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 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 Ntot ∼ (0.2–1) × 1013 cm−2, with Ntot ∼ (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.