Detection of Interstellar Thioformaldehyde

The 211 +-212 transition of thioformaldehyde (HCHS) has been observed in absorption in the direction of SagittariusB2. For a rest frequency of 3139· 38 MHz the peak absorption occurs at a radial velocity of 60 ± 4 km s -1. The half-width of the absorption profile is equivalent to 20 kms-1 and the column density of HCHS is greater than 1016 molecules cm - 2. Comparison with the 211 +-212 absorption of formaldehyde (HCHO) at 15 GHz allows the relative abundance of the two molecular species to be computed as a function of the rotational excitation temperature. For thermal equilibrium the strength of the 110-111 absorption in SgrB2 by thioformaldehyde predicted from the 211+-212 HCHS observations is considerably greater than the limits set by a number of observers for the 110-111 transition at 1046 MHz.