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<jats:title>Abstract</jats:title> <jats:p>Unbiased understanding of molecular distributions in a disk/envelope system of a low-mass protostellar source is crucial for investigating physical and chemical evolution processes. We have observed 23 molecular lines toward the Class 0 protostellar source L483 with ALMA and have performed principal component analysis (PCA) for their cube data (PCA-3D) to characterize their distributions and velocity structures in the vicinity of the protostar. The sum of the contributions of the first three components is 63.1%. Most oxygen-bearing complex organic molecule lines have a large correlation with the first principal component (PC1), representing the overall structure of the disk/envelope system around the protostar. Contrary, the C<jats:sup>18</jats:sup>O and SiO emissions show small and negative correlations with PC1. The NH<jats:sub>2</jats:sub>CHO lines stand out conspicuously at the second principal component (PC2), revealing more compact distribution. The HNCO lines and the high-excitation line of CH<jats:sub>3</jats:sub>OH have a similar trend for PC2 to NH<jats:sub>2</jats:sub>CHO. On the other hand, C<jats:sup>18</jats:sup>O is well correlated with the third principal component (PC3). Thus, PCA-3D enables us to elucidate the similarities and the differences of the distributions and the velocity structures among molecular lines simultaneously, so that the chemical differentiation between the oxygen-bearing complex organic molecules and the nitrogen-bearing ones is revealed in this source. We have also conducted PCA for the moment 0 maps (PCA-2D) and that for the spectral line profiles (PCA-1D). While they can extract part of characteristics of the molecular line data, PCA-3D is essential for comprehensive understandings. Characteristic features of the molecular line distributions are discussed on NH<jats:sub>2</jats:sub>CHO.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present long-duration numerical simulations of the tidal disruption of stars modeled with accurate stellar structures and spanning a range of pericenter distances, corresponding to cases where the stars are partially and completely disrupted. We substantiate the prediction that the late-time power-law index of the fallback rate <jats:italic>n</jats:italic> <jats:sub>∞</jats:sub> ≃ −5/3 for full disruptions, while for partial disruptions—in which the central part of the star survives the tidal encounter intact—we show that <jats:italic>n</jats:italic> <jats:sub>∞</jats:sub> ≃ −9/4. For the subset of simulations where the pericenter distance is close to that which delineates full from partial disruption, we find that a stellar core can reform after the star has been completely destroyed; for these events the energy of the zombie core is slightly positive, which results in late-time evolution from <jats:italic>n</jats:italic> ≃ −9/4 to <jats:italic>n</jats:italic> ≃ −5/3. We find that self-gravity can generate an <jats:italic>n</jats:italic>(<jats:italic>t</jats:italic>) that deviates from <jats:italic>n</jats:italic> <jats:sub>∞</jats:sub> by a small but significant amount for several years post-disruption. In one specific case with the stellar pericenter near the critical value, we find that self-gravity also drives the recollapse of the central regions of the debris stream into a collection of several cores while the rest of the stream remains relatively smooth. We also show that it is possible for the surviving stellar core in a partial disruption to acquire a circumstellar disk that is shed from the rapidly rotating core. Finally, we provide a novel analytical fitting function for the fallback rates that may also be useful in a range of contexts beyond tidal disruption events.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In the first two papers of this series, we demonstrated the dynamism of machine learning applied to optical spectral analysis by using neural networks to extract kinematic parameters and emission-line ratios directly from the spectra observed by the SITELLE instrument located at the Canada–France–Hawai’i Telescope. In this third installment, we develop a framework using a convolutional neural network trained on synthetic spectra to determine the number of line-of-sight components present in the SN3 filter (656–683 nm) spectral range of SITELLE. We compare this methodology to standard practice using Bayesian inference. Our results demonstrate that a neural network approach returns more accurate results and uses fewer computational resources over a range of spectral resolutions. Furthermore, we apply the network to SITELLE observations of the merging galaxy system NGC 2207/IC 2163. We find that the closest interacting sector and the central regions of the galaxies are best characterized by two line-of-sight components while the outskirts and spiral arms are well-constrained by a single component. Determining the number of resolvable components is crucial in disentangling different galactic components in merging systems and properly extracting their respective kinematics.