such as TRAPPIST-1, Kepler-223, and TOI-178. Here, by combining the outcome of planet formation simulations with compositional mass-radius relationships and assuming the complete loss of primordial H-rich atmospheres in late giant impacts, we show that the migration model accounts for the exoplanet radius valley and the intrasystem uniformity ("peas in a pod") of Kepler planets. Our results suggest that planets with sizes of ~1.4 R ⊕ are mostly rocky, whereas those with sizes of ~2.4 R ⊕ are mostly water-ice-rich worlds. Our results do not support an exclusively rocky composition for the cores of mini-Neptunes.

Keywords: planet formation; exoplanets
Statistical Signatures of Nanoflare Activity. III. Evidence of Enhanced Nanoflaring Rates in Fully Convective stars as Observed by the NGTS

Previous examinations of fully convective M-dwarf stars have highlighted enhanced rates of nanoflare activity on these distant stellar sources. However, the specific role the convective boundary, which is believed to be present for spectral types earlier than M2.5V, plays on the observed nanoflare rates is not yet known. Here, we utilize a combination of statistical and Fourier techniques to examine M-dwarf stellar lightcurves that lie on either side of the convective boundary. We find that fully convective M2.5V (and later subtypes) stars have greatly enhanced nanoflare rates compared with their pre-dynamo mode-transition counterparts. Specifically, we derive a flaring power-law index in the region of 3.00 ± 0.20, alongside a decay timescale of 200 ± 100 s for M2.5V and M3V stars, matching those seen in prior observations of similar stellar subtypes. Interestingly, M4V stars exhibit longer decay timescales of 450 ± 50 s, along with an increased power-law index of 3.10 ± 0.18, suggesting an interplay between the rate of nanoflare occurrence and the intrinsic plasma parameters, e.g., the underlying Lundquist number. In contrast, partially convective (i.e., earlier subtypes from M0V to M2V) M-dwarf stars exhibit very weak nanoflare activity, which is not easily identifiable using statistical or Fourier techniques. This suggests that fully convective stellar atmospheres favor small-scale magnetic reconnection, leading to implications for the flare-energy budgets of these stars. Understanding why small-scale reconnection is enhanced in fully convective atmospheres may help solve questions relating to the dynamo behavior of these stellar sources.

Keywords: computational methods; optical flares; stellar flares; flare stars
Laboratory Observations of Ultra-low-frequency Analog Waves Driven by the Right-hand Resonant Ion Beam Instability

The right-hand resonant instability (RHI) is one of several electromagnetic ion/ion beam instabilities responsible for the formation of parallel magnetized collisionless shocks and the generation of ultra-low frequency (ULF) waves in their foreshocks. This instability has been observed for the first time under foreshock-relevant conditions in the laboratory through the repeatable interaction of a pre-formed magnetized background plasma and a super-Alfvénic laser-produced plasma. This platform has enabled unprecedented volumetric measurements of waves generated by the RHI, revealing filamentary current structures in the transverse plane. These measurements are made in the plasma rest frame with both high spatial and temporal resolution, providing a perspective that is complementary to spacecraft observations. Direct comparison of data from both the experiment and the Wind spacecraft to 2D hybrid simulations demonstrates that the waves produced are analogous to the ULF waves observed upstream of the terrestrial bow shock.

Keywords: laboratory astrophysics; planetary bow shocks; space plasmas; plasma astrophysics; shocks
The Influence of Hydrogen Sulfide on the Optical Properties of Planetary Organic Hazes: Implications for Exoplanet Climate Modeling

Planetary organic hazes can play a significant role in influencing a planet's radiative balance and climate, with their impact determined by the optical properties of the haze. The optical properties, in turn, are partly influenced by particle composition. Our previous work, Reed et al., demonstrated that trace amounts of hydrogen sulfide (H2S) in haze chemistry can substantially affect the haze's composition by generating organosulfur compounds and increasing the amount of organic haze produced. However, no study has measured the optical properties of an H2S-influenced organic haze. Here we present results from laboratory experiments measuring the real (scattering, n) and imaginary (absorbing, k) refractive indices of haze analogs produced from photochemistry of gas mixtures composed of 0.1% CH4 and variable H2S (0-10 ppmv) in N2. The optical properties of the aerosol produced were measured in real-time using coupled photoacoustic and cavity ring-down spectroscopy with 405 and 532 nm wavelengths of light. Our findings show that the total extinction of light (scattering plus absorption) by the aerosol increases as a function of H2S mixing ratio. We provide our best-fit equations for predicting n and k at 405 and 532 nm as a function of the sulfur to carbon molar ratio (S:C) of the precursor gas mixture. Further, we demonstrate how these changes in optical properties could alter the transmittance of 405 and 532 nm light through a haze layer. These results have potential implications for modeling the climate, habitability, and spectra for exoplanets exhibiting organic haze.

Keywords: exoplanet atmospheres; optical constants (dust); exoplanet atmospheric composition; planetary climates; planetary atmospheres
MOSAIC: A Satellite Constellation to Enable Groundbreaking Mars Climate System Science and Prepare for Human Exploration

The Martian climate system has been revealed to rival the complexity of Earth's. Over the last 20 yr, a fragmented and incomplete picture has emerged of its structure and variability; we remain largely ignorant of many of the physical processes driving matter and energy flow between and within Mars' diverse climate domains. Mars Orbiters for Surface, Atmosphere, and Ionosphere Connections (MOSAIC) is a constellation of ten platforms focused on understanding these climate connections, with orbits and instruments tailored to observe the Martian climate system from three complementary perspectives. First, low-circular near-polar Sun-synchronous orbits (a large mothership and three smallsats spaced in local time) enable vertical profiling of wind, aerosols, water, and temperature, as well as mapping of surface and subsurface ice. Second, elliptical orbits sampling all of Mars' plasma regions enable multipoint measurements necessary to understand mass/energy transport and ion-driven escape, also enabling, with the polar orbiters, dense radio occultation coverage. Last, longitudinally spaced areostationary orbits enable synoptic views of the lower atmosphere necessary to understand global and mesoscale dynamics, global views of the hydrogen and oxygen exospheres, and upstream measurements of space weather conditions. MOSAIC will characterize climate system variability diurnally and seasonally, on meso-, regional, and global scales, targeting the shallow subsurface all the way out to the solar wind, making many first-of-their-kind measurements. Importantly, these measurements will also prepare for human exploration and habitation of Mars by providing water resource prospecting, operational forecasting of dust and radiation hazards, and ionospheric communication/positioning disruptions.

