{
  "id": "1710.11409",
  "version": "v1",
  "published": "2017-10-31T11:16:24.000Z",
  "updated": "2017-10-31T11:16:24.000Z",
  "title": "The Abundance of SiC$_{2}$ in Carbon Star Envelopes$^\\star$: Evidence that SiC$_{2}$ is a gas-phase precursor of SiC dust",
  "authors": [
    "S. Massalkhi",
    "M. Agúndez",
    "J. Cernicharo",
    "L. Velilla Prieto",
    "J. R. Goicoechea",
    "G. Quintana-Lacaci",
    "J. P. Fonfría",
    "J. Alcolea",
    "V. Bujarrabal"
  ],
  "comment": "Accepted for publication in A&A. 16 pages and 10 figures",
  "categories": [
    "astro-ph.SR"
  ],
  "abstract": "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. 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. 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 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. 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. 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.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2017-10-31T11:16:24.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "c-rich agb stars",
      "carbon star envelopes",
      "silicon carbide dust",
      "important gas-phase precursor",
      "iram 30m telescope"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 16,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}