2 Observations

The observations were carried out in the period from May 14 to May 31, 1995, with the 20-m radio telescope in Onsala (Sweden). The rest frequency of the CS(2-1) line is 97980.968 MHz and that of the C³⁴S(2-1) line is 96412.982 MHz (Lovas 1992). The aperture efficiency is 47 $\pm$ 3% and the half-power beam width is 39'' at 98 GHz. The pointing accuracy is 3'' rms in azimuth and elevation. The beam efficiency $\eta_{\rm mb}=0.56$. The observations were performed in a frequency-switching mode with a 12-MHz frequency throw. A cryogenically cooled low-noise SIS mixer was used as receiver frontend. The system noise temperature, corrected for atmospheric absorption, rear ward spillover and radome losses, varied during observations between 250 and 500 K, depending on weather conditions and elevation of the sources. The data were calibrated using the standard chopper-wheel method of Kutner & Ulich (1981). An antenna temperature of 1 K corresponds to 18.7 Jy. The backend was a 64 MHz-wide 256-channel filter spectrometer with a frequency resolution of 250 kHz (velocity resolution 0.7 km s^-1 at 98 GHz).