Journal Article

The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream*

Eric Lagadec, Albert A. Zijlstra, Nicolas Mauron, Gary Fuller, Eric Josselin, G. C. Sloan and A. J. E. Riggs

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 403, issue 3, pages 1331-1338
Published in print April 2010 | ISSN: 0035-8711
Published online April 2010 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.16088.x
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream*

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We report the detection, from observations using the James Clerk Maxwell Telescope, of CO J= 3 → 2 transition lines in six carbon stars, selected as members of the Galactic Halo and having similar infrared colours. Just one Halo star had been detected in CO before this study. Infrared observations show that these stars are red (JK > 3), due to the presence of large dusty circumstellar envelopes. Radiative transfer models indicate that these stars are losing mass with rather large dust mass-loss rates in the range 1–3.3 × 10−8 M yr−1, similar to what can be observed in the Galactic disc. We show that two of these stars are effectively in the Halo, one is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr dSph), and the other three stars certainly belong to the thick disc. The wind expansion velocities of the observed stars are lower compared to carbon stars in the thin disc and are lower for the stars in the Halo and the Sgr dSph stream than in the thick disc. We discuss the possibility that the low expansion velocities result from the low metallicity of the Halo carbon stars. This implies that metal-poor carbon stars lose mass at a rate similar to metal-rich carbon stars, but with lower expansion velocities, as predicted by recent theoretical models. This result implies that the current estimates of mass-loss rates from carbon stars in Local Group galaxies will have to be reconsidered.

Keywords: stars: carbon; circumstellar matter; infrared: stars; stars: mass loss

Journal Article.  4593 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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