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The nature of the silicon carbide in carbon star outflows

Speck, AK; Barlow, MJ; Skinner, CJ; (1997) The nature of the silicon carbide in carbon star outflows. Monthly Notices of the Royal Astronomical Society , 288 (2) pp. 431-456. 10.1093/mnras/288.2.431. Green open access

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Abstract

We present 7.5-13.5 μm UKIRT CGS3 spectra of 32 definite or candidate carbon stars. In addition to the extreme carbon star AFGL 3068, the only carbon star previously known to show the 11-μm silicon carbide (SiC) feature in absorption, we have discovered three further examples of sources that show SiC in net absorption, namely IRAS 02408 + 5458, AFGL 2477 and AFGL 5625. We investigate the mineralogy of carbon star SiC and its relationship to meteoritic dust by using a χ2-minimization routine to fit the observed SiC features, and laboratory optical constants that have been published for a variety of SiC samples. With the exception of R For, all of the observed SiC features are best fitted by α-SiC grains. Excluding V414 Per, all of the sources with 8–13 μm colour temperatures > 1200 K (corresponding to mass-loss rates at the bottom end of the range) are best fitted by μ-SiC in pure emission, whereas all but one of the sources with 8–13 μm colour temperatures < 1200 K (corresponding to higher mass-loss rates) are best fitted using self-absorbed a-SiC emission. The four sources whose SiC features are in net absorption (and which have the lowest 8–13 μm colour temperatures and therefore presumably the highest mass-loss rates) are also well fitted by self-absorbed α-SiC emission, but with higher optical depths. Given that β-SiC is the form most commonly found in meteorites, we have searched for evidence of β-SiC in the circumstellar shells of all these carbon stars. However, our observations provide no unambiguous evidence for the presence of β-SiC around these stars, with all of the observed SiC features being best explained in terms of α-SiC grains. The self- absorption that we find in the observed SiC emission features has not previously been taken into account in radiative transfer modelling, and so the amount of SiC present in the outflows has probably been underestimated in the past.

Type: Article
Title: The nature of the silicon carbide in carbon star outflows
Open access status: An open access version is available from UCL Discovery
DOI: 10.1093/mnras/288.2.431
Publisher version: https://doi.org/10.1093/mnras/288.2.431
Language: English
Additional information: This version is the version of record. For information on re-use, please refer to the publisher’s terms and conditions.
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy
URI: https://discovery.ucl.ac.uk/id/eprint/10096096
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