Astron. Astrophys. 342, 257-270 (1999)

2. Heterodyne observations

The heterodyne observations of IC 5146 were carried out with the IRAM 30m radiotelescope at Pico Veleta near Granada, Spain, during 8 nights in April 1996, and 2 nights in August 1997. The (10) and (21) rotational transitions of were observed simultaneously with two SIS receivers tuned to single sideband mode: the image-sideband was suppressed better than -22dB. The same setup was used for the two transitions. As backends we used the autocorrelators with a bandwidth of 40 (80) MHz and a resolution of 40 (80) kHz at 110 (220) GHz, leading to a velocity resolution of
kms^-1. All observations in 1996 were conducted by frequency switching with a period of 3 seconds and a throw of 7.7 MHz. This frequency throw was chosen to minimize a possible baseline ripple of the same frequency caused by reflections between the telescope receiver and parts of the subreflector (Thum et al. 1995). In 1997, we observed at several additional positions by position switching to an off-position at (,) relative to our (,) position. The off-position was checked to be free of emission with an rms()=0.25 K.

Data were corrected online for sky transmission by measuring the radiation temperature of the sky at the elevation of the astronomical source and by then using an atmospherical model to fit a model atmosphere to the detected sky emission (Cernicharo 1985) in order to derive the transmission. During the 1996 observing time, zenith opacities at 220 GHz ranged between 0.1 and 0.3. In 1997, opacities were about 0.6.

To derive main beam temperatures (Downes 1989) we multiplied antenna temperatures with the ratio of forward efficiency over main beam efficiency . These ratios are 1.35 and 2.10 at 110 and 220 GHz respectively (Kramer 1997, Greve et al. 1998). The main beam widths at 110 and 220 GHz are and corresponding to 0.05 and 0.025 pc respectively.

The map (Fig. 1) comprises 594 positions and covers a region of (pc²). The central region was observed on a grid to sample at half the FWHM of the beam at 220 GHz. Baselines of order 3 to 5 were subtracted from the frequency switched spectra before and after folding leading to average rms values of 0.09 K at 110 GHz and 0.13 K at 220 GHz after about 1.5 minutes of integration. To check the relative calibration uncertainty, we took spectra at the center position about every hour. The rms of the integrated intensities at this position was found to be 7% and 17% at 110 GHz and 220 GHz respectively. We also observed (10) and (21) at 24 positions in the mapped region (Fig. 1b) reaching an rms value of K after minutes of total integration time. In addition, we conducted pointed observations towards the positions of 94 background stars in the direction of the Northern Streamer (Fig. 10). was observed towards 7 stars with mag (Table 1). Integration times were the same as for the mapped region.

Fig. 1. a  The velocity integrated (21) emission in greyscales. Superimposed in contours is the dust extinction from the new NIR results with a resolution of (Lada et al. 1998). Contours of extinctions are 3 () to 27 mag by 2 mag. For (21) the velocity range of integration is 2 to 4.5 kms-1. Grey levels are 0.2 () by 0.2 to 1.4 Kkms-1 (). The (0,0) position lies at = , = . Small crosses mark the positions of the observations. A diagonal cut in NW-SE direction is indicated by a line (cf. Fig. 8). b  The velocity integrated (10) emission in greyscales. The velocity range of integration is 1.5 to 5.5 kms-1. Grey levels are 0.18 () by 0.18 to 2.52 Kkms-1 (), which is the peak intensity. Crosses mark the positions of the observations.

Table 1. (10) opacities at all positions within the mapped region observed in (upper table). Opacities are derived via Eq. 1 from the ratio of integrated and line intensities. and are in [Kkms^-1] on a scale. Errors in are %. The last column gives the optical extinctions derived from the NIR data of Lada et al. (1998). The lower table gives the absolute positions and parameters of all 7 background stars at which C¹⁷O was observed. The last column of the lower table gives optical extinctions derived from the NIR data of LLCB94.

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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