2 Observations

The Southern sources in the sample were observed in several sessions between December 1998 and November 2000 with the 15 m SEST at La Silla, Chile. For HCN  $1\rightarrow 0$ and CO  $1\rightarrow 0$ we used the 100 GHz (SESIS) and 115 GHz (IRAM) receivers, respectively. Since the 115 GHz can be used simultaneously with the 230 GHz receiver, we also mapped the CO $2\rightarrow 1$transition. All receivers were tuned to single-sideband mode and typical system temperatures, on the $T_{\rm A}^*$-scale, were 200 K at 89 GHz, 200 to 300 K at 115 GHz and 300 K at 230 GHz. The backends were acousto-optical spectrometers with 1440 channels and a channel width of 0.7 MHz. We used dual-beam switching with a throw of about $12\hbox{$^\prime$ }$ in azimuth, and pointing errors were typically $3\hbox{$^{\prime\prime}$ }$ rms on each axis. The intensity was calibrated using the chopper-wheel method. For all of the observing runs the weather was excellent, and only the removal of linear baselines was required.

The Northern sources in the sample were observed over one continuous 7 day session in September 2000 with the 20 m telescope at Onsala Space Observatory (OSO). Both the CO $1\rightarrow 0$ and HCN  $1\rightarrow 0$ transitions were observed with the SIS 100 GHz receiver. The backend was a filter-bank with a bandwidth of 512 MHz and a channel separation of 1 MHz. We used a similar dual-beam switching as the SEST observations and obtained similar pointing errors. The weather was good (typical system temperatures were around 250 to 400 K at 89 GHz and 450 to 600 K at 115 GHz) and, as with the SEST data, only linear baselines were removed. The beam sizes and efficiences for the two telescopes at various transitions are given in Table 1.

 

 

Table 1: Beam sizes and efficiencies

Transition	$\nu$ [GHz]	HPBW ['']		$\eta _{\rm mb}$
		OSO	SEST	OSO	SEST
HCN  $1\rightarrow 0$	89	44	57	0.59	0.75
CO  $1\rightarrow 0$	115	33	45	0.50	0.70
CO  $2\rightarrow 1$	230	-	22	-	0.50