2010ApJ...725..561A

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 ∼20-25 km/s. Column densities derived for HSCN in Sgr B2 are typically N_tot∼ (0.2-1)x10¹³/cm2, with N_tot ∼ (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.