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<jats:title>Abstract</jats:title> <jats:p>Thermal instability is a fundamental process of astrophysical plasmas. It is expected to occur whenever the cooling is dominated by radiation and cannot be compensated for by heating. In this work, we conduct 2.5D radiation MHD simulations with the <jats:italic>Bifrost</jats:italic> code of an enhanced activity network in the solar atmosphere. Coronal loops are produced self-consistently, mainly through Joule heating, which is sufficiently stratified and symmetric to produce thermal nonequilibrium. During the cooling and driven by thermal instability, coronal rain is produced along the loops. Due to flux freezing, the catastrophic cooling process leading to a rain clump produces a local enhancement of the magnetic field, thereby generating a distinct magnetic strand within the loop up to a few Gauss stronger than the surrounding coronal field. These strands, which can be considered fundamental, are a few hundred kilometers in width, span most of the loop leg, and emit strongly in the UV and extreme UV (EUV), thereby establishing a link between the commonly seen rain strands in the visible spectrum with the observed EUV coronal strands at high resolution. The compression downstream leads to an increase in temperature that generates a plume-like structure, a strongly emitting spicule-like feature, and short-lived brightening in the UV during the rain impact, providing an explanation for similar phenomena seen with IRIS. Thermal instability and nonequilibrium can therefore be associated with localized and intermittent UV brightening in the transition region and chromosphere, as well as contribute to the characteristic filamentary morphology of the solar corona in the EUV.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The empirical relationship between the nonthermal 5 GHz radio luminosity and the soft X-ray luminosity of active stellar coronae, canonically called the Güdel–Benz relationship, has been a cornerstone of stellar radio astronomy, as it explicitly ties the radio emission to the coronal heating mechanisms. The relationship extends from microflares on the Sun to the coronae of the most active stars suggesting that active coronae are heated by a flare-like process. The relationship is thought to originate from a consistent partition of the available flare energy into relativistic charges, which emit in the radio-band via the <jats:italic>incoherent</jats:italic> gyrosynchrotron mechanism, and heating of the bulk coronal plasma, which emits in the X-ray band via the Bremsstrahlung mechanism. Consequently, <jats:italic>coherent</jats:italic> emission from stellar and substellar objects is not expected to adhere to this empirical relationship, as it is observed in ultracool dwarf stars and brown dwarfs. Here we report a population of radio-detected chromospherically active stars that surprisingly follow the Güdel–Benz relationship despite their radio emission being classified as coherent emission by virtue of its high circularly polarized fraction and high brightness temperature. Our results prompt a reexamination of the physics behind the Güdel–Benz relationship, its implication for the mechanism of coronal heating and particle acceleration in active stars, and the phenomenological connection between solar and stellar flares.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The detection of a binary neutron star (BNS) merger in 2017 through both gravitational waves and electromagnetic emission opened a new era of multimessenger astronomy. The understanding of the magnetic field amplification triggered by the Kelvin–Helmholtz instability during the merger is still a numerically unresolved problem because of the relevant small scales involved. One of the uncertainties comes from the simplifications usually assumed in the initial magnetic topology of merging neutron stars. We perform high-resolution, convergent large-eddy simulations of BNS mergers, following the newly formed remnant for up to 30 ms. Here we specifically focus on the comparison between simulations with different initial magnetic configurations, going beyond the widespread-used aligned dipole confined within each star. The results obtained show that the initial topology is quickly forgotten, in a timescale of a few milliseconds after the merger. Moreover, at the end of the simulations, the average intensity (<jats:italic>B</jats:italic> ∼ 10<jats:sup>16</jats:sup> <jats:italic>G</jats:italic>) and the spectral distribution of magnetic energy over spatial scales barely depend on the initial configuration. This is expected due to the small-scale efficient dynamo involved, and thus it holds as long as (i) the initial large-scale magnetic field is not unrealistically high (as often imposed in mergers studies), and (ii) the turbulent instability is numerically (at least partially) resolved, so that the amplified magnetic energy is distributed across a wide range of scales and becomes orders of magnitude larger than the initial one.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A population of more than 50 binary black hole mergers has now been observed by the LIGO and Virgo gravitational-wave observatories. While neutron stars are known to have large velocities associated with impulsive kicks imparted to them at birth in supernovae, whether black holes receive similar kicks, and of what magnitude, remains an open question. Recently, Callister et al. analyzed the binary black hole population under the hypothesis that they were all formed through isolated binary evolution and claimed that large black hole kicks (greater than 260 km s<jats:sup>−1</jats:sup> at 99% confidence) were required for the spin distribution of merging binary black holes to match observations. Here we highlight that a key assumption made by Callister et al.