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1. GBT/Argus Observations of Molecular Gas in the Inner Regions of IC 342

   https://doi.org/10.3847/1538-4357/ad2171  

   Li 李, Jialu 佳璐 ; Harris, Andrew I. ; Rosolowsky, Erik ; Kepley, Amanda A. ; Frayer, David ; Bolatto, Alberto D. ; Leroy, Adam K. ; Donovan Meyer, Jennifer ; Church, Sarah ; Gundersen, Joshua Ott ; et al ( March 2024 , The Astrophysical Journal)

   We report observations of the ground state transitions of¹²CO,¹³CO, C¹⁸O, HCN, and HCO⁺at 88–115 GHz in the inner region of the nearby galaxy IC 342. These data were obtained with the 16 pixel spectroscopic focal plane array Argus on the 100 m Robert C. Byrd Green Bank Telescope (GBT) at 6″–9″ resolution. In the nuclear bar region, the intensity distributions of¹²CO(1–0) and¹³CO(1–0) emission trace moderate densities, and differ from the dense gas distributions sampled in C¹⁸O(1–0), HCN(1–0), and HCO⁺(1–0). We observe a constant HCN(1–0)-to-HCO⁺(1–0) ratio of 1.2 ± 0.1 across the whole ∼1 kpc bar. This indicates that the HCN(1–0) and HCO⁺(1–0) lines have intermediate optical depth, and that the corresponding$n_{{\mathrm{H}}_2}$of the gas producing the emission is of order 10^4.5−6cm⁻³. We show that HCO⁺(1–0) is thermalized and HCN(1–0) is close to thermalization. The very tight correlation between the HCN(1–0) and HCO⁺(1–0) intensities across the 1 kpc bar suggests that this ratio is more sensitive to the relative abundance of the two species than to the gas density. We confirm an angular offset (∼10″) between the spatial distribution of molecular gas and the star formation sites. Finally, we find a breakdown of theL_IR–$L_{\mathrm{HCN}}$correlation at high spatial resolution due to the effect of incomplete sampling of star-forming regions by HCN emission in IC 342. The scatter of theL_IR–$L_{\mathrm{HCN}}$relation decreases as the spatial scale increases from 10″ to 30″ (170–510 pc), and is comparable to the scatter of the global relation at a scale of 340 pc.

    

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2. Elusive Iron: Detection of the FeC Radical (X 3 Δ i ) in the Envelope of IRC+10216

   https://doi.org/10.3847/2041-8213/ad0899  

   Koelemay, L. A. ; Ziurys, L. M. ( November 2023 , The Astrophysical Journal Letters)

   A new interstellar molecule, FeC (X³Δ_i), has been identified in the circumstellar envelope of the carbon-rich asymptotic giant branch star IRC+10216. FeC is the second iron-bearing species conclusively observed in the interstellar medium, in addition to FeCN, also found in IRC+10216. TheJ= 4 → 3, 5 → 4, and 6 → 5 rotational transitions of this free radical near 160, 201, and 241 GHz, respectively, were detected in the lowest spin–orbit ladder, Ω = 3, using the Submillimeter Telescope of the Arizona Radio Observatory (ARO) for the 1 mm lines and the ARO 12 m at 2 mm. Because the ground state of FeC is inverted, these transitions are the lowest energy lines. The detected features exhibit slight U shapes with LSR velocities nearV_LSR≈ −26 km s⁻¹and linewidths of ΔV_1/2≈ 30 km s⁻¹, line parameters characteristic of IRC+10216. Radiative transfer modeling of FeC suggests that the molecule has a shell distribution with peak radius near 300R_*(∼6″) extending out to ∼500R_*(∼10″) and a fractional abundance, relative to H₂, off∼ 6 × 10⁻¹¹. The previous FeCN spectra were also modeled, yielding an abundance off∼ 8 × 10⁻¹¹in a larger shell situated near 800R_*. These distributions suggest that FeC may be the precursor species for FeCN. Unlike cyanides and carbon-chain molecules, diatomic carbides with a metallic element are rare in IRC+10216, with FeC being the first such detection.

    

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3. Resolved molecular line observations reveal an inherited molecular layer in the young disk around TMC1A

   https://doi.org/10.1051/0004-6361/202038697  

   Harsono, D. ; van der Wiel, M. H. ; Bjerkeli, P. ; Ramsey, J. P. ; Calcutt, H. ; Kristensen, L. E. ; Jørgensen, J. K. ( February 2021 , Astronomy & Astrophysics)

   null (Ed.)

