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<jats:title>Abstract</jats:title> <jats:p>The cosmic-ray ionization rate (CRIR) is a key parameter in understanding the physical and chemical processes in the interstellar medium. Cosmic rays are a significant source of energy in star formation regions, impacting the physical and chemical processes that drive the formation of stars. Previous studies of the circum-molecular zone of the starburst galaxy NGC 253 have found evidence for a high CRIR value: 10<jats:sup>3</jats:sup>–10<jats:sup>6</jats:sup> times the average CRIR within the Milky Way. This is a broad constraint, and one goal of this study is to determine this value with much higher precision. We exploit ALMA observations toward the central molecular zone of NGC 253 to measure the CRIR. We first demonstrate that the abundance ratio of H<jats:sub>3</jats:sub>O<jats:sup>+</jats:sup> and SO is strongly sensitive to the CRIR. We then combine chemical and radiative transfer models with nested sampling to infer the gas properties and CRIR of several star-forming regions in NGC 253 from emission from their transitions. We find that each of the four regions modeled has a CRIR in the range (1–80) × 10<jats:sup>−14</jats:sup> s<jats:sup>−1</jats:sup> and that this result adequately fits the abundances of other species that are believed to be sensitive to cosmic rays, including C<jats:sub>2</jats:sub>H, HCO<jats:sup>+</jats:sup>, HOC<jats:sup>+</jats:sup>, and CO. From shock and photon-dominated/X-ray dominated region models, we further find that neither UV-/X-ray-driven nor shock-dominated chemistry is a viable single alternative as none of these processes can adequately fit the abundances of all of these species.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Accretion of interplanetary dust onto gas giant exoplanets is considered. Poynting–Robertson drag causes dust particles from distant reservoirs to slowly inspiral toward the star. Orbital simulations for the three-body system of the star, planet, and dust particle show that a significant fraction of the dust may accrete onto massive planets in close orbits. The deceleration of the supersonic dust in the planet’s atmosphere is modeled, including ablation by thermal evaporation and sputtering. The fraction of the accreted dust mass deposited as gas-phase atoms is found to be large for close-in orbits and massive planets. If mass outflow and vertical mixing are sufficiently weak, the accreted dust produces a constant mixing ratio of atoms and remnant dust grains below the stopping layer. When vertical mixing is included along with settling, the solutions interpolate between the mixing ratio due to the meteoric source above the homopause, and that of the well-mixed deeper atmosphere below the homopause. The line opacity from atoms and continuum opacity from remnant dust may be observable in transmission spectra for sufficiently large dust accretion rates, a grain size distribution tilted toward the blowout size, and sufficiently weak vertical mixing. If mixing is strong, the meteoric source may still act to augment heavy elements mixed up from the deep atmosphere as well as provide nucleation sites for the formation of larger particles. The possible role of the Lorentz drag force in limiting the flow speeds and mixing coefficient for pressures ≲1 mbar is discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Some dozen supernovae (SNe) associated with long gamma-ray bursts (GRBs) have been confirmed. Most of the previous studies derive the physical properties of the GRB-SNe by fitting the constructed (pseudo-)bolometric light curves. However, many GRB-SNe only have a few filter data, for which the (pseudo-)bolometric light curves are very difficult to construct. Additionally, constructing (pseudo-)bolometric light curves rely on some assumptions. In this paper, we use the multiband broken power-law plus <jats:sup>56</jats:sup>Ni model to fit the multiband light curves of the afterglows and the SNe (SN 2001ke, SN 2013dx, and SN 2016jca) associated with three GRBs (GRB 011121, GRB 130702A, and GRB 161219B). We find our model can account for the multiband light curves of the three GRB-SNe (except for the late-time <jats:italic>z</jats:italic>-band light curve of two events), indicating that the model is a reliable model. The <jats:sup>56</jats:sup>Ni masses we derive are higher than those in the literature. This might be due to the fact that the <jats:sup>56</jats:sup>Ni masses in the literature are usually obtained by fitting the pseudo-bolometric light curves whose luminosities are usually (significantly) underestimated. We suggest that the multiband model can not only be used to fit the multiband light curves of GRB-SNe that have many filter observations, but also fit those having sparse data.