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<jats:title>Abstract</jats:title> <jats:p>Double-peaked light curves are observed for some Type Ic supernovae (SNe Ic), including LSQ14efd, iPTF15dtg, and SN 2020bvc. One possible explanation of the first peak would be shock-cooling emission from massive extended material around the progenitor, which is produced by mass eruption or rapid expansion of the outermost layers of the progenitor shortly before the supernova explosion. We investigate the effects of such circumstellar matter (CSM) on the multiband optical light curves of SNe Ic using the radiation hydrodynamics code STELLA. Two different SNe Ic progenitor masses at the pre-SN stage (3.93 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and 8.26 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>) are considered in the SN models. The adopted parameter space consists of the CSM mass of <jats:italic>M</jats:italic> <jats:sub>CSM</jats:sub> = 0.05–0.3<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, the CSM radius of <jats:italic>R</jats:italic> <jats:sub>CSM</jats:sub> = 10<jats:sup>13</jats:sup>–10<jats:sup>15</jats:sup> cm, and the explosion energy of <jats:italic>E</jats:italic> <jats:sub>burst</jats:sub> = (1.0–12.0) × 10<jats:sup>51</jats:sup> erg. We also investigate the effects of the radioactive nickel distribution on the overall shape of the light curve and the color evolution. Comparison of our SN models with the double-peaked SNe Ic LSQ14efd, iPTF15dtg, and SN 2020bvc indicates that these three SNe Ic had a similar CSM structure (i.e., <jats:italic>M</jats:italic> <jats:sub>CSM</jats:sub> ≈ 0.1–0.2<jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and <jats:italic>R</jats:italic> <jats:sub>CSM</jats:sub> = 10<jats:sup>13</jats:sup>–10<jats:sup>14</jats:sup> cm), which might imply a common mechanism for their CSM formation. The implied mass-loss rate of <jats:inline-formula> <jats:tex-math> <?CDATA $\dot{M}\gtrsim 1.0\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>≳</mml:mo> <mml:mn>1.0</mml:mn> <mml:mspace width="0.25em" /> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi>yr</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabe0b1ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> is too high to be explained by the previously suggested scenarios for pre-SN eruption, which calls for a novel mechanism.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The emission feature of intermittent pulsars is significant for understanding the pulsar emission mechanism. Using the observational evidence of radio emissions turning on and off and the corresponding spin-down rates in these two states of an intermittent pulsar, we will examine the polar-cap potential drop, gap height, and curvature radii of a few intermittent pulsars within the regime of the pulsar polar-cap emission theory by applying the current loss and energy flux of particle flow to pulsar braking, which are generally associated with radio emission from the polar cap. It is seen that the polar-cap parameters of the intermittent pulsars are almost equal to their maximum values, which is the main prediction of the pulsar polar-cap theory with respect to the breaking of the radio emission. It is also noticed that the intermittent pulsars are distributed near the dipole death line in the <jats:inline-formula> <jats:tex-math> <?CDATA $P\mbox{--}\dot{P}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabdb37ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> diagram, which is consistent with their emission features and the calculated polar-cap parameters. To further confirm the state switching of the intermittent pulsar, the relationships among spin-down rate, gap height, potential drop, and activity duty cycles of PSR B1931+24 are discussed. It is found that the gap height has an anticorrelation with the activity duty cycle, which indicates that the intermittency of the radio emission has a close connection to the gap height, as indicated by the pulsar polar-cap emission theory.