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

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4. Discussion

Fig. 2a shows the SiCN line profile obtained by averaging the 4 non-blended components of Fig. 1. The profile has the same cusped shape than the line profiles of the carbon chain molecules and radicals (see e.g. the C4H line on Fig. 2b). In the case of a spherical envelope expanding with a constant velocity, cusped line profiles are characteristic of optically thin lines arising from a resolved hollow shell. The emission in the horns arises from the blueshifted and redshifted polar caps and the emission at the center of the line from the meridian ring perpendicular to the line of sight. If the shell thickness is constant, the horn-to-center intensity ratio depends primarily on the shell diameter relative to the telescope beam: the larger the shell, the larger the ratio.

The millimeter line emission from the ground and the first vibrationally excited states of C4H have been mapped with the IRAM interferometer (see e.g. Guélin et al. 1993). They are found to arise from a 4" thick shell of radius 15". This shell is also the source of emission of the carbon chain molecules and of the MgNC radical, all of which show cusped line profiles, when observed with the 30-m telescope. In contrast, the optically thin 30SiS line (Fig. 2d) which arises from a compact region close to the central star, is flat-topped.

The C4H line in Fig. 2 arises from the [FORMULA] bending state, which lies in energy 260 cm-1 above the ground state (Yamamoto et al. 1987); the SiCN line arises from the ground vibrational state. The conditions for the excitation of these two lines are quite different. The similarity of their line profiles cannot be a mere excitation effect. It rather implies that both species coexist spatially, and that SiCN is confined in the same thin shell as C4H.

The molecules confined in the thin outer shell of IRC +10216 have mm-line intensities which agree with those expected for a Blotzmann distribution of the rotational level populations (see Kawaguchi et al. 1995 and CGK). This probably means that they are collisionally excited, and that the physical conditions in the thin shell are fairly uniform. The rotation temperatures derived for the mole-
cules with moments of inertia and dipole moments similar to those of SiCN (HC3N, C3N, MgNC) are all in the range 20-30 K.

Assuming that the rotation temperature of SiCN is also in this range, we derive an SiCN column density along the line of sight (twice the radial column density accross the shell) of [FORMULA] cm-2. For this estimate, we have neglected the population of the [FORMULA] rotational ladder, which lies higher in energy by 71 cm-1 above the [FORMULA] ground state. The derived column density is [FORMULA]20 times smaller than the column density of SiC (Cernicharo et al. 1989) and MgNC, and about equal to that of MgCN (Ziurys et al. 1995). The abundance of SiCN relative to H2, assuming it is confined to the thin 15" radius shell where MgNC is observed, is [FORMULA] inside the shell.

We have also searched for the [FORMULA] rotational transition of SiCCH, without success. None of its [FORMULA]-doublets could be detected, down to a limit of 38 mK.kms-1 (3[FORMULA]), Taking into account the different dipole moments of the two species, the 3[FORMULA] upper limit on the SiCCH abundance we derive is 0.8 of that of SiCN.

An Si-bearing molecule especially abundant in the outer envelope is SiC2. The emission of its [FORMULA] transition, at 94.2 GHz, has been mapped with the IRAM interferometer (Lucas et al. 1994). Although SiC2 is also observed in the inner circumstellar shell, its abundance peaks in the outer shell at a radius of 15". This suggests that SiC2 and SiCN could be formed simultaneously: the peak fractional abundance of SiC2 is close to 10-6 and is two orders of magnitude larger than that of SiCN; hence, if SiC2 is formed in situ, it may not be difficult to form enough SiCN by a parallel path.

In the ion-molecule scheme, SiC2 comes from the reaction of Si+ with C2H or C2H2, leading to SiC2H+, followed by dissociative recombination of the latter ion (Glassgold et al. 1986). SiCN could result from parallel reactions of Si+ with HCN, followed by the recombination of SiCNH+. The reaction rate of Si+ with HCN, however, is a factor of 103 smaller than that with C2H (Millar et al. 1991). In this scheme, the low abundance of SiCCH can be explained by the difficulty of forming [FORMULA].

SiC2 and SiCN could also both be formed by radical-radical reactions involving the photodissociation products C2H and CN. Alternately, these species could be synthesized on the surface of grains and released in space when the grains reach the outer envelope and are exposed to interstellar UV radiation (Guélin et al. 1993).

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000