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<jats:title>Abstract</jats:title> <jats:p>The explosion mechanism of core-collapse supernovae is not fully understood yet. In this work, we give constraints on the explosion timescale based on <jats:sup>56</jats:sup>Ni synthesized by supernova explosions. First, we systematically analyze multiband light curves of 82 stripped-envelope supernovae (SESNe) to obtain bolometric light curves, which is among the largest samples of the bolometric light curves of SESNe derived from the multiband spectral energy distribution. We measure the decline timescale and the peak luminosity of the light curves and estimate the ejecta mass (<jats:italic>M</jats:italic> <jats:sub>ej</jats:sub>) and <jats:sup>56</jats:sup>Ni mass (<jats:italic>M</jats:italic> <jats:sub>Ni</jats:sub>) to connect the observed properties with the explosion physics. We then carry out one-dimensional hydrodynamics and nucleosynthesis calculations, varying the progenitor mass and the explosion timescale. From the calculations, we show that the maximum <jats:sup>56</jats:sup>Ni mass that <jats:sup>56</jats:sup>Ni-powered SNe can reach is expressed as <jats:italic>M</jats:italic> <jats:sub>Ni</jats:sub> ≲ 0.2 <jats:italic>M</jats:italic> <jats:sub>ej</jats:sub>. Comparing the results from the observations and the calculations, we show that the explosion timescale shorter than 0.3 s explains the synthesized <jats:sup>56</jats:sup>Ni mass of the majority of the SESNe.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We examine the UV/X-ray properties of 1378 quasars in order to link empirical correlations to theoretical models of the physical mechanisms dominating quasars as a function of mass and accretion rate. The clarity of these correlations is improved when (1) using C <jats:sc>iv</jats:sc> broad emission line equivalent width (EQW) and blueshift (relative to systemic) values calculated from high signal-to-noise ratio reconstructions of optical/UV spectra and (2) removing quasars expected to be absorbed based on their UV/X-ray spectral slopes. In addition to using the traditional C <jats:sc>iv</jats:sc> parameter space measures of C <jats:sc>iv</jats:sc> EQW and blueshift, we define a “C <jats:sc>iv</jats:sc> ∥ distance” along a best-fit polynomial curve that incorporates information from both C <jats:sc>iv</jats:sc> parameters. We find that the C <jats:sc>iv</jats:sc> ∥ distance is linearly correlated with both the optical-to-X-ray slope, <jats:italic>α</jats:italic> <jats:sub>ox</jats:sub>, and broad-line He <jats:sc>ii</jats:sc> EQW, which are known spectral energy distribution indicators, but does not require X-ray or high spectral resolution UV observations to compute. The C <jats:sc>iv</jats:sc> ∥ distance may be a better indicator of the mass-weighted accretion rate, parameterized by <jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub>, than the C <jats:sc>iv</jats:sc> EQW or blueshift alone, as those relationships are known to break down at the extrema. Conversely, there is only a weak correlation with the X-ray energy index (Γ), an alternate <jats:italic>L</jats:italic>/<jats:italic>L</jats:italic> <jats:sub>Edd</jats:sub> indicator. We find no X-ray or optical trends in the direction perpendicular to the C <jats:sc>iv</jats:sc> distance that could be used to reveal differences in accretion disk, wind, or corona structure that could be widening the C <jats:sc>iv</jats:sc> EQW–blueshift distribution. A different parameter (such as metallicity) not traced by these data must come into play.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Sgr B region, including Sgr B1 and Sgr B2, is one of the most active star-forming regions in the Galaxy. Hasegawa et al. originally proposed that Sgr B2 was formed by a cloud–cloud collision (CCC) between two clouds with velocities of ∼45 km s<jats:sup>−1</jats:sup> and ∼75 km s<jats:sup>−1</jats:sup>. However, some recent observational studies conflict with this scenario. We have reanalyzed this region, by using recent, fully sampled, dense-gas data and by employing a recently developed CCC identification methodology, with which we have successfully identified more than 50 CCCs and compared them at various wavelengths. We found two velocity components that are widely spread across this region and that show clear signatures of a CCC, each with a mass of ∼10<jats:sup>6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. Based on these observational results, we suggest an alternative scenario, in which contiguous collisions between two velocity features with a relative velocity of ∼20 km s<jats:sup>−1</jats:sup> created both Sgr B1 and Sgr B2. The physical parameters, such as the column density and the relative velocity of the colliding clouds, satisfy a relation that has been found to apply to the most massive Galactic CCCs, meaning that the triggering of high-mass star formation in the Galaxy and starbursts in external galaxies can be understood as being due to the same physical CCC process.