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the analysis of the diffuse, low column density H <jats:sc>i</jats:sc> environment of 18 MHONGOOSE galaxies. We obtained deep observations with the Robert C. Byrd Green Bank Telescope and reached down to a 3<jats:italic>σ</jats:italic> column density detection limit of <jats:italic>N</jats:italic> <jats:sub>HI</jats:sub> = 6.3 × 10<jats:sup>17</jats:sup> cm<jats:sup>−2</jats:sup> over a 20 km s<jats:sup>−1</jats:sup> line width. We analyze the environment around these galaxies, with a focus on H <jats:sc>i</jats:sc> gas that reaches column densities below <jats:italic>N</jats:italic> <jats:sub>HI</jats:sub> = 10<jats:sup>19</jats:sup> cm<jats:sup>−2</jats:sup>. We calculate the total amount of H <jats:sc>i</jats:sc> gas in and around the galaxies, revealing that nearly all of these galaxies contained excess H <jats:sc>i</jats:sc> outside of their disks. We quantify the amount of diffuse gas in the maps of each galaxy, defined by H <jats:sc>i</jats:sc> gas with column densities below 10<jats:sup>19</jats:sup> cm<jats:sup>−2</jats:sup>, and find a large spread in percentages of diffuse gas. However, by binning the percentage of diffuse H <jats:sc>i</jats:sc> into quarters, we find that the bin with the largest number of galaxies is the lowest quartile (0%–25% diffuse H <jats:sc>i</jats:sc>). We identified several galaxies that may be undergoing gas accretion onto the galaxy disk using multiple methods of analysis, including azimuthally averaging column densities beyond the disk, and identifying structure within our integrated intensity (moment 0) maps. We measured H <jats:sc>i</jats:sc> mass outside the disks of most of our galaxies, with rising cumulative flux even at large radii. We also find a strong correlation between the fraction of diffuse gas in a galaxy and its baryonic mass, and we test this correlation using both Spearman and Pearson correlation coefficients. We see evidence of a dark matter halo mass threshold of <jats:italic>M</jats:italic> <jats:sub>halo</jats:sub> ∼ 10<jats:sup>11.1</jats:sup> <jats:italic>M</jats:italic> <jats:sub>☉</jats:sub> in which galaxies with high fractions of diffuse H <jats:sc>i</jats:sc> all reside below. It is in this regime that cold-mode accretion should dominate. Finally, we suggest a rotation velocity of <jats:italic>v</jats:italic> <jats:sub>rot</jats:sub> ∼ 80 km s<jats:sup>−1</jats:sup> as an upper threshold to find diffuse-gas-dominated galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We explore upper limits for the largest avalanches or catastrophes in nonlinear energy dissipation systems governed by self-organized criticality. We generalize the idealized “straight” power-law size distribution and Pareto distribution functions in order to accommodate incomplete sampling, limited instrumental sensitivity, finite system-size effects, and “Black Swan” and “Dragon King” extreme events. Our findings are as follows. (i) Solar flares show no finite system-size limits up to <jats:italic>L</jats:italic> ≲ 200 Mm, but solar flare durations reveal an upper flare duration limit of ≲6 hr. (ii) Stellar flares observed with Kepler exhibit inertial ranges of <jats:italic>E</jats:italic> ≈ 10<jats:sup>34</jats:sup>–10<jats:sup>37</jats:sup> erg, finite system-size ranges of <jats:italic>E</jats:italic> ≈ 10<jats:sup>37</jats:sup>–10<jats:sup>38</jats:sup> erg, and extreme events at <jats:italic>E</jats:italic> ≈ (1–5) × 10<jats:sup>38</jats:sup> erg. (iii) The maximum flare energies of different spectral type stars (M, K, G, F, A, giants) reveal a positive correlation with the stellar radius, which indicates a finite system-size limit imposed by the stellar surface area. Fitting our finite system-size models to terrestrial data sets (earthquakes, wildfires, city sizes, blackouts, terrorism, words, surnames, web links) yields evidence (in half of the cases) for finite system-size limits and extreme events, which can be modeled with dual power-law size distributions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Understanding the origin of chondritic components and their accretion pathways is critical to unraveling the magnitude of mass transport in the protoplanetary disk, as well as the accretionary history of the terrestrial planet region and, by extension, its prebiotic inventory. Here we trace the heritage of pristine components from the relatively unaltered CV chondrite Leoville through their mass-independent Cr and mass-dependent Zn isotope compositions. Investigating these chondritic fractions in such detail reveals an onion-shell structure of chondrules, which is characterized by <jats:sup>54</jats:sup>Cr- and <jats:sup>66</jats:sup>Zn-poor cores surrounded by increasingly <jats:sup>54</jats:sup>Cr- and <jats:sup>66</jats:sup>Zn-rich igneous rims and an outer coating of fine-grained dust. This is interpreted as a progressive addition of <jats:sup>54</jats:sup>Cr- and <jats:sup>66</jats:sup>Zn-rich, CI-like material to the accretion region of these carbonaceous chondrites. Our findings show that the observed Cr isotopic range in chondrules from more altered CV chondrites is the result of chemical equilibration between the chondrules