3 Observations and data reduction

  The observations were carried out in July 1994 using the IRAM 30-m radiotelescope at Pico Veleta (Spain). Spectra were obtained in position switching with a reference position at 30$^\prime$ offset or using the wobbler (i.e. a nutating secondary) with a beam-throw of 240$^{\prime\prime}$ and a phase duration of 2 s. The calibration was performed with the chopper wheel method with both hot (sky and absorber) and cold (nitrogen) loads. The calibration was verified on well-known molecular sources and by continuum scans across Jupiter, Uranus, Mars, and Saturn: we found it to be accurate to within 20% at all frequencies. The absolute pointing was good to 4$^{\prime\prime}$.The conversion factor from main beam brightness temperature ($T_{\rm MB}$) to flux density is 4.7 Jy K^-1. The backends were: 1) two filterbanks spanning 512 MHz and providing a frequency resolution of 1 MHz; 2) a filterbank with 25.6 MHz bandwidth and 100 kHz resolution; and 3) an autocorrelator used in various high spectral resolution configurations, so that three bands could be observed simultaneously.

Two $3\times3$ point raster maps centred on the H₂O maser position (see Table 1) were made; one with 24$^{\prime\prime}$ sampling, the other with 12$^{\prime\prime}$. In some cases, larger fields were covered to complete the mapping of the cloud. The integration time (ON + OFF) per point was 1 min and the maps were repeated up to three times and then averaged. The pointing and the focus were verified regularly (every hour) on nearby continuum sources. A total of 7 molecular species and 11 rotational transitions were mapped. They are summarised in Table 4 together with the corresponding half power beam width (HPBW) of the telescope.

The data were reduced with the CLASS and GRAPHIC programs of the Grenoble Astrophysical Group (GAG) package. A polynomial baseline was removed from each spectrum and then spectra taken towards the same position were averaged; however, in cases where more than one map had been taken in the same tracer towards a given source, the different maps were compared before average, to ensure consistency between them. Finally, channel maps and integrated maps in suitable velocity intervals were created.

  

Table 4: Frequencies of observed molecular transitions and telescope HPBWs

^a Frequency of the 3₀-2₀ E transition.
^b Frequency of the K=0 transition.
^c Frequency of the 5₀-4₀ E1 transition.