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In recent years, phosphorus monoxide (PO), an important molecule for prebiotic chemistry, has been detected in star-forming regions and in the comet 67P/Churyumov-Gerasimenko. These studies have revealed that, in the interstellar medium (ISM), PO is systematically the most abundant P-bearing species, with abundances that are about one to three times greater than those derived for phosphorus nitride (PN), the second-most abundant P-containing molecule. The reason why PO is more abundant than PN remains still unclear. Experimental studies with phosphorus in the gas phase are not available, probably because of the difficulties in dealing with its compounds. Therefore, the reactivity of atomic phosphorus needs to be investigated using reliable computational tools. To this end, state-of-the-art quantum-chemical computations have been employed to evaluate accurate reaction rates and branching ratios for the P + OH → PO + H and P + H<jats:sub>2</jats:sub>O → PO + H<jats:sub>2</jats:sub> reactions in the framework of a master equation approach based on ab initio transition state theory. The hypothesis that OH and H<jats:sub>2</jats:sub>O can be potential oxidizing agents of atomic phosphorus is based on the ubiquitous presence of H<jats:sub>2</jats:sub>O in the ISM. Its destruction then produces OH, which is another very abundant species. While the reaction of atomic phosphorus in its ground state with water is not a relevant source of PO because of emerged energy barriers, the P + OH reaction represents an important formation route of PO in the ISM. Our kinetic results show that this reaction follows an Arrhenius–Kooij behavior, and thus its rate coefficients (<jats:italic>α</jats:italic> = 2.28 × 10<jats:sup>−10</jats:sup> cm<jats:sup>3</jats:sup> molecule<jats:sup>−1</jats:sup> s<jats:sup>−1</jats:sup>, <jats:italic>β</jats:italic> = 0.16 and <jats:italic>γ</jats:italic> = 0.37 K) increase by increasing the temperature.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Less abundant but still dynamically important solar wind components are the proton beam and alpha particles, which usually contribute similarly to the total ion momentum. The main characteristics of alpha particles are determined by the solar wind source region, but the origin of the proton beam and its properties are still not fully explained. We use the plasma data measured in situ on the path from 0.3 to 1 au (Helios 1 and 2) and focus on the proton beam development with an increasing radial distance as well as on the connection between the proton beam and alpha particle properties. We found that the proton beam relative abundance increases with increasing distance from the Sun in the collisionally young streams. Among the mechanisms suggested for beam creation, we have identified the wave–particle interactions with obliquely propagating Alfvén modes being consistent with observations. As the solar wind streams get collisionally older, the proton beam decay gradually dominates and the beam abundance is reduced. In search for responsible mechanisms, we found that the content of alpha particles is correlated with the proton beam abundance, and this effect is more pronounced in the fast solar wind streams during the solar maximum. We suggest that Coulomb collisions are the main agent leading to merging of the proton beam and core. We are also showing that the variations of the proton beam abundance are correlated with a decrease of the alpha particle velocity in order to maintain the total momentum balance in the solar wind frame.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Stellar population models produce radiation fields that ionize oxygen up to O<jats:sup>+2</jats:sup>, defining the limit of standard H <jats:sc>ii </jats:sc>region models (&lt;54.9 eV). Yet, some extreme emission-line galaxies, or EELGs, have surprisingly strong emission originating from much higher ionization potentials. We present UV HST/COS and optical LBT/MODS spectra of two nearby EELGs that have very high-ionization emission lines (e.g., He <jats:sc>ii</jats:sc> <jats:italic>λλ</jats:italic>1640,4686 C <jats:sc>iv</jats:sc> <jats:italic>λλ</jats:italic>1548,1550, [Fe <jats:sc>v</jats:sc>]<jats:italic>λ</jats:italic>4227, [Ar <jats:sc>iv</jats:sc>]<jats:italic>λλ</jats:italic>4711,4740). We define a four-zone ionization model that is augmented by a very high-ionization zone, as characterized by He<jats:sup>+2</jats:sup> (&gt;54.4 eV). The four-zone model has little to no effect on the measured total nebular abundances, but does change the interpretation of other EELG properties: we measure steeper central ionization gradients; higher volume-averaged ionization parameters; and higher central <jats:italic>T</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>, <jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>, and log <jats:italic>U</jats:italic> values. Traditional three-zone estimates of the ionization parameter can underestimate the average log <jats:italic>U</jats:italic> by up to 0.5 dex. Additionally, we find a model-independent dichotomy in the abundance patterns, where the <jats:italic>α</jats:italic>/H abundances are consistent but N/H, C/H, and Fe/H are relatively deficient, suggesting these EELGs are <jats:italic>α</jats:italic>/Fe-enriched by more than three times. However, there still is a high-energy ionizing photon production problem (HEIP<jats:sup>3</jats:sup>). Even for such <jats:italic>α</jats:italic>/Fe enrichment and very high log <jats:italic>U</jats:italic> s, photoionization models cannot reproduce the very high-ionization emission lines observed in EELGs.