—that all secondary black holes can be tidally spun up—is not motivated by physical models and may lead to a bias in their estimate of the magnitudes of black hole kicks. We make only minor changes to the Callister et al. model, accounting for a population of wider merging binaries where tidal synchronization is ineffective. We show that this naturally produces a bimodal spin distribution for secondary black holes and that the spin–orbit misalignments observed in the binary black hole population can be explained by more typical black hole kicks of order 100 km s<jats:sup>−1</jats:sup>, consistent with kicks inferred from Galactic X-ray binaries containing black holes. We conclude that the majority of the binary black hole population is consistent with forming through isolated binary evolution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the discovery of neutral gas detected in both damped Ly<jats:italic>α</jats:italic> absorption (DLA) and H <jats:sc>i</jats:sc> 21 cm emission outside of the stellar body of a galaxy, the first such detection in the literature. A joint analysis between the Cosmic Ultraviolet Baryon Survey and the MeerKAT Absorption Line Survey reveals an H <jats:sc>i</jats:sc> bridge connecting two interacting dwarf galaxies (log (<jats:italic>M</jats:italic> <jats:sub>star</jats:sub>/<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) = 8.5 ± 0.2) that host a <jats:italic>z</jats:italic> = 0.026 DLA with log[<jats:italic>N</jats:italic>(H <jats:sc>i</jats:sc>)/cm<jats:sup>−2</jats:sup>] = 20.60 ± 0.05 toward the QSO J2339−5523 (<jats:italic>z</jats:italic> <jats:sub>QSO</jats:sub> = 1.35). At impact parameters of <jats:italic>d</jats:italic> = 6 and 33 kpc, the dwarf galaxies have no companions more luminous than ≈0.05<jats:italic>L</jats:italic> <jats:sub>*</jats:sub> within at least Δ<jats:italic>v</jats:italic> = ±300 km s<jats:sup>−1</jats:sup> and <jats:italic>d</jats:italic> ≈ 350 kpc. The H <jats:sc>i</jats:sc> 21 cm emission is spatially coincident with the DLA at the 2<jats:italic>σ</jats:italic>–3<jats:italic>σ</jats:italic> level per spectral channel over several adjacent beams. However, H <jats:sc>i</jats:sc> 21 cm absorption is not detected against the radio-bright QSO; if the background UV and radio sources are spatially aligned, the gas is either warm or clumpy (with a spin temperature to covering factor ratio <jats:italic>T</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>/<jats:italic>f</jats:italic> <jats:sub> <jats:italic>c</jats:italic> </jats:sub> &gt; 1880 K). Observations with VLT-MUSE demonstrate that the <jats:italic>α</jats:italic>-element abundance of the ionized interstellar medium (ISM) is consistent with the DLA (≈10% solar), suggesting that the neutral gas envelope is perturbed ISM gas. This study showcases the impact of dwarf–dwarf interactions on the physical and chemical state of neutral gas outside of star-forming regions. In the SKA era, joint UV and H <jats:sc>i</jats:sc> 21 cm analyses will be critical for connecting the cosmic neutral gas content to galaxy environments.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We identify Gaia 0007–1605 A,C as the first inner brown dwarf–white dwarf binary of a hierarchical triple system in which the outer component is another white dwarf (Gaia 0007–1605 B). From optical/near-infrared spectroscopy obtained at the Very Large Telescope with the X-Shooter instrument and/or from Gaia photometry plus spectral energy distribution fitting, we determine the effective temperatures and masses of the two white dwarfs (12,018 ± 68 K and 0.54 ± 0.01 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> for Gaia 0007–1605 A and 4445 ± 116 K and 0.56 ± 0.05 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> for Gaia 0007–1605 B) and the effective temperature of the brown dwarf (1850 ± 50 K; corresponding to spectral type L3 ± 1). By analyzing the available TESS light curves of Gaia 0007–1605 A,C we detect a signal at 1.0446 ± 0.0015 days with an amplitude of 6.25 ppt, which we interpret as the orbital period modulated from irradiation effects of the white dwarf on the brown dwarf’s surface. This drives us to speculate that the inner binary evolved through a common-envelope phase in the past. Using the outer white dwarf as a cosmochronometer and analyzing the kinematic properties of the system, we conclude that the triple system is about 10 Gyr old.