Keywords: mars; planetary atmospheres; surface ices; planetary ionospheres; planetary magnetospheres; thermosphere; upper atmosphere; space weather; planetary climates; space vehicle instruments; space plasmas; mesosphere
Interstellar Objects from Broken Dyson Spheres

Without extensive maintenance, Dyson spheres inevitably disintegrate by asteroid impacts over billions of years. The resulting fragments would appear as anomalous interstellar objects, potentially sharing the unusual shape and motion of 1I/'Oumuamua or the unusual material strength of the first two interstellar meteors, IM1 and IM2. If the Dyson sphere's tiles are light sails, the number of fragments could exceed that of interstellar asteroids because of their reduced escape speed from the star and the increase in stellar luminosity during the red giant phase.

Keywords: asteroids; asteroid dynamics
Atmospheric Overturning Circulation on Dry, Tidally Locked Rocky Planets Is Mainly Driven by Radiative Cooling

In this study, we examine the driving mechanism for the atmospheric overturning circulation on dry, tidally locked rocky planets without the condensation of water vapor or other species. We find that the main driving process is the radiative cooling of CO2 (or other noncondensable greenhouse gases) rather than CO2 greenhouse warming or stellar radiation. Stellar radiation is the ultimate mechanism but not the direct mechanism. Due to the combination of the uneven distribution in the stellar radiation and effective horizontal energy transports in the free troposphere, there is strong temperature inversion in the area away from the substellar region. This inversion makes CO2 have a radiative cooling effect rather than a radiative warming effect for the atmosphere, the same as that in the stratosphere of Earth's atmosphere. This cooling effect produces negative buoyancy and drives large-scale downwelling, supporting the formation of a global-scale overturning circulation. If CO2 is excluded from the atmosphere, the overturning circulation becomes very weak, regardless of the level of stellar radiation. This mechanism is completely different from that for the atmospheric overturning circulation on Earth or on moist, tidally locked rocky planets, where latent heat release and/or baroclinic instability are the dominated mechanisms. Our study improves the understanding of the atmospheric circulation on tidally locked exoplanets and also on other dry planets, such as Venus and Mars in the solar system.

Keywords: planetary atmospheres; planetary science; atmospheric circulation; exoplanet atmospheres
Possible Disintegration of Main-belt Asteroid 2008 BJ22

Active asteroids are a mysterious population of objects on asteroid-like orbits that display comet-like activity produced by a diverse variety of mechanisms, providing glimpses of the ongoing evolution of the asteroid belt and near-Earth objects. We report the identification of a new active asteroid 2008 BJ22, from images obtained by the Subaru Telescope in 2008. The object was not detected in earlier Canada-France-Hawaii Telescope images obtained in 2005 down to a 3σ limiting magnitude of m r ~ 24, corresponding to a V-band absolute magnitude of H V > 18.4, more than 3 mag fainter than the absolute magnitude computed from 2008 data. Analysis of later images obtained by the Dark Energy Camera on the 4.0 m Victor M. Blanco Telescope in 2013 set the object's current V-band absolute magnitude at H V > 19.3. These results suggest that 2008 BJ22 was discovered in an active state in 2008 January.

Keywords: asteroid belt; main-belt comets; comets
Isotropic Scaling Features Measured Locally in the Solar Wind Turbulence with Stationary Background Field

The scaling anisotropy is crucial to interpret the nonlinear interactions in solar wind turbulence. Previous observations provide diverse results and the structure function analyses are also reported to be an approach to investigate the scaling anisotropy based on a local magnetic field. However, the determination of the sampling angle with respect to the local background magnetic field requires that the observed time series for the average are time stationary. Whether or not this required time stationarity is compatible with the measurements has not been investigated. Here we utilize the second-order structure function method to study the scaling anisotropy with a time-stationary background field. We analyze 88 fast solar wind intervals each with time durations ≥ 2 days measured by WIND spacecraft in the period 2005-2018. We calculate the local magnetic field as the average of the time series B(t^{'} ) whose time stationarity is fulfilled by our criterion φ < 10° (φ is the angle between the two averaged magnetic fields after cutting B(t^{'} ) into two halves). We find for the first time the isotropic scaling feature of the magnetic-trace structure functions with scaling indices -0.63± 0.08 and 0.70 ± 0.04, respectively, with the local magnetic field parallel and perpendicular to the solar wind velocity directions. The scaling for the velocity-trace structure functions is also isotropic and the indices are -0.47 ± 0.10 and 0.51 ± 0.09. We also find that with increasing φ threshold to 90°, the scaling index of the magnetic-trace structure function in the parallel direction decreases to -0.81, while the rms of the instantaneous angle between magnetic field and solar wind velocity increases up to 45° at the timescale 150 s, indicating a mix of perpendicular measurements into parallel ones at large scales.

Keywords: solar wind; interplanetary shocks; magnetic fields; space plasmas