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<jats:title>Abstract</jats:title> <jats:p>Sub-subgiant stars (SSGs) fall below the subgiant branch and/or red of the giant branch in open and globular clusters, an area of the color–magnitude diagram (CMD) not populated by standard stellar evolution tracks. One hypothesis is that SSGs result from rapid rotation in subgiants or giants due to tidal synchronization in a close binary. The strong magnetic fields generated inhibit convection, which in turn produces large starspots, radius inflation, and lower-than-expected average surface temperatures and luminosities. Here we cross-reference a catalog of active giant binaries (RS CVns) in the field with Gaia EDR3. Using the Gaia photometry and parallaxes, we precisely position the RS CVns in a CMD. We identify stars that fall below a 14 Gyr, metal-rich isochrone as candidate field SSGs. Out of a sample of 1723 RS CVn, we find 448 SSG candidates, a dramatic expansion from the 65 SSGs previously known. Most SSGs have rotation periods of 2–20 days, with the highest SSG fraction found among RS CVn with the shortest periods. The ubiquity of SSGs among this population indicates that SSGs are a normal phase in evolution for RS CVn-type systems, not rare by-products of dynamical encounters found only in dense star clusters as some have suggested. We present our catalog of 1723 active giants, including Gaia photometry and astrometry, and rotation periods from the Transiting Exoplanet Survey Satellite and International Variable Star Index (VSX). This catalog can serve as an important sample to study the impacts of magnetic fields in evolved stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Ultrastripped supernovae (USSNe) with a relatively low ejecta mass of ∼0.1 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> (e.g., iPTF 14gqr and SN 2019dge) are considered to originate from ultrastripped carbon–oxygen cores in close binary systems and are likely to be progenitors of binary neutron stars. Here we conduct the explosion simulations of ultrastripped progenitors with various masses (1.45 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> ≤ <jats:italic>M</jats:italic> <jats:sub>CO</jats:sub> ≤ 2.0 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) based on results of neutrino-radiation hydrodynamics simulations, and consistently calculate the nucleosynthesis and the supernova light curves. We find that a USSN from a more massive progenitor has a larger ejecta mass but a smaller <jats:sup>56</jats:sup>Ni mass mainly due to the fallback that leads to the light curve being dimmer and slower. By comparing the synthetic light curves with the observed ones, we show that SN 2019dge can be solely powered by <jats:sup>56</jats:sup>Ni synthesized during the explosion of a progenitor with <jats:italic>M</jats:italic> <jats:sub>CO</jats:sub> ≲ 1.6 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> while iPTF 14gqr cannot be explained by the <jats:sup>56</jats:sup>Ni-powered model; ∼0.05 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> of <jats:sup>56</jats:sup>Ni inferred from the light-curve fitting is argued to be difficult to synthesize for ultrastripped progenitors. We consider fallback accretion onto and rotation-powered relativistic wind from the newborn neutron star (NS) as alternative energy sources and show that iPTF 14gqr could be powered by a newborn NS with a magnetic field of <jats:italic>B</jats:italic> <jats:sub> <jats:italic>p</jats:italic> </jats:sub> ∼ 10<jats:sup>15</jats:sup> G and an initial rotation period of <jats:italic>P</jats:italic> <jats:sub> <jats:italic>i</jats:italic> </jats:sub> ∼ 0.1 s.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A dozen X-ray supernova shock breakout (SN SBO) candidates were reported recently based on XMM-Newton archival data, which increased the X-ray-selected SN SBO sample by an order of magnitude. Assuming that they are genuine SN SBOs, we study the luminosity function (LF) by improving on the method used in our previous work. The light curves and the spectra of the candidates were used to derive the maximum volume within which these objects could be detected with XMM-Newton by simulation. The results show that the SN SBO LF can be described by either a broken power law (BPL) with indices (at the 68% confidence level) of 0.48 ± 0.28 and 2.11 ± 1.27 before and after the break luminosity at <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}({L}_{b}/\mathrm{erg}\,{{\rm{s}}}^{-1})$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>b</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>erg</mml:mi> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo stretchy="false">)</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5328ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> = 45.32 ± 0.55 or a single power law (SPL) with an index of 0.80 ± 0.16. The local event rate densities of SN SBOs above 5 × 10<jats:sup>42</jats:sup> erg s<jats:sup>−1</jats:sup> are consistent for two models, i.e., <jats:inline-formula> <jats:tex-math> <?CDATA ${4.6}_{-1.3}^{+1.7}\times {10}^{4}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>4.6</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.7</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5328ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> Gpc<jats:sup>−3</jats:sup> yr<jats:sup>−1</jats:sup> and <jats:inline-formula> <jats:tex-math> <?CDATA ${4.9}_{-1.4}^{+1.9}\times {10}^{4}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>4.9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.9</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac5328ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> Gpc<jats:sup>−3</jats:sup> yr<jats:sup>−1</jats:sup> for BPL and SPL models, respectively. The number of fast X-ray transients of SN SBO origin can be significantly increased by wide-field X-ray telescopes such as the Einstein Probe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Fermi Bubbles (FBs) are a pair of large-scale ellipsoidal structures extending above and below the Galactic plane almost symmetrically aligned with the Galactic center. After more than 10 yr since their discovery, their nature and origin remain unclear. Unveiling the primary emission mechanisms, whether hadronic or leptonic, is considered to be the main tool to shed light on the topic. We explore the potential key role of MeV observations of the FB, and we provide a recipe to determine the sensitivity of Compton and Compton-pair telescopes to the extended emission of the FB. We illustrate the capabilities of the Imaging Compton Telescope COMPTEL, the newly selected NASA MeV mission Compton Spectrometer and Imager, as well as the expectations for a potential future Compton-pair telescope such as the All-sky Medium Energy Gamma-ray Observatory eXplorer.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The dust mass of the well-known supernova remnant IC 443 is estimated from both the infrared emission and the visual extinction. With photometry to the images taken by Spitzer, WISE, IRAS, AKARI, and Planck, the spectral energy distribution (SED) of the dust is obtained after subtracting synchrotron radiation and considering the spectral line emission. The dust mass is derived by fitting the SED with a two-component model, which results in a warm component with a temperature of ∼53 K and mass of 0.1 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and a cold component with a temperature of ∼17 K and mass of 46 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. On the other hand, the dust mass is derived to be ∼66 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> from the visual extinction of IC 443, which is identified from the 3D Bayestar extinction map and its coincidence with the infrared emission morphology. The dust mass derived from the infrared emission is in rough agreement with that derived from extinction. However, the two can be adjusted to be more consistent by using a different dust opacity or by considering optically thick radiation. In addition, the distribution of dust temperature and mass is analyzed by fitting the SED pixel-by-pixel.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The discovery of changing-look active galactic nuclei (CLAGNs) with a significant change in optical broad emission lines (optical CLAGNs) and/or strong variation of line-of-sight column densities (X-ray CLAGNs) challenges the orientation-based AGN unification model. We explore mid-infrared (mid-IR) properties for a sample of 57 optical CLAGNs and 11 X-ray CLAGNs based on the Wide-field Infrared Survey Explorer archive data. We find that Eddington-scaled mid-IR luminosities of both optical and X-ray CLAGNs stay just between those of low-luminosity AGNs and luminous QSOs. The average Eddington-scaled mid-IR luminosities for optical and X-ray CLAGNs are ∼0.4% and ∼0.5%, respectively, which roughly correspond to the bolometric luminosity of transition between a radiatively inefficient accretion flow and a Shakura–Sunyaev disk. We estimate the time lags of the variation in the mid-IR behind that in the optical band for 13 CLAGNs with strong mid-IR variability, where the tight correlation between the time lag and the bolometric luminosity (<jats:italic>τ</jats:italic>–<jats:italic>L</jats:italic>) for CLAGNs roughly follows that found in the luminous QSOs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Dwarf galaxies are missing nearly all of their baryons and metals from the stellar disk, which are presumed to be in a bound halo or expelled beyond the virial radius. The virial temperature for galaxies with <jats:italic>M</jats:italic> <jats:sub>h</jats:sub> ∼ 10<jats:sup>9</jats:sup>–10<jats:sup>10</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> is similar to the collisional ionization equilibrium temperature for the C <jats:sc>iv</jats:sc> ion. We search for UV absorption from C <jats:sc>iv</jats:sc> in six sightlines toward three dwarf galaxies in the anti-M31 direction and at the periphery of the Local Group (<jats:italic>D</jats:italic> ≈ 1.3 Mpc; Sextans A, Sextans B, and NGC 3109). The C <jats:sc>iv</jats:sc> doublet is detected in only one of six sightlines, toward Sextans A, with <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}N({\rm{C}}\,{\rm\small{IV}})=13.06\pm 0.08$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mi>N</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mspace width="0.25em" /> <mml:mi mathsize="small" mathvariant="normal">IV</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>13.06</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.08</mml:mn> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac51dfieqn1.gif" xlink:type="simple" /> </jats:inline-formula>. This is consistent with our gaseous halo models, where the halo gas mass is determined by the cooling rate, feedback, and the star formation rate; the inclusion of photoionization is an essential ingredient. This model can also reproduce the higher detection rate of O <jats:sc>vi</jats:sc> absorption in other dwarf samples (beyond the Local Group), with C <jats:sc>iv</jats:sc> only