adsabs/SciX_UAT_keywords huggingface dataset:
Particle-in-Cell Simulations of Sunward and Anti-sunward Whistler Waves in the Solar Wind

We present particle-in-cell simulations of a combined whistler heat flux and temperature anisotropy instability that is potentially operating in the solar wind. The simulations are performed in a uniform plasma and initialized with core and halo electron populations typical of the solar wind beyond about 0.3 au. We demonstrate that the instability produces whistler-mode waves propagating both along (anti-sunward) and opposite (sunward) to the electron heat flux. The saturated amplitudes of both sunward and anti-sunward whistler waves are strongly correlated with their initial linear growth rates, ${B}_{w}/{B}_{0}\sim {(\gamma /{\omega }_{{ce}})}^{\nu }$ , where for typical electron betas we have 0.6 ≲ ν ≲ 0.9. We show that because of the relatively large spectral width of the whistler waves, the instability saturates through the formation of quasi-linear plateaus around the resonant velocities. The revealed correlations of whistler wave amplitudes and spectral widths with electron beta and temperature anisotropy are consistent with solar wind observations. We show that anti-sunward whistler waves result in an electron heat flux decrease, while sunward whistler waves actually lead to an electron heat flux increase. The net effect is the electron heat flux suppression, whose efficiency is larger for larger electron betas and temperature anisotropies. The electron heat flux suppression can be up to 10%-60% provided that the saturated whistler wave amplitudes exceed about 1% of the background magnetic field. The experimental applications of the presented results are discussed.

Keywords: space plasmas; solar wind
A JWST NIRSpec Phase Curve for WASP-121b: Dayside Emission Strongest Eastward of the Substellar Point and Nightside Conditions Conducive to Cloud Formation

We present the first exoplanet phase-curve measurement made with the JWST NIRSpec instrument, highlighting the exceptional stability of this newly commissioned observatory for exoplanet climate studies. The target, WASP-121b, is an ultrahot Jupiter with an orbital period of 30.6 hr. We analyze two broadband light curves generated for the NRS1 and NRS2 detectors, covering wavelength ranges of 2.70-3.72 μm and 3.82-5.15 μm, respectively. Both light curves exhibit minimal systematics, with approximately linear drifts in the baseline flux level of 30 ppm hr-1 (NRS1) and 10 ppm hr-1 (NRS2). Assuming a simple brightness map for the planet described by a low-order spherical harmonic dipole, our light-curve fits suggest that the phase curve peaks coincide with orbital phases 3.°36 ± 0.°11 (NRS1) and 2.°66 ± 0.°12 (NRS2) prior to mideclipse. This is consistent with the strongest dayside emission emanating from eastward of the substellar point. We measure planet-to-star emission ratios of 3924 ± 7 ppm (NRS1) and 4924 ± 9 ppm (NRS2) for the dayside hemisphere and 136 ± 8 ppm (NRS1) and 630 ± 10 ppm (NRS2) for the nightside hemisphere. The latter nightside emission ratios translate to planetary brightness temperatures of 926 ± 12 K (NRS1) and 1122 ± 10 K (NRS2), which are low enough for a wide range of refractory condensates to form, including enstatite and forsterite. A nightside cloud deck may be blocking emission from deeper, hotter layers of the atmosphere, potentially helping to explain why cloud-free 3D general circulation model simulations systematically overpredict the nightside emission for WASP-121b.

Keywords: exoplanet astronomy; exoplanet atmospheres
Using AI for Wave-front Estimation with the Rubin Observatory Active Optics System

The Vera C. Rubin Observatory will, over a period of 10 yr, repeatedly survey the southern sky. To ensure that images generated by Rubin meet the quality requirements for precision science, the observatory will use an active-optics system (AOS) to correct for alignment and mirror surface perturbations introduced by gravity and temperature gradients in the optical system. To accomplish this, Rubin will use out-of-focus images from sensors located at the edge of the focal plane to learn and correct for perturbations to the wave front. We have designed and integrated a deep-learning (DL) model for wave-front estimation into the AOS pipeline. In this paper, we compare the performance of this DL approach to Rubin's baseline algorithm when applied to images from two different simulations of the Rubin optical system. We show the DL approach is faster and more accurate, achieving the atmospheric error floor both for high-quality images and low-quality images with heavy blending and vignetting. Compared to the baseline algorithm, the DL model is 40× faster, the median error 2× better under ideal conditions, 5× better in the presence of vignetting by the Rubin camera, and 14× better in the presence of blending in crowded fields. In addition, the DL model surpasses the required optical quality in simulations of the AOS closed loop. This system promises to increase the survey area useful for precision science by up to 8%. We discuss how this system might be deployed when commissioning and operating Rubin.

