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<jats:title>Abstract</jats:title> <jats:p>To study the nuclear (≲1 kpc) dust of nearby (<jats:italic>z</jats:italic> &lt; 0.1) quasi-stellar objects (QSOs), we obtained new near-infrared (NIR) high angular resolution (∼0.″3) photometry in the <jats:italic>H</jats:italic> and <jats:italic>Ks</jats:italic> bands for 13 QSOs with available mid-infrared (MIR) high angular resolution spectroscopy (∼7.5–13.5 <jats:italic>μ</jats:italic>m). We find that in most QSOs, the NIR emission is unresolved. We subtract the contribution from the accretion disk, which decreases from NIR (∼35%) to MIR (∼2.4%). We also estimate these percentages assuming a bluer accretion disk and find that the contribution in the MIR is nearly seven times larger. We find that the majority of objects (64%, 9/13) are better fitted by the disk+wind H17 model, while others can be fitted by the smooth F06 (14%, 2/13), clumpy N08 (7%, 1/13), clumpy H10 (7%, 1/13), and two-phase media S16 (7%, 1/13) models. However, if we assume the bluer accretion disk, the models fit only 2/13 objects. We measured two NIR-to-MIR spectral indexes, <jats:italic>α</jats:italic> <jats:sub>NIR−MIR(1.6–8.7 <jats:italic>μ</jats:italic>m)</jats:sub> and <jats:italic>α</jats:italic> <jats:sub>NIR−MIR(2.2–8.7 <jats:italic>μ</jats:italic>m)</jats:sub>, and two MIR spectral indexes, <jats:italic>α</jats:italic> <jats:sub>MIR(7.8–9.8 <jats:italic>μ</jats:italic>m)</jats:sub> and <jats:italic>α</jats:italic> <jats:sub>MIR(9.8–11.7 <jats:italic>μ</jats:italic>m)</jats:sub>, from models and observations. From observations, we find that the NIR-to-MIR spectral indexes are ∼−1.1, and the MIR spectral indexes are ∼−0.3. Comparing the synthetic and observed values, we find that none of the models simultaneously match the measured NIR-to-MIR and 7.8–9.8 <jats:italic>μ</jats:italic>m slopes. However, we note that measuring <jats:italic>α</jats:italic> <jats:sub>MIR(7.8–9.8 <jats:italic>μ</jats:italic>m)</jats:sub> on the starburst-subtracted Spitzer/IRS spectrum gives values of the slopes (∼−2) that are similar to the synthetic values obtained from the models.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Parker Solar Probe (PSP) spacecraft is performing the first in situ exploration of the solar wind within 0.2 au of the Sun. Initial observations confirmed the Alfvénic nature of aligned fluctuations of the magnetic field <jats:bold> <jats:italic>B</jats:italic> </jats:bold> and velocity <jats:bold> <jats:italic>V</jats:italic> </jats:bold> in solar wind plasma close to the Sun, in domains of nearly constant magnetic field magnitude ∣<jats:bold> <jats:italic>B</jats:italic> </jats:bold>∣, i.e., approximate magnetic pressure balance. Such domains are interrupted by particularly strong fluctuations, including but not limited to radial field (polarity) reversals, known as switchbacks. It has been proposed that nonlinear Kelvin–Helmholtz instabilities form near magnetic boundaries in the nascent solar wind leading to extensive shear-driven dynamics, strong turbulent fluctuations including switchbacks, and mixing layers that involve domains of approximate magnetic pressure balance. In this work we identify and analyze various aspects of such domains using data from the first five PSP solar encounters. The filling fraction of domains, a measure of Alfvénicity, varies from median values of 90% within 0.2 au to 38% outside 0.9 au, with strong fluctuations. We find an inverse association between the mean domain duration and plasma <jats:italic>β</jats:italic>. We examine whether the mean domain duration is also related to the crossing time of spatial structures frozen into the solar wind flow for extreme cases of the aspect ratio. Our results are inconsistent with long, thin domains aligned along the radial or Parker spiral direction, and compatible with isotropic domains, which is consistent with prior observations of isotropic density fluctuations or <jats:italic>flocculae</jats:italic> in the solar wind.