and matrix during secondary alteration. The <jats:sup>54</jats:sup>Cr-poor nature of the cores of Leoville chondrules implies formation in the inner solar system and subsequent massive outward chondrule transport past the Jupiter barrier. At the same time, CI-like dust is transferred inward. We propose that the accreting Earth acquired CI-like dust through this mechanism within the lifetime of the disk. This radial mixing of the chondrules and matrix shows the limited capacity of Jupiter to act as an efficient barrier and maintain the proposed noncarbonaceous and carbonaceous chondrite dichotomy over time. Finally, also considering current astrophysical models, we explore both inner and outer solar system origins for the CV chondrite parent body.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent high-resolution simulations of early structure formation have shown that externally enriched halos may form some of the first metal-enriched stars. This study utilizes a 1 comoving Mpc<jats:sup>3</jats:sup> high-resolution simulation to study the enrichment process of metal-enriched halos down to <jats:italic>z</jats:italic> = 9.3. Our simulation uniquely tracks the metals ejected from Population III stars, and we use this information to identify the origin of metals within metal-enriched halos. These halos show a wide range of metallicities, but we find that the source of metals for ≳50% of metal-enriched halos is supernova explosions of Population III stars occurring outside their virial radii. The results presented here indicate that external enrichment by metal-free stars dominates the enrichment process of halos with virial mass below 10<jats:sup>6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> down to <jats:italic>z</jats:italic> = 9.3. Despite the prevalence of external enrichment in low-mass halos, Population II stars forming due to external enrichment are rare because of the small contribution of low-mass halos to the global star formation rate combined with low metallicities toward the center of these halos resulting from metal ejecta from external sources mixing from the outside in. The enriched stars that do form through this process have absolute metallicities below 10<jats:sup>−3</jats:sup> <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>. We also find that the fraction of externally enriched halos increases with time: ∼90% of halos that are externally enriched have <jats:italic>M</jats:italic> <jats:sub>vir</jats:sub> &lt; 10<jats:sup>6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, and that pair-instability supernovae contribute the most to the enrichment of the intergalactic medium as a whole and are thus are the predominant supernova type contributing to the external enrichment of halos.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Ecliptic-poles Stellar Survey (EclipSS) collected far-ultraviolet (FUV: 1160–1420 Å) spectra of 49 nearby (<jats:italic>d</jats:italic> ≲ 100 pc) F3–K3 main-sequence stars, located at high ecliptic latitudes (north and south), using the Cosmic Origins Spectrograph of the Hubble Space Telescope. The ecliptic poles receive higher exposures from scanning missions like the Transiting Exoplanet Survey Satellite (high-precision optical photometry) and Extended Roentgen Survey with an Imaging Telescope Array (X-ray monitoring), which can deliver crucial contextual information, not otherwise easily secured. The objective was to support theoretical studies of stellar hot outer atmospheres—chromospheres (∼10<jats:sup>4</jats:sup> K) and coronae (≳1 MK)—which, among other things, can adversely impact exoplanets via host-star “space weather.” Flux–flux diagrams (e.g., C <jats:sc>ii</jats:sc> 1335 Å versus O <jats:sc>i</jats:sc> 1306 Å) were constructed for the EclipSS stars, solar Cycle 23/24 irradiances, and long-term FUV records of <jats:italic>α</jats:italic> Cen A (G2 V) and B (K1 V). The EclipSS cohort displays similar minimum (“basal”) fluxes to the Sun and solar twin <jats:italic>α</jats:italic> Cen A, in chromospheric O <jats:sc>i</jats:sc> 1306 Å. In hotter C <jats:sc>ii</jats:sc> 1335 Å, a downward slump of the basal fluxes—noted in previous, less controlled surveys—can now be explained as an effect of subsolar abundances. The consistent basal minima in chromospheric and higher temperature species at solar metallicity favor the idea that stellar analogs of the solar supergranulation network provide a baseline of high-energy emissions. The magnetic network is replenished by a “local dynamo” independently of the stellar spin. It can operate even when the starspot-spawning internal dynamo has ceased cycling, as during the Sun’s 17th century Maunder Minimum.