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We identify members of 65 open clusters in the solar neighborhood using the machine-learning algorithm <jats:monospace>StarGO</jats:monospace> based on Gaia EDR3 data. After adding members of 20 clusters from previous studies we obtain 85 clusters, and study their morphology and kinematics. We classify the substructures outside the tidal radius into four categories: filamentary (f1) and fractal (f2) for clusters &lt;100 Myr, and halo (h) and tidal tail (t) for clusters &gt;100 Myr. The kinematical substructures of f1-type clusters are elongated; these resemble the disrupted cluster Group X. Kinematic tails are distinct in t-type clusters, especially Pleiades. We identify 29 hierarchical groups in four young regions (Alessi 20, IC 348, LP 2373, LP 2442); 10 among these are new. The hierarchical groups form filament networks. Two regions (Alessi 20, LP 2373) exhibit global <jats:italic>orthogonal</jats:italic> expansion (stellar motion perpendicular to the filament), which might cause complete dispersal. Infalling-like flows (stellar motion along the filament) are found in UBC 31 and related hierarchical groups in the IC 348 region. Stellar groups in the LP 2442 region (LP 2442 gp 1–5) are spatially well mixed but kinematically coherent. A merging process might be ongoing in the LP 2442 subgroups. For younger systems (≲30 Myr), the mean axis ratio, cluster mass, and half-mass–radius tend to increase with age values. These correlations between structural parameters may imply two dynamical processes occurring in the hierarchical formation scenario in young stellar groups: (1) filament dissolution and (2) subgroup mergers.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Are WO-type Wolf–Rayet (WR) stars in the final stage of massive star evolution before core-collapse? Although WC- and WO-type WRs have very similar spectra, WOs show a much stronger O <jats:sc>vi</jats:sc> <jats:italic>λλ</jats:italic>3811,34 emission-line feature. This has usually been interpreted to mean that WOs are more oxygen rich than WCs, and thus further evolved. However, previous studies have failed to model this line, leaving the relative abundances uncertain, and the relationship between the two types unresolved. To answer this fundamental question, we modeled six WCs and two WOs in the LMC using UV, optical, and NIR spectra with the radiative transfer code <jats:sc>cmfgen</jats:sc> in order to determine their physical properties. We find that WOs are not richer in oxygen; rather, the O <jats:sc>vi</jats:sc> feature is insensitive to the abundance. However, the WOs have a significantly higher carbon and lower helium content than the WCs, and hence are further evolved. A comparison of our results with single-star Geneva and binary BPASS evolutionary models show that, while many properties match, there is more carbon and less oxygen in the WOs than either set of evolutionary model predicts. This discrepancy may be due to the large uncertainty in the <jats:sup>12</jats:sup>C+<jats:sup>4</jats:sup>He → <jats:sup>16</jats:sup>O nuclear reaction rate; we show that if the Kunz et al. rate is decreased by a factor of 25%–50%, then there would be a good match with the observations. It would also help explain the LIGO/VIRGO detection of black holes whose masses are in the theoretical upper mass gap.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>During the transition phase from a prestellar to a protostellar cloud core, one or several protostars can form within a single gas core. The detailed physical processes of this transition, however, remain unclear. We present 1.3 mm dust continuum and molecular line observations with the Atacama Large Millimeter/submillimeter Array toward 43 protostellar cores in the Orion molecular cloud complex (<jats:italic>λ</jats:italic> Orionis, Orion B, and Orion A) with an angular resolution of ∼0.″35 (∼140 au). In total, we detect 13 binary/multiple systems. We derive an overall multiplicity frequency (MF) of 28% ± 4% and a companion star fraction (CSF) of 51% ± 6%, over a separation range of 300–8900 au. The median separation of companions is about 2100 au. The occurrence of stellar multiplicity may depend on the physical characteristics of the dense cores. Notably, those containing binary/multiple systems tend to show a higher gas density and Mach number than cores forming single stars. The integral-shaped filament of the Orion A giant molecular cloud (GMC), which has the highest gas density and hosts high-mass star formation in its central region (the Orion Nebula cluster), shows the highest MF and CSF among the Orion GMCs. In contrast, the <jats:italic>λ</jats:italic> Orionis GMC has a lower MF and CSF than the Orion B and Orion A GMCs, indicating that feedback from H <jats:sc>ii</jats:sc> regions may suppress the formation of multiple systems. We also find that the protostars comprising a binary/multiple system are usually at different evolutionary stages.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The advent of sensitive gravitational-wave (GW) detectors, coupled with wide-field, high-cadence optical time-domain surveys, raises the possibility of the first joint GW–electromagnetic detections of core-collapse supernovae (CCSNe). For targeted searches of GWs from CCSNe, optical observations can be used to increase the sensitivity of the search by restricting the relevant time interval, defined here as the GW search window (GSW). The extent of the GSW is a critical factor in determining the achievable false alarm probability for a triggered CCSN search. The ability to constrain the GSW from optical observations depends on how early a CCSN is detected, as well as the ability to model the early optical emission. Here we present several approaches to constrain the GSW, ranging in complexity from model-independent analytical fits of the early light curve, model-dependent fits of the rising or entire light curve, and a new data-driven approach using existing well-sampled CCSN light curves from Kepler and the Transiting Exoplanet Survey Satellite. We use these approaches to determine the time of core-collapse and its associated uncertainty (i.e., the GSW). We apply our methods to two Type II SNe that occurred during LIGO/Virgo Observing Run 3: SN 2019fcn and SN 2019ejj (both in the same galaxy at <jats:italic>d</jats:italic> = 15.7 Mpc). Our approach shortens the duration of the GSW and improves the robustness of the GSW compared to the techniques used in past GW CCSN searches.