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The maximum mass of neutron stars (NSs) is of great importance for constraining equations of state of NSs and understanding the mass gap between NSs and stellar-mass black holes. NSs in X-ray binaries increase in mass by accreting material from their companions (known as the recycling process), and the uncertainties in the accretion process make studying the NS mass at birth a challenge. In this work, we investigate the NS accreted mass while considering the effect of NS spin evolution and provide the maximum accreted mass for NSs in the recycling process. By exploring a series of binary evolution calculations, we obtain the final NS mass and the maximum accreted mass for a given birth mass of an NS and a mass transfer efficiency. Our results show that NSs can accrete relatively more material for binary systems with donor masses in the range of 1.8 ∼ 2.4 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, NSs accrete relatively more mass when the remnant WD mass is in the range of ∼ 0.25–0.30 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, and the maximum accreted mass is positively correlated with the initial NS mass. For a 1.4 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> NS at birth with a moderate mass transfer efficiency of 0.3, the maximum accreted mass could be 0.27 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. The results can be used to estimate the minimum birth mass for systems with massive NSs in observations.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>With the presence of evermore complex S-bearing molecules being detected lately, studies of their chemical formation routes need to keep up the pace to rationalize observations, suggest new candidates for detection, and provide input for chemical evolution models. In this paper, we theoretically characterize the hydrogenation channels of OCS on top of amorphous solid water (ASW) as an interstellar dust grain analog in molecular clouds. Our results show that the significant reaction outcome is trans-HC(O)SH, a recently detected prebiotic molecule toward G+0.693. The reaction is diastereoselective, explaining the apparent absence of the <jats:italic>cis</jats:italic> isomer in astronomical observations. We found that the reaction proceeds through a highly localized radical intermediate (cis-OCSH), which could be essential in the formation of other sulfur-bearing complex organic molecules due to its slow isomerization dynamics on top of ASW.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the joint Chandra, XMM-Newton, and NuSTAR analysis of two nearby Seyfert galaxies, NGC 3081 and ESO 565-G019. These are the only two having Chandra data in a larger sample of 10 low-redshift (<jats:italic>z</jats:italic> ≤ 0.05), candidates Compton-thick (CT) Active Galactic Nuclei selected in the 15–150 keV band with Swift-BAT that were still lacking NuSTAR data. Our spectral analysis, performed using physically motivated models, provides an estimate of both the line-of-sight (l.o.s.) and average (<jats:italic>N</jats:italic> <jats:sub>H,<jats:italic>S</jats:italic> </jats:sub>) column densities of the two torii. NGC 3081 has a Compton-thin l.o.s. column density <jats:italic>N</jats:italic> <jats:sub>H,<jats:italic>z</jats:italic> </jats:sub> = [0.58–0.62] × 10<jats:sup>24</jats:sup> cm<jats:sup>−2</jats:sup>, but the <jats:italic>N</jats:italic> <jats:sub>H,<jats:italic>S</jats:italic> </jats:sub>, beyond the CT threshold (<jats:italic>N</jats:italic> <jats:sub>H,<jats:italic>S</jats:italic> </jats:sub> = [1.41–1.78] × 10<jats:sup>24</jats:sup> cm<jats:sup>−2</jats:sup>), suggests a “patchy” scenario for the distribution of the circumnuclear matter. ESO 565-G019 has both CT l.o.s. and <jats:italic>N</jats:italic> <jats:sub>H,<jats:italic>S</jats:italic> </jats:sub> column densities (<jats:italic>N</jats:italic> <jats:sub>H,<jats:italic>z</jats:italic> </jats:sub> &gt; 2.31 × 10<jats:sup>24</jats:sup> cm<jats:sup>−2</jats:sup> and <jats:italic>N</jats:italic> <jats:sub>H,<jats:italic>S</jats:italic> </jats:sub> &gt; 2.57 × 10<jats:sup>24</jats:sup> cm<jats:sup>−2</jats:sup>, respectively). The use of physically motivated models, coupled with the broad energy range covered by the data (0.6–70 keV and 0.6–40 keV, for NGC 3081 and ESO 565-G019, respectively) allows us to constrain the covering factor of the obscuring material, which is <jats:italic>C</jats:italic> <jats:sub>TOR</jats:sub> = [0.63–0.82] for NGC 3081, and <jats:italic>C</jats:italic> <jats:sub>TOR</jats:sub> = [0.39–0.65] for ESO 565-G019.