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The Abundance of SiC 2 in Carbon Star Envelopes: Evidence that SiC 2 is a gas-phase precursor of SiC dust.
- Source :
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Astronomy and astrophysics [Astron Astrophys] 2018 Mar; Vol. 611. Date of Electronic Publication: 2018 Mar 20. - Publication Year :
- 2018
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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 <subscript>2</subscript> , SiC, and Si <subscript>2</subscript> C. To date, the ring molecule SiC <subscript>2</subscript> has been observed in a handful of evolved stars, while SiC and Si <subscript>2</subscript> C have only been detected in the C-star envelope IRC +10216.<br />Aims: We aim to study how widespread and abundant SiC <subscript>2</subscript> , SiC, and Si <subscript>2</subscript> 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.<br />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 <subscript>2</subscript> , SiC, and Si <subscript>2</subscript> 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 <subscript>2</subscript> and to derive SiC <subscript>2</subscript> fractional abundances in the observed envelopes.<br />Results: We detect SiC <subscript>2</subscript> in most of the sources, SiC in about half of them, and do not detect Si <subscript>2</subscript> 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 <subscript>2</subscript> line emission, which suggests that both species are chemically linked, the SiC radical probably being the photodissociation product of SiC <subscript>2</subscript> in the external layer of the envelope. We find a clear trend in which the denser the envelope, the less abundant SiC <subscript>2</subscript> is. The observed trend is interpreted as an evidence of efficient incorporation of SiC <subscript>2</subscript> onto dust grains, a process which is favored at high densities owing to the higher rate at which collisions between particles take place.<br />Conclusions: The observed behavior of a decline in the SiC <subscript>2</subscript> abundance with increasing density strongly suggests that SiC <subscript>2</subscript> is an important gas-phase precursor of SiC dust in envelopes around carbon stars.
Details
- Language :
- English
- ISSN :
- 0004-6361
- Volume :
- 611
- Database :
- MEDLINE
- Journal :
- Astronomy and astrophysics
- Publication Type :
- Academic Journal
- Accession number :
- 29628518
- Full Text :
- https://doi.org/10.1051/0004-6361/201732038