Abstract
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.