The Abundance of SiC 2 in Carbon Star Envelopes: Evidence that SiC 2 is a gas-phase precursor of SiC dust.

Context: Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich AGB stars. However, the main gas-phase precursors leading to the formation of SiC dust have not yet been identified. The most obvious candidates among the molecules containing an Si-C bond detected in C-rich AGB stars are SiC ₂ , SiC, and Si ₂ C. To date, the ring molecule SiC ₂ has been observed in a handful of evolved stars, while SiC and Si ₂ C have only been detected in the C-star envelope IRC +10216.
Aims: We aim to study how widespread and abundant SiC ₂ , SiC, and Si ₂ C are in envelopes around C-rich AGB stars and whether or not these species play an active role as gas-phase precursors of silicon carbide dust in the ejecta of carbon stars.
Methods: We carried out sensitive observations with the IRAM 30m telescope of a sample of 25 C-rich AGB stars to search for emission lines of SiC ₂ , SiC, and Si ₂ C in the λ 2 mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC ₂ and to derive SiC ₂ fractional abundances in the observed envelopes.
Results: We detect SiC ₂ in most of the sources, SiC in about half of them, and do not detect Si ₂ C in any source, at the exception of IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC ₂ line emission, which suggests that both species are chemically linked, the SiC radical probably being the photodissociation product of SiC ₂ in the external layer of the envelope. We find a clear trend in which the denser the envelope, the less abundant SiC ₂ is. The observed trend is interpreted as an evidence of efficient incorporation of SiC ₂ onto dust grains, a process which is favored at high densities owing to the higher rate at which collisions between particles take place.
Conclusions: The observed behavior of a decline in the SiC ₂ abundance with increasing density strongly suggests that SiC ₂ is an important gas-phase precursor of SiC dust in envelopes around carbon stars.