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′ × 3′ region, centered near Sgr B2(M). Comparativemapping observations were also done for the J_Ka,Kc =40,4→ 30,3 line of HNCO and HOCN near 84-87 GHz. In addition, the J _Ka,Kc = 7_0,7 → 60,6 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 ∼20-25 km s^-1. Column densities derived for HSCN in Sgr B2 are typically N_tot ∼ (0.2-1) × 10¹³ cm^-2,with Ntot ∼(0.8-5)×10 ¹³ 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.