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The eruptive young star V899 Mon shows characteristics of both FUors and EXors. It reached a peak brightness in 2010, then briefly faded in 2011, followed by a second outburst. We conducted multifilter optical photometric monitoring, as well as optical and near-infrared spectroscopic observations, of V899 Mon. The light curves and color–magnitude diagrams show that V899 Mon has been gradually fading after its second outburst peak in 2018, but smaller accretion bursts are still happening. Our spectroscopic observations taken with Gemini/IGRINS and VLT/MUSE show a number of emission lines, unlike during the outbursting stage. We used the emission line fluxes to estimate the accretion rate and found that it has significantly decreased compared to the outbursting stage. The mass-loss rate is also weakening. Our 2D spectroastrometric analysis of emission lines recovered jet and disk emission of V899 Mon. We found that the emission from permitted metallic lines and the CO bandheads can be modeled well with a disk in Keplerian rotation, which also gives a tight constraint for the dynamical stellar mass of 2 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. After a discussion of the physical changes that led to the changes in the observed properties of V899 Mon, we suggest that this object is finishing its second outburst.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We have analyzed the data from a large-scale CO survey toward the northern region of the Small Magellanic Cloud (SMC) obtained with the Atacama Compact Array (ACA) stand-alone mode of ALMA. The primary aim of this study is to comprehensively understand the behavior of CO as an H<jats:sub>2</jats:sub> tracer in a low-metallicity environment (<jats:italic>Z</jats:italic> ∼ 0.2 <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>). The total number of mosaic fields is ∼8000, which results in a field coverage of 0.26 deg<jats:sup>2</jats:sup> (∼2.9 ×10<jats:sup>5</jats:sup> pc<jats:sup>2</jats:sup>), corresponding to ∼10% of the area of the galaxy. The sensitive ∼2 pc resolution observations reveal the detailed structure of the molecular clouds previously detected in the single-dish NANTEN survey. We have detected a number of compact CO clouds within lower H<jats:sub>2</jats:sub> column density (∼10<jats:sup>20</jats:sup> cm<jats:sup>−2</jats:sup>) regions whose angular scale is similar to the ACA beam size. Most of the clouds in this survey also show peak brightness temperature as low as &lt;1 K, which for optically thick CO emission implies an emission size much smaller than the beam size, leading to beam dilution. The comparison between an available estimation of the total molecular material traced by thermal dust emission and the present CO survey demonstrates that more than ∼90% of H<jats:sub>2</jats:sub> gas cannot be traced by the low-<jats:italic>J</jats:italic> CO emission. Our processed data cubes and 2D images are publicly available.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey has obtained high-resolution spectra for thousands of red giant stars distributed among the massive satellite galaxies of the Milky Way (MW): the Large and Small Magellanic Clouds (LMC/SMC), the Sagittarius Dwarf Galaxy (Sgr), Fornax (Fnx), and the now fully disrupted Gaia Sausage/Enceladus (GSE) system. We present and analyze the APOGEE chemical abundance patterns of each galaxy to draw robust conclusions about their star formation histories, by quantifying the relative abundance trends of multiple elements (C, N, O, Mg, Al, Si, Ca, Fe, Ni, and Ce), as well as by fitting chemical evolution models to the [<jats:italic>α</jats:italic>/Fe]–[Fe/H] abundance plane for each galaxy. Results show that the chemical signatures of the starburst in the Magellanic Clouds (MCs) observed by Nidever et al. in the <jats:italic>α</jats:italic>-element abundances extend to C+N, Al, and Ni, with the major burst in the SMC occurring some 3–4 Gyr before the burst in the LMC. We find that Sgr and Fnx also exhibit chemical abundance patterns suggestive of secondary star formation epochs, but these events were weaker and earlier (∼5–7 Gyr ago) than those observed in the MCs. There is no chemical evidence of a second starburst in GSE, but this galaxy shows the strongest initial star formation as compared to the other four galaxies. All dwarf galaxies had greater relative contributions of AGB stars to their enrichment than the MW. Comparing and contrasting these chemical patterns highlight the importance of galaxy environment on its chemical evolution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Many high-energy astrophysical systems contain magnetized collisionless plasmas with relativistic particles, in which turbulence can be driven by an arbitrary mixture of solenoidal and compressive motions. For example, turbulence in hot accretion flows may be driven solenoidally by the magnetorotational instability or compressively by spiral shock waves. It is important to understand the role of the driving mechanism on kinetic turbulence and the associated particle energization. In this work, we compare particle-in-cell simulations of solenoidally driven turbulence with similar simulations of compressively driven turbulence. We focus on plasma that has an initial beta of unity, relativistically hot electrons, and varying ion temperature. Apart from strong large-scale density fluctuations in the compressive case, the turbulence statistics are similar for both drives, and the bulk plasma is described reasonably well by an isothermal equation of state. We find that nonthermal particle acceleration is more efficient when turbulence is driven compressively. In the case of relativistically hot ions, both driving mechanisms ultimately lead to similar power-law particle energy distributions, but over a different duration. In the case of nonrelativistic ions, there is significant nonthermal particle acceleration only for compressive driving. Additionally, we find that the electron-to-ion heating ratio is less than unity for both drives, but takes a smaller value for compressive driving. We demonstrate that this additional ion energization is associated with the collisionless damping of large-scale compressive modes via perpendicular electric fields.</jats:p>