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Astron. Astrophys. 363, L9-L12 (2000)

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2. Observations

The three just mentioned silicon radicals are linear and have a regular [FORMULA] electronic ground state. In either case, their rotational spectra consist of nearly harmonic [FORMULA] doublets separated by [FORMULA] GHz. Each rotational transition is split into several components by [FORMULA]-doubling and magnetic hyperfine structure, but for the large J transitions of interest here, the hyperfine structure is unresolved, and the spectrum reduces to a series of doublets with a constant splitting. The two [FORMULA]-doublet components are denoted e and f (see AMGT). By analogy with MgNC and HC3N, which have similar moments of inertia and dipole moments 1, the rotational lines are expected to peak in intensity in IRC +10216 near a wavelength of 3 mm.

The observations were made with the IRAM 30-m telescope in July 2000. Two SIS mixer receivers with orthogonal polarizations were used to observe the 3-mm rotational transitions of SiCN and SiCCH. The receivers were tuned to a single sideband, with rejections of the upper sideband [FORMULA] dB. (The rejection level was measured against a frequency-modulated load and checked by recording the intensity of strong astronomical lines from the image sideband.) The system noise temperature was 110-150 K.

The data were taken in the balanced wobbler-switching mode, with a wobbling period of 0.5 Hz and a throw of 2'. This mode yielded baselines so flat that only first degree polynomials had to be removed across the 0.5 GHz-wide band of the spectrometer. The channel resolution was 1 MHz. The telescope pointing and focus were checked every 1-2 hours on the nearby continuum source OJ287. After calibration and baseline removal, the spectra observed in July 2000 were averaged with spectra previously taken in the course of a spectral survey of IRC +10216 (Cernicharo, Guélin & Kahane 2000, hereafter CGK).

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000