adsabs/SciX_UAT_keywords huggingface dataset:
Inhomogeneous Kinetic Alfvén Waves in the Near-Sun Solar Wind

Intervals of intense electromagnetic, broadband plasma waves are reported in the near-Sun solar wind. These waves are identified as kinetic Alfvén waves (KAWs), based on comparison between data and theory for their observed electric- to magnetic-field ratio, and magnetic compressibility, as a function of frequency. In contrast to KAW observations at 1 au, KAWs in the near-Sun solar wind are found to be spatially inhomogeneous, preferentially occurring where the ambient magnetic field experiences strong deviations from the Parker spiral direction, including during magnetic switchbacks and clusters of switchbacks. To the extent that turbulent fluctuations in the solar wind at small scales are characterized as KAWs, the observations reported here create an expectation that KAW-driven particle heating should also be inhomogeneous in the near-Sun solar wind, being strongest in regions of intense KAWs. Finally, a discrepancy between KAW observation and theory is identified at frequencies two decades above the frequency where kinetic effects begin to manifest, and possible origins of this discrepancy are considered.

Keywords: solar wind; alfven waves; interplanetary turbulence; space plasmas; solar magnetic fields
Are "Changing-look" Active Galactic Nuclei Special in the Coevolution of Supermassive Black Holes and Their Hosts? I.

The nature of the so-called "changing-look" (CL) active galactic nucleus (AGN), which is characterized by spectral-type transitions within ~10 yr, remains an open question. As the first in our series of studies, we attempt here to understand the CL phenomenon from the view of the coevolution of AGNs and their host galaxies (i.e., if CL-AGNs are at a specific evolutionary stage) by focusing on the SDSS local "partially obscured" AGNs in which the stellar population of the host galaxy can be easily measured in the integrated spectra. A spectroscopic follow-up program using the Xinglong 2.16 m, Lick/Shane 3 m, and Keck 10 m telescopes enables us to identify in total nine CL-AGNs from a sample of 59 candidates selected by their mid-infrared variability. Detailed analysis of these spectra shows that the host galaxies of the CL-AGNs are biased against young stellar populations and tend to be dominated by intermediate-age stellar populations. This motivates us to propose that CL-AGNs are probably particular AGNs at a specific evolutionary stage, such as a transition stage from "feast" to "famine" fueling of the supermassive black hole. In addition, we reinforce the previous claim that CL-AGNs tend to be biased against both a high Eddington ratio and a high bolometric luminosity, suggesting that the disk-wind broad-line-region model is a plausible explanation of the CL phenomenon.

Keywords: active galactic nuclei; active galaxies; supermassive black holes; spectroscopy; galaxy evolution; x-ray active galactic nuclei
Evidence of Dust Grain Evolution from Extinction Mapping in the IC 63 Photodissociation Region

Photodissociation regions (PDRs) are parts of the ISM consisting of predominantly neutral gas, located at the interface between H II regions and molecular clouds. The physical conditions within these regions show variations on very short spatial scales, and therefore PDRs constitute ideal laboratories for investigating the properties and evolution of dust grains. We have mapped IC 63 at high resolution from the UV to the NIR (275 nm to 1.6 μm), using the Hubble Space Telescope WFC3. Using a Bayesian SED fitting tool, we simultaneously derive a set of stellar (T eff, {log}(g), distance) and extinction (A V , R V ) parameters for 520 background stars. We present maps of A V and R V with a resolution of 25 arcsec based on these results. The extinction properties vary across the PDR, with values for A V between 0.5 and 1.4 mag, and a decreasing trend in R V , going from 3.7 at the front of the nebula to values as low as 2.5 further in. This provides evidence for evolution of the dust optical properties. We fit two modified blackbodies to the MIR and FIR SED, obtained by combining the A V map with data from Spitzer and Herschel. We derive effective temperatures (30 and 227 K) and the ratio of opacities at 160 μm to V band κ 160/κ V (7.0 × 10-4 and 2.9 × 10-9) for the two dust populations. Similar fits to individual pixels show spatial variations of κ 160/κ V . The analysis of our HST data, combined with these Spitzer and Herschel data, provides the first panchromatic view of dust within a PDR.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with program GO-14186.

