Observations of the [HNCS]/[HSCN] ratio in Sgr B2 and TMC-1: Evidence for low-temperature gas-phase chemistry

G. R. Adande, D. T. Halfen, L. M. Ziurys, D. Quan, E. Herbst

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21 Scopus citations


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 9 0,9→80,8 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 JKa,Kc =40,4→ 30,3 line of HNCO and HOCN near 84-87 GHz. In addition, the J Ka,Kc = 70,7 → 60,6 and 80,8 → 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 Ntot ∼ (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.

Original languageEnglish (US)
Pages (from-to)561-570
Number of pages10
JournalAstrophysical Journal
Issue number1
StatePublished - Dec 10 2010


  • Astrochemistry
  • ISM: abundances
  • ISM: individual objects (Sgr B2, TMC-1)
  • ISM: molecules
  • Radio lines: ISM

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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