Keywords: neural networks; multiple mirror telescopes; optical telescopes; astronomical optics; astronomical instrumentation
Revised and Extended Analysis of Ar VI

This paper analyzes the emission spectrum of Ar VI and adds new information about this ion. We use a capillary light source to study the Ar VI spectrum, presenting 39 line classifications for the first time, including one line in the ultraviolet region. A total of nine possible Ar VI lines in the ultraviolet between 3288 and 3440 Å are analyzed. We revised the wavelengths of the lines at 1284.01 and 1307.42 Å previously used to identify Ar VI in astronomical objects. The lifetimes, weighted transition rates (gA) and their estimated uncertainties, and cancellation factors are obtained using a Hartree-Fock calculation with energy parameters adjusted to fit the values of the experimental levels and modified to include core polarization effects. We analyze all the experimentally known transitions of the Ar VI spectrum, with particular attention to conflicting information. Isoelectronic sequences of energy levels and transitions, for which the trend is not a smooth curve, are analyzed. We revise previous studies where the information needed to support line classifications and level designations is incomplete.

Keywords: atomic spectroscopy; plasma astrophysics; line intensities; transition probabilities; hot ionized medium
New Candidate Extreme T Subdwarfs from the Backyard Worlds: Planet 9 Citizen Science Project

Schneider et al. presented the discovery of WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5, which appear to be the first examples of extreme T-type subdwarfs (esdTs; metallicity ≤-1 dex, Teff ≲ 1400 K). Here, we present new discoveries and follow-up of three T-type subdwarf candidates, with an eye toward expanding the sample of such objects with very low metallicity and extraordinarily high kinematics, properties that suggest membership in the Galactic halo. Keck/NIRES near-infrared spectroscopy of WISEA J155349.96+693355.2, a fast-moving object discovered by the Backyard Worlds: Planet 9 citizen science project, confirms that it is a mid-T subdwarf. With HW2 = 22.3 mag, WISEA J155349.96+693355.2 has the largest W2 reduced proper motion among all spectroscopically confirmed L and T subdwarfs, suggesting that it may be kinematically extreme. Nevertheless, our modeling of the WISEA J155349.96+693355.2 near-infrared spectrum indicates that its metallicity is only mildly subsolar. In analyzing the J155349.96+693355.2 spectrum, we present a new grid of low-temperature, low-metallicity model atmosphere spectra. We also present the discoveries of two new esdT candidates, CWISE J073844.52-664334.6 and CWISE J221706.28-145437.6, based on their large motions and colors similar to those of the two known esdT objects. Finding more esdT examples is a critical step toward mapping out the spectral sequence and observational properties of this newly identified population.

Keywords: brown dwarfs; t dwarfs; t subdwarfs
Deep Potential: Recovering the Gravitational Potential from a Snapshot of Phase Space

One of the major goals of the field of Milky Way dynamics is to recover the gravitational potential field. Mapping the potential would allow us to determine the spatial distribution of matter-both baryonic and dark-throughout the galaxy. We present a novel method for determining the gravitational field from a snapshot of the phase-space positions of stars, based only on minimal physical assumptions, which makes use of recently developed tools from the field of deep learning. We first train a normalizing flow on a sample of observed six-dimensional phase-space coordinates of stars, obtaining a smooth, differentiable approximation of the distribution function. Using the collisionless Boltzmann equation, we then find the gravitational potential-represented by a feed-forward neural network-that renders this distribution function stationary. This method, which we term "Deep Potential," is more flexible than previous parametric methods, which fit restricted classes of analytic models of the distribution function and potential to the data. We demonstrate Deep Potential on mock data sets and demonstrate its robustness under various nonideal conditions. Deep Potential is a promising approach to mapping the density of the Milky Way and other stellar systems, using rich data sets of stellar positions and kinematics now being provided by Gaia and ground-based spectroscopic surveys.