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Reionization Era Bright Emission Line Survey (REBELS) is a cycle-7 ALMA Large Program (LP) that is identifying and performing a first characterization of many of the most luminous star-forming galaxies known in the <jats:italic>z</jats:italic> &gt; 6.5 universe. REBELS is providing this probe by systematically scanning 40 of the brightest UV-selected galaxies identified over a 7 deg<jats:sup>2</jats:sup> area for bright [C <jats:sc>ii</jats:sc>]<jats:sub>158 <jats:italic>μ</jats:italic>m</jats:sub> and [O <jats:sc>iii</jats:sc>]<jats:sub>88 <jats:italic>μ</jats:italic>m</jats:sub> lines and dust-continuum emission. Selection of the 40 REBELS targets was done by combining our own and other photometric selections, each of which is subject to extensive vetting using three completely independent sets of photometry and template-fitting codes. Building on the observational strategy deployed in two pilot programs, we are increasing the number of massive interstellar medium (ISM) reservoirs known at <jats:italic>z</jats:italic> &gt; 6.5 by ∼4–5× to &gt;30. In this manuscript, we motivate the observational strategy deployed in the REBELS program and present initial results. Based on the first-year observations, 18 highly significant ≥ 7<jats:italic>σ</jats:italic> [C <jats:sc>ii</jats:sc>]<jats:sub>158 <jats:italic>μ</jats:italic>m</jats:sub> lines have already been discovered, the bulk of which (13/18) also show ≥3.3<jats:italic>σ</jats:italic> dust-continuum emission. These newly discovered lines more than triple the number of bright ISM-cooling lines known in the <jats:italic>z</jats:italic> &gt; 6.5 universe, such that the number of ALMA-derived redshifts at <jats:italic>z</jats:italic> &gt; 6.5 rival Ly<jats:italic>α</jats:italic> discoveries. An analysis of the completeness of our search results versus star formation rate (SFR) suggests an ∼79% efficiency in scanning for [C <jats:sc>ii</jats:sc>]<jats:sub>158 <jats:italic>μ</jats:italic>m</jats:sub> when the SFR<jats:sub>UV+IR</jats:sub> is &gt;28 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup>. These new LP results further demonstrate ALMA’s efficiency as a “redshift machine,” particularly in the Epoch of Reionization.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The task of finding the potential of a thin circular disk with power-law radial density profile is revisited. The result, given in terms of infinite Legendre-type series in the above reference, has now been obtained in closed form thanks to the method of Conway employing Bessel functions. Starting from a closed-form expression for the potential generated by the elementary density term <jats:italic>ρ</jats:italic> <jats:sup>2<jats:italic>l</jats:italic> </jats:sup>, we cover more generic—finite solid or infinite annular—thin disks using superposition and/or inversion with respect to the rim. We check several specific cases against the series-expansion form by numerical evaluation at particular locations. Finally, we add a method to obtain a closed-form solution for <jats:italic>finite</jats:italic> annular disks whose density is of “bump” radial shape, as modeled by a suitable combination of several powers of radius. Density and azimuthal pressure of the disks are illustrated on several plots, together with radial profiles of free circular velocity.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Solar extreme ultraviolet (EUV) waves are large-scale propagating disturbances in the corona. It is generally believed that a vital key to the formation of EUV waves is the rapid expansion of the loops that overlie erupting cores in solar eruptions, such as coronal mass ejections (CMEs) and solar jets. However, the details of the interaction between the erupting cores and overlying loops are not clear because the overlying loops always instantly open after energetic eruptions. Here, we present three typical jet-driven EUV waves without CMEs to study the interaction between the jets and the overlying loops that remained closed during the events. All three jets emanated from magnetic flux cancellation sites in the source regions. Interestingly, after the interactions between the jets and overlying loops, three EUV waves respectively formed ahead of the top, the near end (close to the jet source), and the far (another) end of the overlying loops. According to the magnetic field distribution of the loops extrapolated through the potential field source surface method, it is confirmed that the birthplaces of three jet-driven EUV waves were around the parts of the overlying loops with the weakest magnetic field strengths. We suggest that the jet-driven EUV waves preferentially occur at the weakest part of the overlying loops, and the location can be subject to the magnetic field intensity around the ends of the loops.</jats:p>