OBSERVATIONS OF THE [HNCS]/[HSCN] RATIO IN Sgr B2 AND TMC-1: EVIDENCE FOR LOW-TEMPERATURE GAS-PHASE CHEMISTRY

Journal Article · Fri Dec 10 04:00:00 EST 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/725/1/561· OSTI ID:21474409

Adande, G R; Halfen, D T; Ziurys, L M ^[1]; Quan, D ^[2]; Herbst, E ^[3]

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1. Departments of Chemistry and Astronomy, Arizona Radio Observatory, and Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721 (United States)
2. Chemical Physics Program, Ohio State University, Columbus, OH 43210 (United States)
3. Departments of Physics, Chemistry, and Astronomy, Ohio State University, Columbus, OH 43210 (United States)

Millimeter observations of isothiocyanic acid (HNCS) and its higher energy isomer, thiocyanic acid (HSCN), have been carried out toward Sgr B2 and TMC-1 using the 12 m telescope of the Arizona Radio Observatory. For both species, the J{sub Ka,Kc} = 8{sub 0,8} {yields} 7{sub 0,7} and 9{sub 0,9} {yields} 8{sub 0,8} transitions near 91-93 GHz and 103-106 GHz were mapped across a 6' x 3' region, centered near Sgr B2(M). Comparative mapping observations were also done for the J{sub Ka,Kc} = 4{sub 0,4} {yields} 3{sub 0,3} line of HNCO and HOCN near 84-87 GHz. In addition, the J{sub Ka,Kc} = 7{sub 0,7} {yields} 6{sub 0,6} and 8{sub 0,8} {yields} 7{sub 0,7} transitions of both HNCS and HSCN were detected in TMC-1, the first identification of either molecule in a cold, dark cloud. Emission from HNCS and HSCN was found to be extended over the Sgr B2 cloud, with a single velocity component and a linewidth of {approx}20-25 km s{sup -1}. Column densities derived for HSCN in Sgr B2 are typically N{sub tot} {approx} (0.2-1) x 10{sup 13} cm{sup -2}, with N{sub tot} {approx} (0.8-5) x 10{sup 13} cm{sup -2} for the more stable isomer, HNCS. In TMC-1, these species have similar column densities of (6-8) x 10{sup 10} cm{sup -2}. The [HNCS]/[HSCN] abundance ratio ranges from 2 to 7 in Sgr B2, with a value of {approx}1 in TMC-1. In contrast, the [HNCO]/[HOCN] ratio in Sgr B2 is {approx}110-250. Gas-grain chemical models do not reproduce the observed abundances of the sulfur isomers in either source. Given the energy difference of over 3200 K between HNCS and HSCN, these observations suggest that both molecules are produced from gas-phase, ion-molecule chemistry with a common precursor, HNCSH{sup +}. The oxygen analogs, in contrast, probably have a more complex chemical network, perhaps involving the H{sub 2}NCO{sup +} precursor, which preferentially leads to HNCO.

• OSTI ID: 21474409
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 725; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

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