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1. Maser Activity of Organic Molecules toward Sgr B2(N)

   https://doi.org/10.3847/1538-4357/ad4094  

   Xue, Ci; Remijan, Anthony; Faure, Alexandre; Momjian, Emmanuel; Hunter, Todd_R; Loomis, Ryan_A; Herbst, Eric; McGuire, Brett (May 2024, The Astrophysical Journal)

   Abstract At centimeter wavelengths, single-dish observations have suggested that the Sagittarius (Sgr) B2 molecular cloud at the Galactic Center hosts weak maser emission from several organic molecules, including CH₂NH, HNCNH, and HCOOCH₃. However, the lack of spatial distribution information on these new maser species has prevented us from assessing the excitation conditions of the maser emission as well as their pumping mechanisms. Here, we present a mapping study toward Sgr B2 north (N) to locate the region where the complex maser emission originates. We report the first detection of the Class I methanol (CH₃OH) maser at 84 GHz and the first interferometric map of the methanimine (CH₂NH) maser at 5.29 GHz toward this region. In addition, we present a tool for modeling and fitting the unsaturated molecular maser signals with non-LTE radiative transfer models and Bayesian analysis using the Markov Chain Monte Carlo approach. These enable us to quantitatively assess the observed spectral profiles. The results suggest a two-chain-clump model for explaining the intense CH₃OH Class I maser emission toward a region with low continuum background radiation. By comparing the spatial origin and extent of maser emission from several molecular species, we find that the 5.29 GHz CH₂NH maser has a close spatial relationship with the 84 GHz CH₃OH Class I masers. This relationship serves as observational evidence to suggest a similar collisional pumping mechanism for these maser transitions. 

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2. Broadband VLA Spectral-line Survey of a Sample of Ionized Jet Candidates

   https://doi.org/10.3847/1538-4365/acdb5e  

   Sanchez-Tovar, E.; Araya, E. D.; Rosero, V.; Hofner, P.; Kurtz, S. (August 2023, The Astrophysical Journal Supplement Series)

   Abstract The study of the interaction between ionized jets, molecular outflows, and their environments is critical to understanding high-mass star formation, especially because jets and outflows are thought to be key in the transfer of angular momentum outward from accretion disks. We report a low spectral resolution Karl G. Jansky Very Large Array (VLA) survey for OH, NH₃, CH₃OH, and hydrogen radio recombination lines, toward a sample of 58 high-mass star-forming regions that contain numerous ionized jet candidates. The observations are from a survey designed to detect radio continuum; the novel aspect of this work is to search for spectral lines in broadband VLA data (we provide the script developed in this work to facilitate exploration of other data sets). We report detection of 25 GHz CH₃OH transitions toward 10 sources; 5 of them also show NH₃emission. We found that most of the sources detected in CH₃OH and NH₃have been classified as ionized jets or jet candidates and that the emission lines are coincident with, or very near (≲0.1 pc), these sources; hence, these molecular lines could be used as probes of the environment near the launching site of jets/outflows. No radio recombination lines were detected, but we found that the rms noise of stacked spectra decreases following the radiometer equation. Therefore, detecting radio recombination lines in a sample of brighter free–free continuum sources should be possible. This work demonstrates the potential of broadband VLA continuum observations as low resolution spectral-line scans. 

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3. Evidence of a Cloud–Cloud Collision from Overshooting Gas in the Galactic Center

   https://doi.org/10.3847/1538-4357/ad01be  

   Gramze, Savannah R.; Ginsburg, Adam; Meier, David S.; Ott, Juergen; Shirley, Yancy; Sormani, Mattia C.; Svoboda, Brian E. (December 2023, The Astrophysical Journal)

   Abstract The Milky Way is a barred spiral galaxy withbar lanesthat bring gas toward the Galactic center. Gas flowing along these bar lanes often overshoots, and instead of accreting onto the Central Molecular Zone (CMZ), it collides with the bar lane on the opposite side of the Galaxy. We observed G5, a cloud that we believe is the site of one such collision, near the Galactic center at (ℓ,b) = ( +5.4, −0.4) with the Atacama Large Millimeter/submillimeter Array/Atacama Compact Array. We took measurements of the spectral lines¹²COJ= 2 → 1,¹³COJ= 2 → 1, C¹⁸OJ= 2 → 1, H₂COJ= 3₀₃→ 2₀₂, H₂COJ= 3₂₂→ 2₂₁, CH₃OHJ= 4₂₂→ 3₁₂, OCSJ= 18 → 17, and SiOJ= 5 → 4. We observed a velocity bridge between two clouds at ∼50 and ∼150 km s⁻¹in our position–velocity diagram, which is direct evidence of a cloud–cloud collision. We measured an average gas temperature of ∼60 K in G5 using H₂CO integrated-intensity line ratios. We observed that the¹²C/¹³C ratio in G5 is consistent with optically thin, or at most marginally optically thick¹²CO. We measured $1.5\times {10}^{19}{\mathrm{cm}}^{-2}{\left(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\right)}^{-1}$for the local X_CO, 10–20× less than the average Galactic value. G5 is strong direct observational evidence of gas overshooting the CMZ and colliding with a bar lane on the opposite side of the Galactic center. 

