Astrophys. J., 725, 561-570 (2010/December-2)
Observations of the [HNCS]/[HSCN] ratio in Sgr B2 and TMC-1: evidence for low-temperature gas-phase chemistry.
ADANDE G.R., HALFEN D.T., ZIURYS L.M., QUAN D. and HERBST E.
Abstract (from CDS):
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' x 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. Column densities derived for HSCN in Sgr B2 are typically Ntot∼ (0.2-1)x1013/cm2, with Ntot ∼ (0.8-5)x1013/cm2 for the more stable isomer, HNCS. In TMC-1, these species have similar column densities of (6-8)x1010/cm2. 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.
astrochemistry - ISM: abundances - ISM: individual (Sgr B2, TMC-1) - ISM: molecules - radio lines: ISM
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