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We study the ionization and excitation structure of the interstellar medium in the late-stage gas-rich galaxy merger NGC 6240 using a suite of emission-line maps at ∼25 pc resolution from the Hubble Space Telescope, Keck/NIRC2 with Adaptive Optics, and the Atacama Large Millimeter/submillimeter Array (ALMA). NGC 6240 hosts a superwind driven by intense star formation and/or one or both of two active nuclei; the outflows produce bubbles and filaments seen in shock tracers from warm molecular gas (H<jats:sub>2</jats:sub> 2.12 <jats:italic>μ</jats:italic>m) to optical ionized gas ([O <jats:sc>iii</jats:sc>], [N <jats:sc>ii</jats:sc>], [S <jats:sc>ii</jats:sc>], and [O <jats:sc>i</jats:sc>]) and hot plasma (Fe <jats:sc>XXV</jats:sc>). In the most distinct bubble, we see a clear shock front traced by high [O <jats:sc>iii</jats:sc>]/H<jats:italic>β</jats:italic> and [O <jats:sc>iii</jats:sc>]/[O <jats:sc>i</jats:sc>]. Cool molecular gas (CO(2−1)) is only present near the base of the bubble, toward the nuclei launching the outflow. We interpret the lack of molecular gas outside the bubble to mean that the shock front is not responsible for dissociating molecular gas, and conclude that the molecular clouds are partly shielded and either entrained briefly in the outflow, or left undisturbed while the hot wind flows around them. Elsewhere in the galaxy, shock-excited H<jats:sub>2</jats:sub> extends at least ∼4 kpc from the nuclei, tracing molecular gas even warmer than that between the nuclei, where the two galaxies’ interstellar media are colliding. A ridgeline of high [O <jats:sc>iii</jats:sc>]/H<jats:italic>β</jats:italic> emission along the eastern arm aligns with the southern nucleus’ stellar disk minor axis; optical integral field spectroscopy from WiFeS suggests this highly ionized gas is centered at systemic velocity and likely photoionized by direct line of sight to the southern active galactic nucleus.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A detailed study of the BL Lacertae PKS 0903-57 has been done for the first time with 12 yr of Fermi Large Area Telescope data. We have identified two bright gamma-ray flares in 2018 and 2020. Many substructures were observed during multiple time binning of these flares. We performed a detailed temporal and spectral study on all the substructures separately. A single-zone emission model is used for time-dependent leptonic modeling of the multiwavelength spectral energy distributions. Our estimated values of variability timescale, magnetic field in the emission region, and the jet power obtained from leptonic modeling of PKS 0903-57 are presented in this work. Currently, we have a minimal number of observations in X-rays and other bands. Hence, further simultaneous multiwavelength monitoring of this source is required to have a better understanding of the physical processes occurring in the jet of the blazar PKS 0903-57.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Becker et al. measured the mean free path of Lyman-limit photons in the intergalactic medium (IGM) at <jats:italic>z</jats:italic> = 6. The short value suggests that absorptions may have played a prominent role in reionization. Here we study physical properties of ionizing photon sinks in the wake of ionization fronts (I-fronts) using radiative hydrodynamic simulations. We quantify the contributions of gaseous structures to the Lyman-limit opacity by tracking the column-density distributions in our simulations. Within Δ<jats:italic>t</jats:italic> = 10 Myr of I-front passage, we find that self-shielding