   Context. Physical processes that govern the star and planet formation sequence influence the chemical composition and evolution of protoplanetary disks. Recent studies allude to an early start to planet formation already during the formation of a disk. To understand the chemical composition of protoplanets, we need to constrain the composition and structure of the disks from whence they are formed. Aims. We aim to determine the molecular abundance structure of the young disk around the TMC1A protostar on au scales in order to understand its chemical structure and any possible implications for disk formation. Methods. We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of CO, HCO + , HCN, DCN, and SO line emission, as well as dust continuum emission, in the vicinity of TMC1A. Molecular column densities are estimated both under the assumption of optically thin emission from molecules in local thermodynamical equilibrium (LTE) as well as through more detailed non-LTE radiative transfer calculations. Results. Resolved dust continuum emission from the disk is detected between 220 and 260 GHz. Rotational transitions from HCO + , HCN, and SO are also detected from the inner 100 au region. We further report on upper limits to vibrational HCN υ 2 = 1, DCN, and N 2 D + lines. The HCO + emission appears to trace both the Keplerian disk and the surrounding infalling rotating envelope. HCN emission peaks toward the outflow cavity region connected with the CO disk wind and toward the red-shifted part of the Keplerian disk. From the derived HCO + abundance, we estimate the ionization fraction of the disk surface, and find values that imply that the accretion process is not driven by the magneto-rotational instability. The molecular abundances averaged over the TMC1A disk are similar to its protostellar envelope and other, older Class II disks. We meanwhile find a discrepancy between the young disk’s molecular abundances relative to Solar System objects. Conclusions. Abundance comparisons between the disk and its surrounding envelope for several molecular species reveal that the bulk of planet-forming material enters the disk unaltered. Differences in HCN and H 2 O molecular abundances between the disk around TMC1A, Class II disks, and Solar System objects trace the chemical evolution during disk and planet formation. 

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4. Laboratory and Astronomical Detection of the SiP Radical (X 2 Π i ): More Circumstellar Phosphorus

   https://doi.org/10.3847/2041-8213/ac9d9b  

   Koelemay, L. A. ; Burton, M. A. ; Singh, A. P. ; Sheridan, P. M. ; Bernal, J. J. ; Ziurys, L. M. ( November 2022 , The Astrophysical Journal Letters)

   The millimeter-wave spectrum of the SiP radical (X²Π_i) has been measured in the laboratory for the first time using direct-absorption methods. SiP was created by the reaction of phosphorus vapor and SiH₄in argon in an AC discharge. Fifteen rotational transitions (J+ 1 ←J) were measured for SiP in the Ω = 3/2 ladder in the frequency range 151–533 GHz, and rotational, lambda doubling, and phosphorus hyperfine constants determined. Based on the laboratory measurements, SiP was detected in the circumstellar shell of IRC+10216, using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory at 1 mm and 2 mm, respectively. Eight transitions of SiP were searched: four were completely obscured by stronger features, two were uncontaminated (J= 13.5 → 12.5 and 16.5 → 15.5), and two were partially blended with other lines (J= 8.5 → 7.5 and 17.5 → 16.5). The SiP line profiles were broader than expected for IRC+10216, consistent with the hyperfine splitting. From non-LTE radiative transfer modeling, SiP was found to have a shell distribution with a radius ∼300R_*, and an abundance, relative to H₂, off∼ 2 × 10⁻⁹. From additional modeling, abundances of 7 × 10⁻⁹and 9 × 10⁻¹⁰were determined for CP and PN, respectively, both located in shells at 550–650R_*. SiP may be formed from grain destruction, which liberates both phosphorus and silicon into the gas phase, and then is channeled into other P-bearing molecules such as PN and CP.

    

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5. Dense gas in a giant molecular filament

   https://doi.org/10.1051/0004-6361/202037928  

   Wang, Y. ; Beuther, H. ; Schneider, N. ; Meidt, S. E. ; Linz, H. ; Ragan, S. ; Zucker, C. ; Battersby, C. ; Soler, J. D. ; Schinnerer, E. ; et al ( September 2020 , Astronomy & Astrophysics)

   null (Ed.)

   Context. Recent surveys of the Galactic plane in the dust continuum and CO emission lines reveal that large (≳50 pc) and massive (≳10 5 M ⊙ ) filaments, know as giant molecular filaments (GMFs), may be linked to Galactic dynamics and trace the mid-plane of the gravitational potential in the Milky Way. Yet our physical understanding of GMFs is still poor. Aims. We investigate the dense gas properties of one GMF, with the ultimate goal of connecting these dense gas tracers with star formation processes in the GMF. Methods. We imaged one entire GMF located at l ~ 52–54° longitude, GMF54 (~68 pc long), in the empirical dense gas tracers using the HCN(1–0), HNC(1–0), and HCO + (1–0) lines, and their 13 C isotopologue transitions, as well as the N 2 H + (1–0) line. We studied the dense gas distribution, the column density probability density functions (N-PDFs), and the line ratios within the GMF. Results. The dense gas molecular transitions follow the extended structure of the filament with area filling factors between 0.06 and 0.28 with respect to 13 CO(1–0). We constructed the N-PDFs of H 2 for each of the dense gas tracers based on their column densities and assumed uniform abundance. The N-PDFs of the dense gas tracers appear curved in log–log representation, and the HCO + N-PDF has the flattest power-law slope index. Studying the N-PDFs for sub-regions of GMF54, we found an evolutionary trend in the N-PDFs that high-mass star-forming and photon-dominated regions have flatter power-law indices. The integrated intensity ratios of the molecular lines in GMF54 are comparable to those in nearby galaxies. In particular, the N 2 H + / 13 CO ratio, which traces the dense gas fraction, has similar values in GMF54 and all nearby galaxies except Ultraluminous Infrared Galaxies. Conclusions. As the largest coherent cold gaseous structure in our Milky Way, GMFs, are outstanding candidates for connecting studies of star formation on Galactic and extragalactic scales. By analyzing a complete map of the dense gas in a GMF we have found that: (1) the dense gas N-PDFs appear flatter in more evolved regions and steeper in younger regions, and (2) its integrated dense gas intensity ratios are similar to those of nearby galaxies. 

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