Observations of the [HNCS]/[HSCN] Ratio in Sgr B2 and TMC-1: Evidence for Low-temperature Gas-phase Chemistry

Details Observations of the [HNCS]/[HSCN] Ratio in Sgr B2 and TMC-1: Evidence for Low-temperature Gas-phase Chemistry Observations of the [HNCS]/[HSCN] Ratio in Sgr B2 and TMC-1: Evidence for Low-temperature Gas-phase Chemistry

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010apj...725..561a&link_type=abstract

The Astrophysical Journal, Volume 725, Issue 1, pp. 561-570 (2010).

Astronomy and Astrophysics

Astronomy

Astrochemistry, Ism: Abundances, Ism: Individual Objects: Sgr B2 Tmc-1, Ism: Molecules, Radio Lines: Ism

Scientific paper

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 JKa,Kc = 80,8 → 70,7 and 90,9 → 80,8 transitions near 91-93 GHz and 103-106 GHz were mapped across a 6' × 3' region, centered near Sgr B2(M). Comparative mapping observations were also done for the JKa,Kc = 40,4 → 30,3 line of HNCO and HOCN near 84-87 GHz. In addition, the JKa,Kc = 70,7 → 60,6 and 80,8 → 70,7 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 ~20-25 km s-1. Column densities derived for HSCN in Sgr B2 are typically N tot ~ (0.2-1) × 1013 cm-2, with N tot ~ (0.8-5) × 1013 cm-2 for the more stable isomer, HNCS. In TMC-1, these species have similar column densities of (6-8) × 1010 cm-2. The [HNCS]/[HSCN] abundance ratio ranges from 2 to 7 in Sgr B2, with a value of ~1 in TMC-1. In contrast, the [HNCO]/[HOCN] ratio in Sgr B2 is ~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+. The oxygen analogs, in contrast, probably have a more complex chemical network, perhaps involving the H2NCO+ precursor, which preferentially leads to HNCO.

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