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

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3. Results

Three spectra, corresponding to the [FORMULA] and [FORMULA] rotational transitions of SiCN are presented in Fig. 1. All three doublets are detected, even though two components ([FORMULA] at 82990.4 MHz, and [FORMULA] at 105122.0 MHz), appear partly blended with lines of NaCN and H13CC4N.

[FIGURE] Fig. 1. Spectra from the 30-m telescope of three successive rotational transitions of SiCN. The position of the SiCN [FORMULA]-doublet components, at the center of each spectrum, and those of other identified lines are indicated by arrows. The lines of C4H at 105174 MHz and 105231 MHz are the fine-structure components of the [FORMULA] rotational transition in the [FORMULA] bending state. The broad feature around 82880 MHz is a blend of transitions of C8H and of vibrationnally excited HC5N ([FORMULA]=2); note that the noise is larger on the left side of the bottom spectrum, because the integration time was a factor of 3 smaller below 82960 MHz, than above.

Despite the large number of lines observed in IRC +10216 at the 5 mK level, there can be little doubt that the three doublets of Fig. 1 are from SiCN: the rest frequencies of the components are accurately measured; they agree with the laboratory-derived frequencies to within 0.3 MHz, or 1/30th of the linewidth in IRC +10216 (Table 1). Moreover, we have unsuccessfully searched for alternate assignments of these lines in the standard catalogs of molecular transitions of astrophysical interest, as well as in a line catalog specifically designed for IRC +10216 (see CGK). We note in this respect that we could identify with the help of this catalog all but one of the 18 spectral lines in Fig. 1 not from SiCN.


Table 1. Observed line parameters

Among these, we note the [FORMULA] line of NP, at 93979 MHz, the [FORMULA] lines of SiC3, at 105200 MHz and 105202 MHz, the [FORMULA] lines of H2C6 at 105011 MHz, and a weak feature which could be a blend of the [FORMULA] [FORMULA] lines of H2C6 at 94115-120-124 MHz.

SiC3 is a highly polar rhomboidal molecule which was recently discovered in IRC +10216 by Apponi et al. (1999 - see also CGK). The detection of three rotational lines of NP in IRC +10216 was previously described by CGK. The NP line shown in Fig. 1 is now observed with a better signal-to-noise ratio than before and its frequency (Table 1) and profile shape (Fig. 2c) can be accurately derived. The rest frequency agrees very well with the laboratory-derived frequency, leaving little doubt as to the identification of this phosphorus molecule.

[FIGURE] Fig. 2a-d. The SiCN line profile a , obtained by averaging the 4 unblended SiCN components of Fig. 1, compared to the profiles of the C4H [FORMULA] line b , the NP [FORMULA] line c , and the 30SiS [FORMULA] line d . Note that the C4H, NP and 30SiS lines, which are much stronger than the SiCN line, have been multipled by 1/15, 1/2, and 1/30, respectively, in order to appear at the same scale.

In a previous paper (Guélin et al. 1997), we had reported the tentative discovery of H2C6 in IRC +10216. The weak features in Fig. 1 are the fourth and, possibly, fifth spectral lines we assign to this long carbene, confirming our earlier discovery. The detection of H2C6 in another astronomical source, TMC 1, was also reported by Langer et al. (1997), shortly after our first report.

Another astronomical source where Si-bearing molecules are detected is Sgr B2. We searched unsuccessfully for the 94.1 GHz lines of SiCN in this source. For an assumed rotation temperature of 20 K, the [FORMULA] limit on the column density we derive is [FORMULA] cm-2.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000