Keywords: interstellar medium; photodissociation regions; interstellar dust extinction
Detection of Small Flares from the Crab Nebula with Fermi-LAT

Gamma radiation from the Crab pulsar wind nebula (PWN) shows significant variability at ∼100 MeV energies, recently revealed with spaceborne gamma-ray telescopes. Here we report the results of a systematic search for gamma-ray flares using a 7.4 yr data set acquired with the Fermi Large Area Telescope. Analyzing the off-pulse phases of the Crab pulsar, we found seven previously unreported low-intensity flares ("small flares"). The "small flares" originate from the variable synchrotron component of the Crab PWN and show clearly different features from the steady component of the Crab PWN emission. They are characterized by larger fluxes and harder photon indices, similar to previously reported flares. These flares show day-scale time variability and imply a strong magnetic field of Bmin ≈ 1 mG at the site of the gamma-ray production. This result seems to be inconsistent with the typical values revealed with modeling of the nonthermal emission from the nebula. The detection of the "small flares" gives a hint of production of gamma-rays above 100 MeV in a part of the nebula with properties that are different from the main emitters, e.g., due to bulk relativistic motion.

Keywords: pulsars; supernova remnants; magnetohydrodynamics; gamma-ray sources
Spacecraft Geometry Effects on Kinetic Impactor Missions

The DART (Double Asteroid Redirection Test) mission will impact a spacecraft on the secondary (Dimorphos) of the binary asteroid system Didymos in 2022 September, with the goal of altering the orbital period of Dimorphos about Didymos sufficiently to be observed from ground-based observations. Numerical impact modeling is a crucial component in understanding the outcome of the DART experiment, and while many have investigated the effects of target properties, such as material strength and porosity (which remain unknown), an often overlooked factor is the importance of accurately representing the spacecraft itself in such models. Most impact modeling to date has considered simple impactor geometries such as a solid uniform sphere, but in reality the spacecraft is a complex shape full of different components, open spaces, and thin walled structures. At a minimum, a simple solid representation underestimates the surface area of the impact: for a small body such as Dimorphos (approximately 160 m in diameter), the difference between a spacecraft spanning 20 m (including solar arrays) impacting and a sub-1 m idealized shape may be important. In this paper, we compare models impacting high-fidelity models of the spacecraft based on the CAD geometry with various simplified impactors, in order to assess the potential importance of this effect. We find that the difference between the simplest impactor geometries (such as a uniform sphere) and the real spacecraft is measurable, and has an interesting dependence on the material properties of the asteroid itself.

Keywords: impact phenomena; planetary science; asteroids; space vehicles
Bound Debris Expulsion from Neutron Star Merger Remnants

Many studies have found that neutron star mergers leave a fraction of the stars' mass in bound orbits surrounding the resulting massive neutron star or black hole. This mass is a site of r-process nucleosynthesis and can generate a wind that contributes to a kilonova. However, comparatively little is known about the dynamics determining its mass or initial structure. Here we begin to investigate these questions, starting with the origin of the disk mass. Using tracer particle as well as discretized fluid data from numerical simulations, we identify where in the neutron stars the debris came from, the paths it takes in order to escape from the neutron stars' interiors, and the times and locations at which its orbital properties diverge from those of neighboring fluid elements that end up remaining in the merged neutron star.

Keywords: neutron stars; general relativity; black holes; accretion
SNR G292.0+1.8: A Remnant of a Low-mass Progenitor Stripped-envelope Supernova

