Matsuura, M ![]() |
Abstract
We investigate the molecular bands in carbon-rich asymptotic giant branch (AGB) stars in the Large Magellanic Cloud (LMC), using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope (SST) over the 5-38 μm range. All 26 low-resolution spectra show acetylene (C<sub>2</sub>H<sub>2</sub>) bands at 7 and 14 μm. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 μm C<sub>2</sub>H<sub>2</sub> band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 μm C<sub>2</sub>H<sub>2</sub> band by circumstellar dust emission. This 14-μm band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C<sub>2</sub>H<sub>2</sub>column density, derived from the 13.7 μm band, shows a gas mass-loss rate in the range 3 × 10<sup>-6</sup> to 5 × 10<sup>-5</sup>M<sub>solar</sub>yr<sup>-1</sup>. This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 μm C<sub>2</sub>H<sub>2</sub> band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C<sub>2</sub>H<sub>2</sub> abundance among LMC and Galactic stars. This reflects the effect of the third dredge-up bringing self-produced carbon to the surface, leading to high carbon-to-oxygen ratio at low metallicity....
Item Type: | Article |
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Date Type: | Publication |
Status: | Published |
Schools: | Physics and Astronomy |
Publisher: | Oxford University Press |
ISSN: | 0035-8711 |
Date of Acceptance: | 7 June 2006 |
Last Modified: | 22 May 2024 15:45 |
URI: | https://orca.cardiff.ac.uk/id/eprint/168102 |
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