Keywords: milky way dynamics; stellar dynamics; neural networks; gravitational fields; astrostatistics
MG1-688432: A Peculiar Variable System

The short-period variable star MG1-688432 has been discovered to exhibit occasional extremely high energy optical outbursts as high as 1038 erg. Outbursts are typically several hours in duration. These events are often highly structured, resembling sequential associated releases of energy. Twenty years of time sequence photometry are presented, indicating a basic sinusoidal light curve of mean period 6.65 days, with some phase shifting and long-term temporal trends in amplitude and mean brightness. Spectroscopy reveals a peculiar star, best resembling a K3 subgiant that has evolved off the main sequence moderately redward of the giant branch. Spectroscopic and radial velocity analyses indicate a binary system orbiting its barycenter with an unseen companion to the K3 IV primary. This is not an eclipsing system, with the inclination of the orbit precluding eclipse by the secondary. The system is at a distance of 1.5 kpc, and analysis of Gaia observations leads to the conclusion that the H-R diagram position of MG1-688432 is established by an intrinsic feature of the system, most likely either the stellar evolutionary state of the observed star or the presence of small (nongray) dust within the system. Two mechanisms (or combinations thereof) that might give rise to characteristics of the system are (1) magnetically induced chromospheric activity and (2) impacts with tidally disrupted planetary debris.

Keywords: eruptive variable stars; stellar chromospheres
New 26P(p, γ)27S Thermonuclear Reaction Rate and Its Astrophysical Implications in the rp-process

Accurate nuclear reaction rates for 26P(p, γ)27S are pivotal for a comprehensive understanding of the rp-process nucleosynthesis path in the region of proton-rich sulfur and phosphorus isotopes. However, large uncertainties still exist in the current rate of 26P(p, γ)27S because of the lack of nuclear mass and energy level structure information for 27S. We reevaluate this reaction rate using the experimentally constrained 27S mass, together with the shell model predicted level structure. It is found that the 26P(p, γ)27S reaction rate is dominated by a direct capture reaction mechanism despite the presence of three resonances at E = 1.104, 1.597, and 1.777 MeV above the proton threshold in 27S. The new rate is overall smaller than the other previous rates from the Hauser-Feshbach statistical model by at least 1 order of magnitude in the temperature range of X-ray burst interest. In addition, we consistently update the photodisintegration rate using the new 27S mass. The influence of new rates of forward and reverse reaction in the abundances of isotopes produced in the rp-process is explored by postprocessing nucleosynthesis calculations. The final abundance ratio of 27S/26P obtained using the new rates is only 10% of that from the old rate. The abundance flow calculations show that the reaction path 26P(p, γ)27S(β +,ν)27P is not as important as previously thought for producing 27P. The adoption of the new reaction rates for 26P(p, γ)27S only reduces the final production of aluminum by 7.1% and has no discernible impact on the yield of other elements.

Keywords: reaction rates; x-ray bursts; nuclear physics; nuclear abundances; explosive nucleosynthesis
Dust Formation in the Nova V1533 Scorpii

We present infrared spectroscopy from two epochs of V1533 Scorpii (=Nova Scorpii 2013); one from immediately before dust formation and a second, from one month later when dust formation was complete. The pre-dust spectrum shows strong lines of C I, features that weakened considerably but were still present after dust formation. Also present in the post-formation spectrum was O I emission from the neutral region, indicating that dust formation did not full deplete either carbon or oxygen. While thermal emission from the dust, of a temperature very close to 870 K, dominated the later spectrum, the extinction did not change measurably between epochs, indicating that little or no dust blocked the line-of-sight. Although carbon monoxide was not detected in the first epoch spectrum, weak emission from molecular hydrogen may have been present.

