Observations of the 1-0 transition of CO towards southern HII regions.

The observations were made between September and November 1981. The telescope had a beamwidth of 2.8 arcmin and an estimated pointing accuracy of 30-40 arcsec. The cooled mixer receiver had a double-sideband temperature of approximately 500 K. The CO spectra were obtained using an acousto-optical spectrograph (Milne and Cole, 1979) with a radial velocity coverage of 244 km/s and a velocity resolution of 0.6 km/s. It is noted that the typical integration period on source was 4 min. To eliminate baseline curvature in the final spectra, reference spectra were obtained at positions away from the HII regions. The mode of observation produced line intensities corrected for the mean atmospheric absorption for the two receiver sidebands. Since the absorption at the CO frequency was significantly higher than the mean value, a further correction was necessary. This, together with a conversion of intensities to a scale of 'corrected antenna temperature' (which corresponds to a beam brightness temperature in the Rayleigh-Jeans limit), was achieved through CO observations of OMC1 at a series of different elevations, assuming a corrected temperature of 65 K for the CO of this molecular cloud.