3 IRAM 30 m observations

3.1 Observing conditions

The ¹²CO($J=1\rightarrow 0$) and ¹²CO($J=2\rightarrow 1$) observations were carried out simultaneously with the IRAM 30 m telescope, Spain (Baars et al. 1987), in three runs between October 1990 and August 1993.

The telescope was equipped with 3 mm and 1 mm SIS receivers operating simultaneously in single sideband mode. The antenna temperature scale was calibrated every 10 minutes by successively observing a cold load, a room temperature load and the sky. The calibration accuracy, checked on well known molecular sources (Mauersberger et al. 1989) was found to present an uncertainty of less than 20%. The main beam temperatures $T_{\rm MB}$ reported in the figures and in Table 4 are related to the antenna temperature scale $T_{\rm A}^*$ by $T_{\rm MB}$ = $T_{\rm A}^*$/$\eta$, where $\eta$, the ratio of the main beam to the forward beam efficiency, is equal to 0.45 for the ¹²CO($J=2\rightarrow 1$) observations and 0.60 for the ¹²CO($J=1\rightarrow 0$) observations.

The backends were at both frequencies filterbanks of 512 or 256 channels 1MHz wide, providing a velocity resolution of 2.60kms^-1 at the frequency of the ¹²CO($J=1\rightarrow 0$) line (115271.2MHz) and 1.30kms^-1 at the frequency of the ¹²CO($J=2\rightarrow 1$) line (230538.0MHz).

In October 1990 and May 1991, the observations were made in position switching mode, with a reference position 150$^{\prime\prime}$ east to the star position. In August 1993, the secondary mirror was wobbled every 3 s, with an amplitude of 240$^{\prime\prime}$ (this technique provides very flat baselines).

The pointing was checked every hour or less on nearby continuum sources. It was found to remain accurate within 5$^{\prime\prime}$ during this time interval. To avoid distorsion due to pointing drifts, the maps were taken with short integration times (between 1 min and 3 min on each position) and completed in less than 1 hour. When longer integration times were needed to achieve the required signal to noise ratio, the maps were observed several times, with pointing checks in between, and subsequently added, using the shift-and-add technique described in Sect. 3.2.

In October 1990, the maps were observed with a grid spacing of 5$^{\prime\prime}$ in right ascension and declination. In May 1991 and August 1993, the spacing was taken equal to 7.5$^{\prime\prime}$. Considering the telescope half power beam widths of 13$^{\prime\prime}$ for the ¹²CO($J=2\rightarrow 1$) observations and 21$^{\prime\prime}$ for the ¹²CO($J=1\rightarrow 0$) observations, these spacings ensure that the maps are fully sampled.

A total of 46 circumstellar envelopes have been mapped in ¹²CO($J=2\rightarrow 1$) emission. 20 envelopes were mapped in October 1990, under fine weather conditions. The SSB system temperature was typically 600 K at 3 mm and 1000 K at 1 mm. 12 envelopes were observed in May 1991, in less favorable conditions, with system temperatures of the order of 1000 K and 2000 K, respectively. The observations of 11 objects were completed in August 1993, in good weather conditions with system temperatures of the order of 800 K and 1200 K, respectively. In particular, the maps of 5 envelopes (IRC+10011, IK Tau, CIT6, IRC+20370, RAFGL3068) observed by Loup (1991) were extended during this run. For 3 envelopes (RX Boo, $\chi$ Cyg and R Cas), we reproduce and reanalyse here the observations (June 1990) obtained and published by Bujarrabal & Alcolea (1991).

3.2 Data reduction

The single-dish data were reduced using the CLASS software package. Only linear or parabolic baselines were substracted from the spectra. When several maps of the same object were observed, we checked that the lines intensities were in agreement and that the positions of the integrated intensity contours coincided within better than 1.25$^{\prime\prime}$ in right ascension and declination. When the shift between two maps was found to be larger, one of the maps was shifted relative to the other in steps of 2.5$^{\prime\prime}$ in each direction. After relative recentering the maps were added to obtain a single data cube. After this stage, the reductions of the ¹²CO($J=1\rightarrow 0$) and ($J=2\rightarrow 1$) data differ, since the ¹²CO($J=1\rightarrow 0$) observations obtained with the IRAM 30 m telescope were combined with the data from the IRAM interferometer before further processing. This is discussed in Sect. 5.2.

To increase the signal to noise ratio, the ¹²CO($J=2\rightarrow 1$) spectra were smoothed to 2.6kms^-1, the velocity resolution of the ¹²CO($J=1\rightarrow 0$) data, except for 8 envelopes with relatively narrow lines: R And, RAFGL482, R Aur, RS Cnc, Y CVn, V CrB, $\chi$ Cyg and T Cep.