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Active galactic nucleus (AGN) feedback is postulated as a key mechanism for regulating star formation within galaxies. Studying the physical properties of the outflowing gas from AGNs is thus crucial for understanding the coevolution of galaxies and supermassive black holes. Here we report 55 pc resolution ALMA neutral atomic carbon [C <jats:sc>i</jats:sc>] <jats:sup>3</jats:sup> <jats:italic>P</jats:italic> <jats:sub>1</jats:sub>−<jats:sup>3</jats:sup> <jats:italic>P</jats:italic> <jats:sub>0</jats:sub> observations toward the central 1 kpc of the nearby Type 2 Seyfert galaxy NGC 1068, supplemented by 55 pc resolution CO(<jats:italic>J</jats:italic> = 1−0) observations. We find that [C <jats:sc>i</jats:sc>] emission within the central kiloparsec is strongly enhanced by a factor of &gt;5 compared to the typical [C <jats:sc>i</jats:sc>]/CO intensity ratio of ∼0.2 for nearby starburst galaxies (in units of brightness temperature). The most [C <jats:sc>i</jats:sc>]-enhanced gas (ratio &gt; 1) exhibits a kiloparsec-scale elongated structure centered at the AGN that matches the known biconical ionized gas outflow entraining molecular gas in the disk. A truncated, decelerating bicone model explains well the kinematics of the elongated structure, indicating that the [C <jats:sc>i</jats:sc>] enhancement is predominantly driven by the interaction between the ISM in the disk and the highly inclined ionized gas outflow (which is likely driven by the radio jet). Our results strongly favor the “CO dissociation scenario” rather than the “in situ C formation” one, which prefers a perfect bicone geometry. We suggest that the high-[C <jats:sc>i</jats:sc>]/CO intensity ratio gas in NGC 1068 directly traces ISM in the disk that is currently dissociated and entrained by the jet and the outflow, i.e., the “negative” effect of the AGN feedback.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the first detection of the S(1) pure rotational line of ortho-H<jats:sub>2</jats:sub> at 17.04 <jats:italic>μ</jats:italic>m in an asymptotic giant branch star, using observations of IRC+10216 with the Echelon-cross-echelle Spectrograph (EXES) mounted on the Stratospheric Observatory for Infrared Astronomy. This line, which was observed in a very high-sensitivity spectrum (rms noise ≃0.04% of the continuum), was detected in the wing of a strong telluric line and displayed a P Cygni profile. The spectral ranges around the frequencies of the S(5) and S(7) ortho-H<jats:sub>2</jats:sub> transitions were observed as well but no feature was detected in spectra with sensitivities of 0.12% and 0.09% regarding the continuum emission, respectively. We used a radiation transfer code to model these three lines and derived a mass-loss rate of (2.43 ± 0.21) × 10<jats:sup>−5</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup> without using the CO abundance. The comparison of this rate with previous estimates derived from CO observations suggests that the CO abundance relative to H<jats:sub>2</jats:sub> is (6.7 ± 1.4) × 10<jats:sup>−4</jats:sup>. From this quantity and previously reported molecular abundances, we estimate the O/H and C/H ratios to be (3.3 ± 0.7) × 10<jats:sup>−4</jats:sup> and &gt;(5.2 ± 0.9) × 10<jats:sup>−4</jats:sup>, respectively. The C/O ratio is &gt;1.5 ± 0.4. The absence of the S(5) and S(7) lines of ortho-H<jats:sub>2</jats:sub> in our observations can be explained by the opacity of hot dust within 5 <jats:italic>R</jats:italic> <jats:sub>⋆</jats:sub> from the center of the star. We estimate the intensity of the S(0) and S(2) lines of para-H<jats:sub>2</jats:sub> to be ≃0.1% and 0.2% of the continuum, respectively, which are below the detection limit of EXES.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Migration of water molecules into the porous lunar soil can lead to sequestration of adsorbed water or ice at depth. Here, this process is modeled at several potential landing sites in the south polar region of the Moon. Desorption times are parameterized based on the Brunauer–Emmett–Teller (BET) isotherm model. Water sequestration is significant at depths where subsurface temperatures remain below the cold-trapping threshold, a condition satisfied at one of three sites considered, west of Haworth Crater. Model calculations predict a hydrated layer below a desiccated layer. The thickness of the desiccated layer is on the scale of the thermal skin depth, which for dust is typically centimeters. The underlying layer, decimeters thick, can be hydrated over a timescale of a billion years, reaching abundances on the order of 1 wt%. This sequestration process potentially simultaneously explains excess hydrogen concentrations outside of cold traps and the observed presence of a desiccated layer above a hydrogenous layer.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The High Energy Stereoscopic System (H.E.S.S.) observatory has carried out a deep survey of the Galactic plane, in the course of which the existence of a significant number of (∼78) TeV <jats:italic>γ</jats:italic>-ray sources was confirmed, many of which remain unidentified. HESS J1828–099 is a point-like (Gaussian standard deviation &lt; 0.°07) unidentified source among the 17 confirmed point-like sources in the H.E.S.S. Galactic Plane Survey (HGPS) catalog. This source is also unique because it does not seem to have any apparent association with any object detected at other wavelengths. We investigate the nature and association of HESS J1828–099 with multiwavelength observational data. A high-mass X-ray binary (HMXB)—composed of the pulsar XTE J1829–098 and a companion Be star—has been observed earlier in the X-ray and infrared bands, 14′ away from HESS J1828–099. With 12 yr of Fermi-LAT <jats:italic>γ</jats:italic>-ray data, we explore the possibility of 4FGL J1830.2–1005 being the GeV counterpart of HESS J1828–099. Within the RXTE confidence region, a steep-spectrum (<jats:italic>α</jats:italic> <jats:sub>radio</jats:sub> = −0.746 ± 0.284) plausible counterpart is detected in data from existing radio frequency surveys. In this Letter, we probe for the first time, using multiwavelength data, whether HESS J1828–099, 4FGL J1830.2–1005, and the HMXB system have a common origin. Our study indicates that HESS J1828–099 might be a TeV high-mass <jats:italic>γ</jats:italic>-ray binary source.</jats:p>