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Under the right conditions, brown dwarfs that gain enough mass late in their lives to cross the hydrogen-burning limit will not turn into low-mass stars, but rather remain essentially brown dwarf–like. While these objects, called either beige dwarfs or overmassive brown dwarfs, may exist in principle, it remains unclear exactly how they would form astrophysically. We show that accretion from AGB winds, aided by the wind Roche lobe overflow mechanism, is likely to produce a substantial population of observable overmassive brown dwarfs, though other mechanisms are still plausible. Specifically, we predict that Sun-like stars born with a massive brown dwarf companion on an orbit with a semimajor axis of order 10 au will likely produce overmassive brown dwarfs, which may be found today as companions to the donor star's remnant white dwarf. The identification and characterization of such an object would produce unique constraints on binary evolution, because there is a solid upper limit on the brown dwarf's initial mass.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Buckminsterfullerene, C<jats:sub>60</jats:sub>, is the largest molecule observed to date in interstellar and circumstellar environments. The mechanism of formation of this molecule is actively debated. Despite targeted searches in primitive carbonaceous chondrites, no unambiguous detection of C<jats:sub>60</jats:sub> in a meteorite has been reported to date. Here we report the first firm detection of fullerenes, from C<jats:sub>30</jats:sub> to at least C<jats:sub>100</jats:sub>, in the Almahata Sitta (AhS) polymict ureilite meteorite. This detection was achieved using highly sensitive laser desorption laser ionization mass spectrometry. Fullerenes have been unambiguously detected in seven clasts of AhS ureilites. Molecular family analysis shows that fullerenes are from a different reservoir compared to the polycyclic aromatic hydrocarbons detected in the same samples. The fullerene family correlates best with carbon clusters, some of which may have been formed by the destruction of solid carbon phases by the impacting laser. We show that the detected fullerenes are not formed in this way. We suggest that fullerenes are an intrinsic component of a specific carbon phase that has yet to be identified. The nondetection of fullerenes in the Murchison and Allende bulk samples, while using the same experimental conditions, suggests that this phase is absent or less abundant in these primitive chondrites. The former case would support the formation of fullerenes by shock-wave processing of carbonaceous phases in the ureilite parent body. However, there are no experimental data to support this scenario. This leaves open the possibility that fullerenes are an interstellar heritage and a messenger of interstellar processes.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The X8.2-class limb flare on 2017 September 10 is among the best studied solar flare events owing to its great similarity to the standard flare model and the broad coverage by multiple spacecraft and ground-based observations. These multiwavelength observations indicate that electron acceleration and transport are efficient in the reconnection and flare looptop regions. However, there lacks a comprehensive model for explaining and interpreting the multi-faceted observations. In this work, we model the electron acceleration and transport in the early impulsive phase of this flare. We solve the Parker transport equation that includes the primary acceleration mechanism during magnetic reconnection in the large-scale flare region modeled by MHD simulations. We find that electrons are accelerated up to several MeV and fill a large volume of the reconnection region, similar to the observations shown in microwaves. The electron spatial distribution and spectral shape in the looptop region agree well with those derived from the microwave and hard X-ray emissions before magnetic islands grow large and dominate the acceleration. Future emission modelings using the electron maps will enable direct comparison with microwave and hard X-ray observations. These results shed new light on the electron acceleration and transport in a broad region of solar flares within a data-constrained realistic flare geometry.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In this paper we discuss and confront recent results on metallicity variations in the local interstellar medium, obtained from observations of H <jats:sc>ii</jats:sc> regions and neutral clouds of the Galactic thin disk, and compare them with recent high-quality metallicity determinations of other tracers of the chemical composition of the interstellar medium as B-type stars, classical Cepheids, and young clusters. We find that the metallicity variations obtained for these last kinds of objects are consistent with each other and with that obtained for H <jats:sc>ii</jats:sc> regions but significantly smaller than those obtained for neutral clouds. We also discuss the presence of a large population of low-metallicity clouds as the possible origin for large metallicity variations in the local Galactic thin disk. We find that such a hypothesis does not seem compatible with: (a) what is predicted by theoretical studies of gas mixing in galactic disks, and (b) the models and observations on the metallicity of high-velocity clouds and their evolution as they mix with the surrounding medium in their fall onto the Galactic plane. We conclude that most of the evidence favors that the chemical composition of the interstellar medium in the solar neighborhood is highly homogeneous.