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Astron. Astrophys. 357, 669-676 (2000)

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2. Observations and data reduction

The NaCl and AlCl data presented here were obtained with the 30m IRAM telescope in Pico Veleta, Spain between Nov. 29, 1997 and Dec. 1, 1997. The telescope was equipped with three SIS receivers (two in the 3mm range and one in the 1mm range) operating simultaneously, tuned in single side band (SSB) mode. The image sideband rejection was measured to be high (from 20 dB to 30 dB). The antenna temperature scale was calibrated every 10 minutes by the cold load technique. The main beam temperatures, [FORMULA], reported here are related to the antenna temperature scale, [FORMULA], by [FORMULA] = [FORMULA]/[FORMULA] where [FORMULA] is the ratio of the main beam to the forward efficiency, as listed in Table 1. The absolute calibration accuracy was estimated to be of the order of 10%, from comparison with standard calibration spectra and from day to day line intensity variations. The antenna pointing was checked approximately every hour using the 115 GHz continuum receiver on a nearby bright quasar taken from the standard IRAM catalog. The typical pointing drift during one hour was 3", significantly smaller than the telescope beam sizes (HPBW) given in Table 1. The signal from each receiver was sent to a filter bank with 256 channels of 1 MHz and to an autocorrelator with a resolution of about 80 kHz for the 3mm range lines and about 300 kHz for the 1mm range lines. The weather conditions were good, with typical SSB system noise temperatures between 250 and 300 K in the 3mm range and between 550 and 700 K in the 1mm range.


Table 1. Telescope and spectra characteristics for the observed lines.

The observed NaCl and AlCl spectra are shown in Fig. 1 and Fig. 2. The NaCl spectra have been smoothed to a velocity resolution ([FORMULA]) of about 1 km s-1 to increase the signal to noise ratio, whereas the AlCl spectra show their original resolution of about 0.44 km s-1. Only linear or parabolic baselines have been subtracted from the spectra.

[FIGURE] Fig. 1. NaCl (7-6) isotopomers lines observed towards IRC+10216 with the IRAM 30m telescope. The source position is [FORMULA](1950) =09h45m14.8s and [FORMULA](1950) = 13o30´40". The spectral resolutions are 1.03 and 1.05 km s-1, respectively. Only linear or parabolic baselines have been subtracted and the solid lines represent the model profiles fitted to the lines.

[FIGURE] Fig. 2. The same as Fig. 1 for the AlCl (15-14) lines. The spectral resolutions are respectively 0.43 and 0.44 km s-1.

In order to derive the line characteristics, the observed profiles have been fitted to model profiles, which are, as usual for spherical uniformly expanding envelopes, truncated parabolas, with positive or negative curvatures. The results and 1[FORMULA] errorbars of the fits (frequencies, intensities at the line center, [FORMULA], integrated intensities, [FORMULA], envelope expansion velocity, [FORMULA]) are reported in Table 1, and the fitted profiles are plotted in Fig. 1 and Fig. 2.

The line profiles are quite rectangular, as expected for unresolved, optically thin, expanding envelope. As already mentioned by Cernicharo et al. (1987), the expansion velocities derived from the linewidths are smaller than the terminal velocity of 14.5 km s-1 derived for most molecular lines in IRC+10216, indicating that the lines arise from inner layers of the envelope, where the terminal velocity is not yet reached.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000