SiS in outflow regions - More high-temperature silicon chemistry

The recent detection of silicon sulfide toward several regions where outflows are present is reported. Toward Orion-KL, where the 3 mm lines of SiS were previously observed, the J = 15 to 14 line of SiS at 272 GHz was detected as well. The SiS spectra were found to qualitatively resemble those observed for SiO, especially in Orion-KL, where the J = 15 to 14 line profile is very broad and almost flat-topped. Column densities of about 10 to the 12th - 10 to the 13th sq cm are derived. Toward Orion, where a correction for beam dilution is applied, the SiS column density is about 10 to the 15th/sq cm. The upper limit to the column density in cold cloud TMC-1 is less than 6 x 10 to the 11th/sq cm. The SiO/SiS ratio observed in the outflow sources is typically about 40-80, which is close to the cosmic O/S ratio of 43. It is inferred from the common appearance of SiS in these hot energetic regions that silicon sulfide formation is linked to high temperatures, as is found for SiO.