detectable within ∼0.5<jats:italic>R</jats:italic> <jats:sub>vir</jats:sub>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Gas mass is a fundamental quantity of protoplanetary disks that directly relates to their ability to form planets. Because we are unable to observe the bulk H<jats:sub>2</jats:sub> content of disks directly, we rely on indirect tracers to provide quantitative mass estimates. Current estimates for the gas masses of the observed disk population in the Lupus star-forming region are based on measurements of isotopologues of CO. However, without additional constraints, the degeneracy between H<jats:sub>2</jats:sub> mass and the elemental composition of the gas leads to large uncertainties in such estimates. Here, we explore the gas compositions of seven disks from the Lupus sample representing a range of CO-to-dust ratios. With Band 6 and 7 ALMA observations, we measure line emission for HCO<jats:sup>+</jats:sup>, HCN, and N<jats:sub>2</jats:sub>H<jats:sup>+</jats:sup>. We find a tentative correlation among the line fluxes for these three molecular species across the sample, but no correlation with <jats:sup>13</jats:sup>CO or submillimeter continuum fluxes. For the three disks where N<jats:sub>2</jats:sub>H<jats:sup>+</jats:sup> is detected, we find that a combination of high disk gas masses and subinterstellar C/H and O/H are needed to reproduce the observed values. We find increases of ∼10–100× previous mass estimates are required to match the observed line fluxes. This work highlights how multimolecular studies are essential for constraining the physical and chemical properties of the gas in populations of protoplanetary disks, and that CO isotopologues alone are not sufficient for determining the mass of many observed disks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>This work presents a new physically motivated supervised machine-learning method, <jats:sc>hydro-bam</jats:sc>, to reproduce the three-dimensional Ly<jats:italic>α</jats:italic> forest field in real and redshift space, which learns from a reference hydrodynamic simulation and thereby saves about seven orders of magnitude in computing time. We show that our method is accurate up to <jats:italic>k</jats:italic> ∼ 1 <jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup> in the one- (probability distribution function), two- (power spectra), and three-point (bispectra) statistics of the reconstructed fields. When compared to the reference simulation including redshift-space distortions, our method achieves deviations of ≲2% up to <jats:italic>k</jats:italic> = 0.6 <jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup> in the monopole and ≲5% up to <jats:italic>k</jats:italic> = 0.9 <jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup> in the quadrupole. The bispectrum is well reproduced for triangle configurations with sides up to <jats:italic>k</jats:italic> = 0.8 <jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup>. In contrast, the commonly adopted Fluctuating Gunn–Peterson approximation shows significant deviations, already when peculiar motions are not included (real space) at configurations with sides of <jats:italic>k</jats:italic> = 0.2–0.4 <jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup> in the bispectrum and is also significantly less accurate in the power spectrum (within 5% up to <jats:italic>k</jats:italic> = 0.7 <jats:italic>h</jats:italic> Mpc<jats:sup>−1</jats:sup>). We conclude that an accurate analysis of the Ly<jats:italic>α</jats:italic> forest requires considering the complex baryonic thermodynamical large-scale structure relations. Our hierarchical domain-specific machine-learning method can efficiently exploit this and is ready to generate accurate Ly<jats:italic>α</jats:italic> forest mock catalogs covering the large volumes required by surveys such as DESI and WEAVE.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Current theoretical models predict a mass gap with a dearth of stellar black holes (BHs) between roughly 50 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and 100 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, while above the range accessible through massive star evolution, intermediate-mass BHs (IMBHs) still remain elusive. Repeated mergers of binary BHs, detectable via gravitational-wave emission with the current LIGO/Virgo/Kagra interferometers and future detectors such as LISA or the Einstein Telescope, can form both mass-gap BHs and IMBHs. Here we explore the possibility that mass-gap BHs and IMBHs are born as a result of successive BH mergers in dense star clusters. In particular, nuclear star clusters at the centers of galaxies have deep enough potential wells to retain most of the BH merger products after they receive significant recoil kicks due to anisotropic emission of gravitational radiation. Using for the first time simulations that include full stellar evolution, we show that a massive stellar BH seed can easily grow to ∼10<jats:sup>3</jats:sup>–10<jats:sup>4</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> as a result of repeated mergers with other smaller BHs. We find that lowering the cluster metallicity leads to larger final BH masses. We also show that the growing BH spin tends to decrease in magnitude with the number of mergers so that a negative correlation exists between the final mass and spin of the resulting IMBHs. Assumptions about the birth spins of stellar BHs affect our results significantly, with low birth spins leading to the production of a larger population of massive BHs.</jats:p>