2018A&A...611A..29M

Context. Silicon carbide dust is ubiquitous in circumstellar envelopes around C-rich asymptotic giant branch (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 SiC₂, SiC, and Si₂C. To date, the ring molecule SiC₂ has been observed in a handful of evolved stars, while SiC and Si₂C have only been detected in the C-star envelope IRC +10216. Aim. We aim to study how widespread and abundant SiC₂, SiC, and Si₂C 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 SiC₂, SiC, and Si₂C in the λ 2mm band. We performed non-LTE excitation and radiative transfer calculations based on the LVG method to model the observed lines of SiC₂ and to derive SiC₂ fractional abundances in the observed envelopes.
Results. We detect SiC₂ in most of the sources, SiC in about half of them, and do not detect Si₂C in any source except IRC +10216. Most of these detections are reported for the first time in this work. We find a positive correlation between the SiC and SiC₂ line emission, which suggests that both species are chemically linked; the SiC radical is probably the photodissociation product of SiC₂ in the external layer of the envelope. We find a clear trend where the denser the envelope, the less abundant SiC₂ is. The observed trend is interpreted as evidence of efficient incorporation of SiC₂ onto dust grains, a process that 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 SiC₂ abundance with increasing density strongly suggests that SiC₂ is an important gas-phase precursor of SiC dust in envelopes around carbon stars.