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The diffuse interstellar medium (ISM) is dynamic, and its chemistry and evolution are determined by shock fronts as well as photodissociation. Shocks are implied by the supersonic motions and velocity dispersion, often statistically called “turbulence”. We compare models of magnetohydrodynamic (MHD) shocks, with speeds typical of cloud motions through the ISM (3–25 km s<jats:sup>−1</jats:sup>) and densities typical of cold neutral gas (∼10<jats:sup>2</jats:sup> cm<jats:sup>−3</jats:sup>), to archival observations of the H <jats:sc>i</jats:sc> 21 cm line for gas kinematics, far-infrared emission for dust mass, and mid-infrared emission for high-resolution morphology, to identify shock fronts in three high-latitude cloud pairs with masses of order 50 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. The clouds have “heads” with extended “tails,” and high-resolution images show arcs on the leading edges of the “heads” that could be individual shocks. The H <jats:sc>i</jats:sc> shows higher-velocity gas at the leading edges due to shock-accelerated material. For two cloud pairs, one cloud has an active shock indicated by broad and offset H <jats:sc>i</jats:sc>, while the other cloud has already been shocked and is predominantly “CO-dark” H<jats:sub>2</jats:sub>. Two-dimensional MHD simulations for shocks parallel to the magnetic field for pairs of clouds show a remarkable similarity to observed cloud features, including merged “tails” due to aligned flow and magnetic field, which leads to lateral confinement downstream. A parallel alignment between magnetic field and gas flow may lead to formation of small molecular clouds.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We propose a new model for treating solid-phase photoprocesses in interstellar ice analogs. In this approach, photoionization and photoexcitation are included in more detail, and the production of electronically excited (suprathermal) species is explicitly considered. In addition, we have included nonthermal, nondiffusive chemistry to account for the low-temperature characteristic of cold cores. As an initial test of our method, we have simulated two previous experimental studies involving the UV irradiation of pure solid O<jats:sub>2</jats:sub>. In contrast to previous solid-state astrochemical model calculations, which have used gas-phase photoabsorption cross-sections, we have employed solid-state cross-sections in our calculations. This method allows the model to be tested using well-constrained experiments rather than poorly constrained gas-phase abundances in interstellar medium regions. Our results indicate that inclusion of nonthermal reactions and suprathermal species allows for reproduction of low-temperature solid-phase photoprocessing that simulates interstellar ices within cold (∼10 K) dense cores such as TMC-1.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Solar active regions and sunspots are believed to be formed by the emergence of strong toroidal magnetic flux from the solar interior. Modeling of such events has focused on the dynamics of compact magnetic entities, colloquially known as “flux tubes,” often considered to be isolated magnetic structures embedded in an otherwise field-free environment. In this paper, we show that relaxing such idealized assumptions can lead to surprisingly different dynamics. We consider the rise of tube-like <jats:italic>flux concentrations</jats:italic> embedded in a large-scale volume-filling horizontal field in an initially quiescent adiabatically stratified compressible fluid. In a previous letter, we revealed the unexpected major result that concentrations whose twist is aligned with the background field at the bottom of the tube are more likely to rise than the opposite orientation (for certain values of parameters). This bias leads to a selection rule which, when applied to solar dynamics, is in agreement with the observations known as the solar hemispheric helicity rule(s) (SHHR). Here, we examine this selection mechanism in more detail than was possible in the earlier letter. We explore the dependence on parameters via simulations, delineating the Selective Rise Regime, where the bias operates. We provide a theoretical model to predict and explain the simulation dynamics. Furthermore, we create synthetic helicity maps from Monte Carlo simulations to mimic the SHHR observations, and to demonstrate that our mechanism explains the observed scatter in the rule, as well as its variation over the solar cycle.</jats:p>