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The central strong activities in core-collapse supernovae are expected to produce the overturning of the Fe- and Si/O-rich ejecta during the supernova explosion based on multidimensional simulations. X-ray observations of the supernova remnant Cassiopeia A have indicated that the Fe-rich ejecta lies outside the Si-rich materials in the southeastern region, which is consistent with the hypothesis on the inversion of the ejecta. We investigate the kinematic and nucleosynthetic properties of the inverted ejecta layers in detail to understand its formation process using the data taken by the Chandra X-Ray Observatory. Three-dimensional velocities of Fe- and Si/O-rich ejecta are obtained as &gt;4500 km s<jats:sup>−1</jats:sup> and ∼2000–3000 km s<jats:sup>−1</jats:sup>, respectively, by combining proper motion and line-of-sight velocities, indicating that the velocity of the Si/O-rich ejecta is slower than that of the Fe-rich ejecta from the early stages of the explosion. To constrain their burning regime, the Cr/Fe mass ratios are evaluated as <jats:inline-formula> <jats:tex-math> <?CDATA ${0.51}_{-0.10}^{+0.11}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.51</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac6e63ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>% in the outermost Fe-rich region and <jats:inline-formula> <jats:tex-math> <?CDATA ${1.24}_{-0.20}^{+0.19}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>1.24</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.20</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.19</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac6e63ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>% in the inner Fe/Si-rich region, suggesting that the complete Si burning layer is invertedly located to the incomplete Si burning layer. All the results support the ejecta overturning at the early stages of the remnant’s evolution or during the supernova explosion of Cassiopeia A.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In the solar corona, magnetic reconnection occurs due to the finite resistivity of the plasma. At the same time, this resistivity leads to ohmic heating. Therefore, the reconnecting current sheet should heat the surrounding plasma. This paper presents experimental evidence of such plasma heating being caused by magnetic reconnection. We observed the effect during a C1.4 solar flare on 2003 February 16 at the active region NOAA 10278, near the solar limb. Thanks to such a location, we successfully identified all the principal elements of the flare: the flare arcade, the flux rope, and, most importantly, the presumed position of the current sheet. By analyzing the monochromatic X-ray images of the Sun obtained by the CORONAS-F/SPIRIT instrument in the Mg <jats:sc>xii</jats:sc> 8.42 Å spectral line, we detected a high-temperature (<jats:italic>T</jats:italic> ≥ 4 MK) emission at the predicted location of the current sheet. The high-temperature emission appeared during the CME’s impulsive acceleration phase. We believe that this additionally confirms that the plasma heating around the current sheet and the magnetic reconnection inside the current sheet are strongly connected.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Solar flares may be the best-known examples of the explosive conversion of magnetic energy into bulk motion, plasma heating, and particle acceleration via magnetic reconnection. The energy source for all flares is the highly sheared magnetic field of a filament channel above a polarity inversion line (PIL). During the flare, this shear field becomes the so-called reconnection guide field (i.e., the nonreconnecting component), which has been shown to play a major role in determining key properties of the reconnection, including the efficiency of particle acceleration. We present new high-resolution, three-dimensional, magnetohydrodynamics simulations that reveal the detailed evolution of the magnetic shear/guide field throughout an eruptive flare. The magnetic shear evolves in three distinct phases: shear first builds up in a narrow region about the PIL, then expands outward to form a thin vertical current sheet, and finally is transferred by flare reconnection into an arcade of sheared flare loops and an erupting flux rope. We demonstrate how the guide field may be inferred from observations of the sheared flare loops. Our results indicate that initially the guide field is larger by about a factor of 5 than the reconnecting component, but it weakens by more than an order of magnitude over the course of the flare. Instantaneously, the guide field also varies spatially over a similar range along the three-dimensional current sheet. We discuss the implications of the remarkable variability of the guide field for the timing and localization of efficient particle acceleration in flares.</jats:p>