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4. A Broad Line-width, Compact, Millimeter-bright Molecular Emission Line Source near the Galactic Center

   https://doi.org/10.3847/2041-8213/ad47fa  

   Ginsburg, Adam; Bally, John; Barnes, Ashley_T; Battersby, Cara; Budaiev, Nazar; Butterfield, Natalie_O; Caselli, Paola; Colzi, Laura; Dutkowska, Katarzyna_M; García, Pablo; et al (June 2024, The Astrophysical Journal Letters)

   Abstract A compact source, G0.02467–0.0727, was detected in Atacama Large Millimeter/submillimeter Array 3 mm observations in continuum and very broad line emission. The continuum emission has a spectral indexα≈ 3.3, suggesting that the emission is from dust. The line emission is detected in several transitions of CS, SO, and SO₂and exhibits a line width FWHM ≈ 160 km s⁻¹. The line profile appears Gaussian. The emission is weakly spatially resolved, coming from an area on the sky ≲1″ in diameter (≲10⁴au at the distance of the Galactic center, GC). The centroid velocity isv_LSR≈ 40–50 km s⁻¹, which is consistent with a location in the GC. With multiple SO lines detected, and assuming local thermodynamic equilibrium (LTE) conditions, the gas temperature isT_LTE= 13 K, which is colder than seen in typical GC clouds, though we cannot rule out low-density, subthermally excited, warmer gas. Despite the high velocity dispersion, no emission is observed from SiO, suggesting that there are no strong (≳10 km s⁻¹) shocks in the molecular gas. There are no detections at other wavelengths, including X-ray, infrared, and radio. We consider several explanations for the millimeter ultra-broad-line object (MUBLO), including protostellar outflow, explosive outflow, a collapsing cloud, an evolved star, a stellar merger, a high-velocity compact cloud, an intermediate-mass black hole, and a background galaxy. Most of these conceptual models are either inconsistent with the data or do not fully explain them. The MUBLO is, at present, an observationally unique object. 

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5. Living on the edge of the Milky Way's central molecular zone: G1.3 is the more likely candidate for gas accretion into the CMZ

   https://doi.org/10.1051/0004-6361/202244870  

   Busch, Laura A.; Riquelme, Denise; Güsten, Rolf; Menten, Karl M.; Pillai, Thushara G.; Kauffmann, Jens (December 2022, Astronomy & Astrophysics)

   Context. The 1°.3 (G1.3) and 1°.6 (G1.6) cloud complexes in the central molecular zone (CMZ) of our Galaxy have been proposed to possibly reside at the intersection region of the X1 and X2 orbits for several reasons. This includes the detection of co-spatial low- and high-velocity clouds, high velocity dispersion, high fractional molecular abundances of shock-tracing molecules, and kinetic temperatures that are higher than for usual CMZ clouds. Aims. By investigating the morphology and deriving physical properties as well as chemical composition, we want to find the origin of the turbulent gas and, in particular, whether evidence of an interaction between clouds can be identified. Methods. We mapped both cloud complexes in molecular lines in the frequency range from 85 to 117 GHz with the IRAM 30 m telescope. The APEX 12m telescope was used to observe higher frequency transitions between 210 and 475 GHz from selected molecules that are emitted from higher energy levels. We performed non-local thermodynamic equilibrium (non-LTE) modelling of the emission of an ensemble of CH 3 CN lines to derive kinetic temperatures and H 2 volume densities. These were used as starting points for non-LTE modelling of other molecules, for which column densities and abundances were determined and compared with values found for other sources in the CMZ. Results. The kinematic structure of G1.3 reveals an ‘emission bridge’ at intermediate velocities (~150 km s −1 ) connecting low-velocity (~100 km s −1 ) and high-velocity (~180 km s −1 ) gas and an overall fluffy shell-like structure. These may represent observational evidence of cloud-cloud interactions. Low- and high-velocity gas components in G1.6 do not show this type of evidence of an interaction, suggesting that they are spatially separated. We selected three positions in each cloud complex for further analysis. Each position reveals several gas components at various peak velocities and of various line widths. We derived kinetic temperatures of 60–100 K and H 2 volume densities of 10 4 –10 5 cm −3 in both complexes. Molecular abundances relative to H 2 suggest a similar chemistry of the two clouds, which is moreover similar to that of other GC clouds and, especially, agrees well with that of G+0.693 and G−0.11. Conclusions. We conclude that G1.3 may indeed exhibit signs of cloud-cloud interactions. In particular, we propose an interaction of gas that is accreted from the near-side dust lane to the CMZ, with gas pre-existing at this location. Low- and high-velocity components in G1.6 are rather coincidentally observed along the same line of sight. They may be associated with either overshot decelerated gas from the far-side dust line or actual CMZ gas and high-velocity gas moving on a dust lane. These scenarios would be in agreement with numerical simulations. 

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