systems (<jats:italic>N</jats:italic> <jats:sub>H I</jats:sub> &gt; 10<jats:sup>17.2</jats:sup> cm<jats:sup>−2</jats:sup>) are comprised of two distinct populations: (1) overdensity Δ ∼ 50 structures in photoionization equilibrium with the ionizing background, and (2) Δ ≳ 100 density peaks with fully neutral cores. The self-shielding systems contribute more than half of the opacity at these times, but the IGM evolves considerably in Δ<jats:italic>t</jats:italic> ∼ 100 Myr as structures are flattened by pressure smoothing and photoevaporation. By Δ<jats:italic>t</jats:italic> = 300 Myr, they contribute ≲10% to the opacity in an average 1 Mpc<jats:sup>3</jats:sup> patch of the universe. The percentage can be a factor of a few larger in overdense patches, where more self-shielding systems survive. We quantify the characteristic masses and sizes of self-shielding structures. Shortly after I-front passage, we find <jats:italic>M</jats:italic> = 10<jats:sup>4</jats:sup>–10<jats:sup>8</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and effective diameters <jats:italic>d</jats:italic> <jats:sub>eff</jats:sub> = 1–20 ckpc <jats:italic>h</jats:italic> <jats:sup>−1</jats:sup>. These scales increase as the gas relaxes. The picture herein presented may be different in dark matter models with suppressed small-scale power.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We explore the effect of forcing on the linear shear flow or plane Couette flow, which is also the background flow in the very small region of the Keplerian accretion disk. We show that depending on the strength of forcing and boundary conditions suitable for the systems under consideration, the background plane shear flow, and hence the accretion disk velocity profile, is modified into parabolic flow, which is a plane Poiseuille flow or Couette–Poiseuille flow, depending on the frame of reference. In the presence of rotation, the plane Poiseuille flow becomes unstable at a smaller Reynolds number under pure vertical as well as three-dimensional perturbations. Hence, while rotation stabilizes the plane Couette flow, the same destabilizes the plane Poiseuille flow faster and hence the forced local accretion disk. Depending on the various factors, when the local linear shear flow becomes a Poiseuille flow in the shearing box due to the presence of extra force, the flow becomes unstable even for Keplerian rotation, and hence turbulence will ensue. This helps to resolve the long-standing problem of subcritical transition to turbulence in hydrodynamic accretion disks and the laboratory plane Couette flow.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the far-UV (FUV) photometry of images acquired with UVIT on AstroSat to probe the horizontal branch (HB) population of the Galactic globular cluster NGC 2298. UV-optical color–magnitude diagrams (CMDs) are constructed for member stars in combination with Hubble Space Telescope UV Globular Cluster Survey data for the central region and Gaia and ground-based photometric data for the outer region. A blue HB (BHB) sequence with a spread and four hot HB stars are detected in all FUV-optical CMDs and are compared with theoretical updated BaSTI isochrones and synthetic HB models with a range in helium abundance, suggesting that the hot HB stars are helium enhanced when compared to the BHB. The estimated effective temperature, radius, and luminosity of HB stars, using the best spectral energy distribution fits, were compared with various HB models. BHB stars span a temperature range from 7500 to 12,250 K. Three hot HB stars have 35,000–40,000 K, whereas one star has around ∼100,000 K. We suggest the following evolutionary scenarios: two stars are likely to be the progeny of extreme HB (EHB) stars formed through an early hot-flasher scenario, one is likely to be an EHB star with probable helium enrichment, and the hottest HB star, which is about to enter the white dwarf cooling phase, could have evolved from the BHB phase. Nevertheless, these are interesting spectroscopic targets to understand the late stages of evolution.</jats:p>