3 Analysis of the spectral survey

3.1 Line identifications

Figure 1 shows an overview of the 2-mm SSB spectral survey at a resolution of 3 MHz ($\simeq 6$ km s^-1). The spectrum is dominated by the lines of half a dozen of species: CS and SiS whose lines have $T_{\rm MB}\geq 10$ K; SiO, SiC₂ and HC₃N with $T_{\rm MB}\geq 5$ K; C₃N and C₄H with $T_{\rm MB}\geq 1$ K. Figure 2 shows the survey at full spectral resolution. The ordinate scale is antenna temperature, corrected for rear sidelobes and atmosphere attenuation. Most spectra are plotted at a fixed intensity scale (-0.1-0.3 K).

There are 380 spectral lines visible in Fig. 2. Their rest frequencies and intensities are given in Table 2. 83% of these lines can unambiguously be assigned to known molecular species; 63 lines (denoted by the letter U) remain unidentified.

While the 2-mm spectrum of IRC+10216 seems now well understood, such was not the case when we started our observations. Only SiO, SiS, H₂S, HC₃N, HC₅N, C₃N, C₃H, C₄H, and SiC₂ were known to have lines in the 2-mm window. As a matter of fact, the first goal of our survey was to search for metal-bearing compounds and for SiC, whose spectroscopic parameters were poorly known.

In 1987, after the first half of the survey had been completed, we realized that most of the 2-mm lines in IRC+10216 were not visible in the spectra of Orion A or Sgr B2 and that 4 out of 5 lines could not be assigned to any known astrophysical molecule. There followed a several year-long effort to identify the carriers of these unidentified lines. We searched for line patterns characteristic of linear carbon chains, such as the harmonically related series of doublets which led to the discoveries of MgNC, C₅H, and vibrationally excited C₄H (Guélin et al. [1986]; Cernicharo et al. [1986a],b, [1987a],b; Guélin et al. [1987a]), and determined the inertia moments of the carriers. Knowing that IRC+10216 is rich in unsaturated C species and poor in oxygen compounds, we guessed on the basis of simple bond-length considerations, of quantum mechanical calculations, or of existing spectroscopic data what species were the most likely carriers. In many cases the avaibility of accurate rotational constants, derived from microwave laboratory measurements, left no doubt as to the identities of the carriers. In other cases (5 out of 22) the carriers were identified prior to the availability of any laboratory data.

3.2 Catalogue of lines of astrophysical interest

To help with the identification of the lines, we compiled a catalog of the mm-wave transitions of all the molecular species with known rotational constants that were likely to be abundant in IRC+10216 (Cernicharo [1988]). The spectroscopic data were taken from the literature - see in particular the compilations by B. Starck & J. Vogt from Universität Ulm (Demaison et al. [1974]; Vogt [1998]) and by Lovas and co-workers (Lovas & Tiemann [1974]; Lovas [1978], [1986]), as well as the references in Poynter & Pickett ([1984]). In several cases, laboratory measurements were triggered by our work (see Cernicharo et al. [1986c], [1989], [1991a]; Cernicharo & Guélin [1987], [1996]; Gottlieb et al. [1986], [1989]; Guélin et al. [1987b], [1995]).

The frequencies of the rare isotopomers of a diatomic molecule can be fairly accurately derived from the spectroscopic constants of their main isotopomers. As is well known (see e.g. Gordy & Cook [1984]), Dunham's molecular constants $Y_{\rm lm}$ scale with the reduced molecular mass $\mu_{\rm r}$ as

$$Y_{\rm lm}\sim \mu_{\rm r}^{-(l+2m)/2},$$

at least in the first order. Applying this relation and the standard formula expressing the rotational transition frequencies in function of the $Y_{\rm lm}$s (see e.g. Lovas & Tiemann [1974]), it was possible to derive the transition frequencies of diatomic molecules from those of a parent isotopomer within few $\times 10^{-6}$. Species calculated in this way were ²⁹Si³⁴S, ²⁹Si³³S, ³⁰Si³⁴S, ³⁰Si³³S, Si¹⁸O, Si¹⁷O, ²⁹Si¹⁸O, ²⁹Si¹⁷O, ³⁰Si¹⁸O, and ³⁰Si¹⁷O. Some unstable isotopes of other molecular species as ²⁶AlF, ³²SiO, Na³⁶Cl, K³⁶Cl and Al³⁶Cl, have been also included in our catalogue.

The catalogue contains now about 560 molecular isotopomers or species and more than 1.8 10⁵ lines in the range 1-300 GHz. The accuracy of the predicted frequencies is better than 1 MHz for most lines.