We present a study of the Galactic supernova remnant (SNR) G292.0+1.8, a classic example of a core-collapse SNR that contains oxygen-rich ejecta, circumstellar material, a rapidly moving pulsar, and a pulsar wind nebula (PWN). We use hydrodynamic simulations of the remnant's evolution to show that the SNR reverse shock is interacting with the PWN and has most likely shocked the majority of the supernova ejecta. In our models, such a scenario requires a total ejecta mass of ≲3 M ⊙ and implies that there is no significant quantity of cold ejecta in the interior of the reverse shock. In light of these results, we compare the estimated elemental masses and abundance ratios in the reverse-shocked ejecta to nucleosynthesis models, and further conclude that they are consistent with a progenitor star with an initial mass of 12-16 M ⊙. We conclude that the progenitor of G292.0+1.8 was likely a relatively low-mass star that experienced significant mass loss through a binary interaction and would have produced a stripped-envelope supernova explosion. We also argue that the region known as the "spur" in G292.0+1.8 arises as a result of the pulsar's motion through the supernova ejecta, and that its dynamical properties may suggest a line-of-sight component to the pulsar's velocity, leading to a total space velocity of ~600 km s-1 and implying a significant natal kick. Finally, we discuss binary mass-loss scenarios relevant to G292.0+1.8 and their implications for the binary companion properties and future searches.

Keywords: supernova remnants; core-collapse supernovae; pulsar wind nebulae; pulsars; circumstellar matter; supernovae; ejecta
Search for Synchrotron Emission from Secondary Electrons of Proton–Proton Interactions in Galactic PeVatron Candidate HESS J1641–463

HESS J1641‑463 is an unidentified gamma-ray source with a hard TeV gamma-ray spectrum, and thus it has been proposed to be a possible candidate for a cosmic-ray (CR) accelerator up to PeV energies (a PeVatron candidate). The source spatially coincides with the radio supernova remnant G338.5+0.1 but has not yet been fully explored in the X-ray band. We analyzed newly taken NuSTAR data, pointing at HESS J1641‑463, with 82 ks effective exposure time. There is no apparent X-ray counterpart of HESS J1641‑463, while nearby stellar cluster, Mercer 81, and stray-light X-rays are detected. Combined with the archival Chandra data, partially covering the source, we derived an upper limit of ∼6 × 10‑13 erg cm‑2 s‑1 in 2–10 keV (∼3 × 10‑13 erg cm‑2 s‑1 in 10–20 keV). If the gamma-ray emission is originated from the decay of π 0 mesons produced in interactions between CR protons and ambient materials, secondary electrons in the proton–proton interactions can potentially emit synchrotron photons in the X-ray band, which can be tested by our X-ray observations. Although the obtained X-ray upper limits cannot place a constraint on the primary proton spectrum, it will be possible with a future hard X-ray mission.

Keywords: galactic cosmic rays; diffuse radiation; gamma-rays; gamma-ray sources
OGLE-2016-BLG-1195 AO: Lens, Companion to Lens or Source, or None of the Above?

We systematically investigate Vandorou et al.'s claim to have detected the host star of the low-mass-ratio (q < 10-4) microlensing planet OGLE-2016-BLG-1195Lb, via Keck adaptive optics (AO) measurements Δt = 4.12 yr after the event's peak (t 0). If correct, this measurement would contradict the microlens-parallax measurement derived from Spitzer observations taken near t 0. We show that this host identification would be in 4σ conflict with the original ground-based relative lens-source proper-motion measurements. By contrast, Gould estimated a probability p = 10% that the "other star" resolved by single-epoch late-time AO would be a companion to the host or the microlensed source, which is much more probable than a 4σ statistical fluctuation. Independent of this proper-motion discrepancy, the kinematics of this host identification are substantially less probable than those of the Spitzer solution. Hence, this identification should not be accepted, pending additional observations that would either confirm or contradict it, which could be taken in 2023. Motivated by this tension, we present two additional investigations. We explore the possibility that Vandorou et al. identified the wrong "star" for their analysis. Astrometry of KMT and Keck images favors a star (or asterism) lying about 175 mas northwest of Vandorou et al.'s star. We also present event parameters from a combined fit to all survey data, which yields a more precise mass ratio, q = (4.6 ± 0.4) × 10-5. Finally, we discuss the broader implications of minimizing such false positives for the first measurement of the planet mass function, which will become possible when AO on next-generation telescopes are applied to microlensing planets.