Keywords: novae; fast novae
Density Turbulence and the Angular Broadening of Outer Heliospheric Radio Sources at High Latitudes and in the Ecliptic Plane

Density irregularities are responsible for the scattering of radio waves in the solar wind and astrophysical plasmas. These irregularities significantly affect the inferred physical properties of radio sources, such as size, direction, and intensity. We present here a theory of angular broadening due to the scattering of radio waves by density irregularities that improves the existing formalism used to investigate radio wave scattering in the outer heliosphere and the very local interstellar medium. The model includes an inner scale and both latitudinal and radial dependencies for the density fluctuation spectra and propagation paths for the radiation both near and out of the ecliptic plane. Based on the pickup-ion-mediated solar wind model (PUI model) of Zank et al., we estimate the turbulence and solar wind quantities for the high-latitude fast solar wind. The predictions include the density variance, inner/dissipation scale, velocity correlation length, mean magnetic field, and proton temperature. The density turbulence amplitude is estimated in two ways. First, a simple scaling technique is used to extend the theoretical predictions of the PUI model for the high-latitude wind beyond the heliospheric termination shock. Second, the solar wind and turbulence quantities are calculated near the ecliptic plane using plasma and magnetometer data from the Voyager 2 spacecraft over the period 1977-2018. Based on the turbulence models and observations, we calculate the scattering angle of the radio sources in the high-latitude and near-ecliptic wind. Finally, we compare the numerical results with the analytic predictions from Cairns and Armstrong et al. and the observed source sizes.

Keywords: solar radio emission; solar wind; heliosphere; interplanetary turbulence; space plasmas
Mass and Color Dependence of the Hubble Spiral Sequence

In the classic Hubble spiral sequence, arm windiness correlates with bulge size; Sa type spiral galaxies with larger bulges also have the most tightly wound spirals. Exceptions to this have long been known, and in recent work using Galaxy Zoo morphologies no strong correlation was seen in a volume limited sample. In this Research Note, we explore the impact of galaxy mass and integrated color upon this correlation in the Galaxy Zoo sample, finding that bluer and lower mass spirals show the "expected" correlation; however, it becomes slightly negative for redder and/or more massive spiral galaxies.

Keywords: spiral arms; spiral galaxies; spiral pitch angle; hubble classification scheme; galaxy classification systems
Making a Supermassive Star by Stellar Bombardment

Approximately 200 supermassive black holes (SMBHs) have been discovered within the first ∼gigayear after the Big Bang. One pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). A possible obstacle to this scenario is that the collapsing gas fragments and forms a cluster of main-sequence stars. Here, we raise the possibility that stellar collisions may be sufficiently frequent and energetic to inhibit the contraction of the massive protostar, avoiding strong UV radiation driven outflows, and allowing it to continue growing into an SMS. We investigate this scenario with semianalytic models incorporating star formation; gas accretion; dynamical friction from stars and gas; stellar collisions; and gas ejection. We find that when the collapsing gas fragments at a density of ≲3 × 1010 cm-3, the central protostar contracts due to infrequent stellar mergers, and in turn photoevaporates the remaining collapsing gas, resulting in the formation of a ≲104 M⊙ object. On the other hand, when the collapsing gas fragments at higher densities (expected for a metal-poor cloud with Z ≲ 10-5 Z⊙ with suppressed H2 abundance) the central protostar avoids contraction and keeps growing via frequent stellar mergers, reaching masses as high as ∼105-106 M⊙. We conclude that frequent stellar mergers represent a possible pathway to form massive BHs in the early universe.

Keywords: population ii stars; pre-main sequence; supermassive black holes; quasars; early universe; primordial galaxies
Transit Analysis of HATS-4 b by Citizen Scientists

Exoplanets are planets that orbit stars other than our Sun, and the most common way of detecting them is by the transit method. This method consists of plotting the relative brightness of the exoplanets host star against a comparison star to make what is known as a light curve. Periodic dips in the light curve could correspond to the exoplanet coming in front of its host star, and blocking some of its light. One time of great importance to constantly keep up to date is the exoplanets mid-transit time. The mid-transit time is the time in which the exoplanet is in the middle of its transit; however, the problem with this value is that it changes over many periods due to the uncertainty in the period of the planet. Thus, it is important to update transit timings as incorrect timings can hinder observations from expensive telescopes. Crowdsourcing exoplanet transit analysis is a great opportunity to help these large telescopes and get the community involved in exoplanet research. In this study, six citizen scientists from the East Bay Astronomical Society (EAS) performed a transit analysis of HATS-4 b using the Exoplanet Transit Interpretation Code (EXOTIC). Using newly calculated mid-transit times, we were able to see how off the expected mid-transit time based on the existing mid-transit time on the NASA Exoplanet Archive was by calculating the phase difference.

