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

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(Ka,Kc) = 8(0,8) -> 7(0,7) and 9(0,9) -> 8(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(Ka, Kc) = 4(0,4) -> 3(0,3) line of HNCO and HOCN near 84-87 GHz. In addition, the J(Ka, Kc) = 7(0,7) -> 6(0,6) and 8(0,8) -> 7(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 similar to 20-25 km s(-1). Column densities derived for HSCN in Sgr B2 are typically N-tot similar to (0.2-1) x 10(13) cm(-2), with N-tot similar to (0.8-5) x 10(13) cm(-2) for the more stable isomer, HNCS. In TMC-1, these species have similar column densities of (6-8) x 10(10) cm(-2). The [HNCS]/[HSCN] abundance ratio ranges from 2 to 7 in Sgr B2, with a value of similar to 1 in TMC-1. In contrast, the [HNCO]/[HOCN] ratio in Sgr B2 is similar to 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.