Keywords: high-resolution microlensing event imaging; satellite microlensing parallax; gravitational microlensing exoplanet detection
Detection of the Red Supergiant Wind from the Progenitor of Cassiopeia A

Cassiopeia A (Cas A) is one of the best studied young Galactic supernova remnants. While it provides a rare opportunity to study in detail the remnant of a supernova (SN) type IIb, questions remain regarding the nature of its progenitor, its mass-loss history, and its pre-SN evolution. Here we present an optical investigation of the circumstellar environment around Cas A and find clumpy and filamentary Hα emission nebulosities concentrated 10-15 pc (10'-15') to the north and east. First reported by Minkowski as a faint H II region, these nebulosities exhibit distinct morphological and spectroscopic properties relative to the surrounding diffuse emissions. Compared to neighboring H II regions, these nebulae show stronger [N II] 6548, 6583 Å and [S II] 6716, 6731 Å emissions relative to Hα. We show that Cas A's highest-velocity ejecta knots are interacting with some of the closest projected emission nebulae, thus providing strong evidence that these nebulae lie at the same distance as the remnant. We interpret these surrounding nebulosities to be the remains of the progenitor's red supergiant wind, which accumulated against the southern edge of a large extended H II region located north of Cas A. Our findings are consistent with the view that Cas A's progenitor underwent considerable mass loss, first from a fast main-sequence wind, then from a slower, clumpy red supergiant wind, and finally from a brief high-velocity wind, like that from a yellow supergiant.

Keywords: supernova remnants; core-collapse supernovae; supernovae; interstellar medium; circumstellar matter; stellar remnants
The Impact of Angular Momentum Loss on the Outcomes of Binary Mass Transfer

We use the rapid binary population synthesis code COMPAS to investigate commonly used prescriptions for the determination of mass transfer stability in close binaries and the orbital separations after stable mass transfer. The degree of orbital tightening during nonconservative mass transfer episodes is governed by the poorly constrained angular momentum carried away by the ejected material. Increased orbital tightening drives systems toward unstable mass transfer leading to a common envelope. We find that the fraction of interacting binaries that will undergo only stable mass transfer throughout their lives fluctuates between a few and ~20% due to uncertainty in the angular momentum loss alone. If mass transfer is significantly nonconservative, stability prescriptions that rely on the assumption of conservative mass transfer underpredict the number of systems which experience unstable mass transfer and stellar mergers. This may substantially impact predictions about the rates of various transients, including luminous red novae, stripped-envelope supernovae, X-ray binaries, and the progenitors of coalescing compact binaries.

Keywords: common envelope binary stars; stellar mergers; interacting binary stars
White Dwarfs with Infrared Excess from LAMOST Data Release 5

Infrared (IR) excess is an important probe for substellar companions and/or debris disks around white dwarfs (WDs). Such systems are still rare, so an in-depth understanding of their formation and long-term evolution has yet to be developed. One of the largest spectroscopic surveys, carried out by the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST), has recently released more than 3000 WDs, a significant fraction of which have not been searched for IR excess. Here, we present a cross-correlation between the LAMOST Data Release 5 WD catalog and Pan-STARRS, the Sloan Digital Sky Survey (SDSS), the UKIRT Infrared Deep Sky Survey, the Two-Micron All Sky Survey (2MASS), and the Wide-field Infrared Survey Explorer (WISE). By performing spectral energy distribution fittings for 846 WDs with WISE detections, we identify 50 candidates with IR excess, including seven candidate WD+M dwarf binaries, 31 candidate WD+brown dwarf (BD) binaries, and 12 candidate WD+dust disk systems. Eight of the dust disk systems are our new identifications. By utilizing a systematic survey with accurate stellar parameters derived from spectral fitting, our work is an important addition to previous searches for IR excess from SDSS and Gaia WDs, and it provides a significant (≳8%) complement to the current database of WDs with candidate BD companions and dust disks. The frequencies of the WD+BD binaries and the WD+dust disk systems are constrained to be ≲3.7% and ~1.4%, respectively. The properties of the candidate dust disk systems are discussed. All of our candidates require follow-up observations for confirmation, owing to the limited spatial resolution of WISE.