Keywords: exoplanet astronomy
An Isolated White Dwarf with 317 s Rotation and Magnetic Emission

We report the discovery of short-period photometric variability and modulated Zeeman-split hydrogen emission in SDSSJ125230.93-023417.72 (EPIC 228939929), a variable white dwarf star observed at long cadence in K2 Campaign 10. The behavior is associated with a magnetic (B = 5.0 MG) spot on the stellar surface, making the 317.278 s period a direct measurement of the stellar rotation rate. This object is therefore the fastest-rotating, apparently isolated (without a stellar companion) white dwarf yet discovered and the second found to exhibit chromospheric Balmer emission after GD 356, in which the emission has been attributed to a unipolar inductor mechanism driven by a possible rocky planet. We explore the properties and behavior of this object, and consider whether its evolution may hold implications for white dwarf mergers and their remnants.

Keywords: white dwarf stars; magnetic variable stars; extrasolar rocky planets; stellar mergers; stellar rotation
A Systematic Study of Galactic Outflows via Fluorescence Emission: Implications for Their Size and Structure

Galactic outflows play a major role in the evolution of galaxies, but the underlying physical processes are poorly understood. This is mainly because we have little information about the outflow structure, especially on large scales. In this paper, we probe the structure of galactic outflows in low-z starbursts using a combination of ultraviolet spectroscopy and imaging of the fluorescence emission lines (associated with transitions to excited fine-structure levels) and spectroscopy of the corresponding strongly blueshifted resonance absorption lines. We find that, in the majority of cases, the observed fluorescence emission lines are much weaker and narrower than the absorption lines, originating in the star-forming interstellar medium and/or the slowest-moving part of the inner outflow. In a minority of cases, the outflowing absorbing material does make a significant contribution to the fluorescence emission. These latter systems are characterized by both strong Lyα emission lines and weak low-ionization absorption lines (both known to be empirical signs of Lyman-continuum leakage). We argue that the observed weakness of emission from the outflow seen in the majority of cases is due to the missing emission arising on scales larger than those encompassed by the aperture of the Cosmic Origins Spectrograph on board the Hubble Space Telescope. This implies shallow radial density profiles in these outflows, and suggests that most of the observed absorbing material must be created/injected at radii much larger than that of the starburst. This has important implications both for our understanding of the physics of galactic outflows and for our estimation of their principal properties.

Keywords: circumgalactic medium; extragalactic astronomy; galaxy formation; galaxy winds; starburst galaxies; interstellar medium
Extreme HBL-like Behavior of Markarian 421 and Its Two-zone Photohadronic Interpretation

Markarian 421 is the nearest high-energy peaked blazar and is also the first extragalactic source to be detected in multi-TeV γ-rays. It has been observed in multiwavelength for an exceptionally long period of time with dense monitoring and several major outbursts have been detected from this source. In 2010 March, the source was in a high state of activity and was observed in multiwavelength by various telescopes for 13 consecutive days. During this period the position of the synchrotron peak was found to be above 1017 Hz and also the position of the second peak was shifted toward higher energy, a signature of extreme HBL-like behavior. We observed that the standard photohadronic model is inadequate to explain the observed spectra. However, a recently proposed two-zone photohadronic model explains very well the GeV-TeV flaring events observed by both MAGIC and VERITAS telescopes. From the observation of the highest energy γ-ray event on MJD 55266 we also estimated the minimum bulk Lorentz factor.

Keywords: high energy astrophysics; blazars; relativistic jets; bl lacertae objects; gamma-rays