Astron. Astrophys. 327, 689-698 (1997)

2. Observations

We have performed maps of several molecular lines in OH 231.8+4.2 using the IRAM-30 m MRT at Pico de Veleta (Granada, Spain) in two observing sessions, October 1995 and November 1996. Three SiS receivers working in the 3, 2 and 1.3 mm bands were used at the same time to observe simultaneously different spectral lines (¹² CO(2-1),¹³ CO(2-1), SiO(5-4) and CS(5-4) at = 1.3 mm; SO₂ (10 -9 ) at = 2 mm; and ¹² CO(1-0), ¹³ CO(1-0), (1-0), HCN(1-0) and HNC(1-0) at = 3 mm). The receivers were always tuned in SSB mode (LSB), with typical system temperatures of 1000-2300 K at = 1.3 mm, 1100 K at = 2 mm and 500 K at = 3 mm (in units of , see below). Each receiver IF was connected to a 1 MHz resolution filter bank. We have observed ¹² CO emission from OH 231.8 towards 47 positions fully covering the extent of the nebula. Points are separated by (see Fig. 2). The data obtained in the first observing session for ¹² CO have been previously reported by Alcolea et al. (1996). We have observed again the ¹² CO J =2-1 line in order to improve the quality of the maps and we present here the final results. ¹³ CO (1-0) and SO₂ emission have also been observed at a large number of points (21) over the whole nebula. For the rest of the molecular lines, we observed 8-13 points along a cross with its largest arm probing the nebular axis.

All the data presented here are calibrated in units of Main Beam Rayleigh-Jeans equivalent Antenna Temperature, , using the chopper wheel method by observing hot (ambient) and cold loads (liquid nitrogen). In addition, observations of the well known evolved star CW Leo (IRC+10216) were used to check the calibration of our data with that of previous works.

Weather conditions were quite good for most observations, with zenith opacities at 230 GHz ranging from 0.3 to 0.6 in October 1995 and from 0.08 to 0.3 in November 1996. The pointing of the telescope was verified every about two hours by observing (cross-scanning) continuum sources close in the sky to our target. In order to minimize the effects of possible receiver missalignments, the pointing was done using the 1.3 mm receiver, since it has the narrowest beam (about including the effects of pointing errors during the observations). Absolute pointing errors at = 1.3 mm smaller than are expected. Receiver alignment was checked by simultaneously observing at all wavelengths strong continuum sources (mainly planets). The pointing discrepancies between the 1.3 and 3 mm receivers were found to be much smaller (between ) than the HPBW at 3 mm (). The missalignment between the 1.3 and 2 mm was also very small (, to be compared with a HPBW of ).

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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