Keywords: white dwarf stars; brown dwarfs; infrared excess; debris disks; circumstellar dust
Solar Chromospheric Network as a Source for Solar Wind Switchbacks

Recent studies suggest that the magnetic switchbacks (SBs) detected by the Parker Solar Probe carry information on the scales of solar supergranulation (large scale) and granulation (medium scale). We test this claim using high-resolution Hα images obtained with the visible spectropolarimeters of the Goode Solar Telescope in Big Bear Solar Observatory. As possible solar sources, we count all the spicule-like features standing along the chromospheric networks near the coronal hole boundary visible in the Hα blue-wing but absent in the red-wing images and measure the geometric parameters of dense sections of individual flux tubes. Intervals between adjacent spicules located along the chromospheric networks are found in the range of 0.4-1.5 Mm (0.°03-0.°12) tending to be smaller than the medium scale of SBs. Interdistances between all pairs of the flux tubes are also counted and they appear in a single peak distribution around 0.7 Mm (0.°06) unlike the waiting-time distribution of SBs in a scale-free single power-law form. The length-to-diameter ratio of the dense section of flux tubes is as high as 6-40, similar to the aspect ratio of SBs. The number of spicules along a network can be as high as 40-100, consistent with numerous SBs within a patch. With these numbers, it is argued that the medium scale of SBs can be understood as an equilibrium distance resulting from a random walk within each diverging magnetic field funnel connected to the chromospheric networks.

Keywords: solar magnetic fields; solar chromosphere; solar wind; interplanetary magnetic fields
Joint Modeling of Radial Velocities and Photometry with a Gaussian Process Framework

Developments in the stability of modern spectrographs have led to extremely precise instrumental radial velocity (RV) measurements. For most stars, the detection limit of planetary companions with these instruments is expected to be dominated by astrophysical noise sources such as starspots. Correlated signals caused by rotationally modulated starspots can obscure or mimic the Doppler shifts induced by even the closest, most massive planets. This is especially true for young, magnetically active stars where stellar activity can cause fluctuation amplitudes of ≳0.1 mag in brightness and ≳100 m s-1 in RV semiamplitudes. Techniques that can mitigate these effects and increase our sensitivity to young planets are critical to improving our understanding of the evolution of planetary systems. Gaussian processes (GPs) have been successfully employed to model and constrain activity signals in individual cases. However, a principled approach of this technique, specifically for the joint modeling of photometry and RVs, has not yet been developed. In this work, we present a GP framework to simultaneously model stellar activity signals in photometry and RVs that can be used to investigate the relationship between both time series. Our method, inspired by the FF ${}^{{\prime} }$ framework of Aigrain et al., models spot-driven activity signals as the linear combinations of two independent latent GPs and their time derivatives. We also simulate time series affected by starspots by extending the starry software to incorporate time evolution of stellar features. Using these synthetic data sets, we show that our method can predict spot-driven RV variations with greater accuracy than other GP approaches.

Keywords: stellar activity; photometry; radial velocity; astronomy data analysis; astronomy data modeling
Particle-in-cell Simulations of the Parallel Proton Firehose Instability Influenced by the Electron Temperature Anisotropy in Solar Wind Conditions

In situ observations of the solar wind show a limited level of particle temperature anisotropy with respect to the interplanetary magnetic field direction. Kinetic electromagnetic instabilities are efficient to prevent the excessive growth of the anisotropy of particle velocity distribution functions. Among them, the firehose instabilities are often considered to prevent the increase of the parallel temperature and hence to shape the velocity distribution functions of electrons and protons in the solar wind. We present a nonlinear modeling of the parallel firehose instability, retaining a kinetic description for both the electrons and protons. One-dimensional (1D) fully kinetic particle-in-cell simulations using the energy conserving semi-implicit method (ECsim) are performed to clarify the role of the electron temperature anisotropy in the development of the parallel proton firehose instability. We found that in the presence of an electron temperature anisotropy, such that the temperature parallel to the background magnetic field is higher than the temperature in the perpendicular direction, the onset of the parallel proton firehose instability occurs earlier and its growth rate is faster. The enhanced wave fluctuations contribute to the particle scattering reducing the temperature anisotropy to a stable, nearly isotropic state. The simulation results compare well with linear theory. A test case of 1D simulations at oblique angles with respect to the magnetic field is also considered, as a first step to study the cumulative effect of protons and electrons on the full spectrum of instabilities.

Keywords: solar wind; plasma astrophysics; space plasmas