The Abundance of SiC$_{2}$ in Carbon Star Envelopes$^\star$: Evidence that SiC$_{2}$ is a gas-phase precursor of SiC dust

摘要

Silicon carbide dust is ubiquitous in circumstellar envelopes around C-richAGB stars. However, the main gas-phase precursors leading to the formation ofSiC dust have not yet been identified. The most obvious candidates among themolecules containing an Si--C bond detected in C-rich AGB stars are SiC$_2$,SiC, and Si$_2$C. We aim to study how widespread and abundant SiC$_2$, SiC, andSi$_2$C are in envelopes around C-rich AGB stars and whether or not thesespecies play an active role as gas-phase precursors of silicon carbide dust inthe ejecta of carbon stars. We carried out sensitive observations with the IRAM30m telescope of a sample of 25 C-rich AGB stars to search for emission linesof SiC$_2$, SiC, and Si$_2$C in the 2 mm band. We performed non-LTE excitationand radiative transfer calculations based on the LVG method to model theobserved lines of SiC$_2$ and to derive SiC$_2$ fractional abundances in theobserved envelopes. We detect SiC$_{2}$ in most of the sources, SiC in abouthalf 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, whichsuggests that both species are chemically linked, the SiC radical probablybeing the photodissociation product of SiC$_2$ in the external layer of theenvelope. We find a clear trend in which the denser the envelope, the lessabundant SiC$_2$ is. The observed trend is interpreted as an evidence ofefficient incorporation of SiC$_2$ onto dust grains, a process which is favoredat high densities owing to the higher rate at which collisions betweenparticles take place. The observed behavior of a decline in the SiC$_2$abundance with increasing density strongly suggests that SiC$_{2}$ is animportant gas-phase precursor of SiC dust in envelopes around carbon stars.