Very Large Telescope three micron spectra of dust-enshrouded red giants in the Large Magellanic Cloud

¹ Astrophysics Group, School of Physical & Geographical Sciences, Keele University, Staffordshire ST5 5BG, UK e-mail: jacco@astro.keele.ac.uk
² Radio Astronomy Lab, 601 Campbell Hall, University of California at Berkeley, Berkeley CA 94720-3411, USA
³ Department of Physics and Astronomy, University of Manchester, Sackville Street, PO Box 88, Manchester M60 1QD, UK
⁴ Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia
⁵ Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan

Received: 18 September 2005
Accepted: 17 October 2005

Abstract

We present ESO/VLT spectra in the 2.9-4.1 μm range for a large sample of infrared stars in the Large Magellanic Cloud (LMC), selected on the basis of MSX and 2MASS colours to be extremely dust-enshrouded AGB star candidates. Out of 30 targets, 28 are positively identified as carbon stars, significantly adding to the known population of optically invisible carbon stars in the LMC. We also present spectra for six IR-bright stars in or near three clusters in the LMC, identifying four of them as carbon stars and two as oxygen-rich supergiants. We analyse the molecular bands of C₂H₂ at 3.1 and 3.8 μm, HCN at 3.57 μm, and sharp absorption features in the 3.70-3.78 μm region that we attribute to C₂H₂. There is evidence for a generally high abundance of C₂H₂ in LMC carbon stars, suggestive of high carbon-to-oxygen abundance ratios at the low metallicity in the LMC. The low initial metallicity is also likely to have resulted in less abundant HCN and CS. The sample of IR carbon stars exhibits a range in C₂H₂:HCN abundance ratio. We do not find strong correlations between the properties of the molecular atmosphere and circumstellar dust envelope, but the observed differences in the strengths and shapes of the absorption bands can be explained by differences in excitation temperature. High mass-loss rates and strong pulsation would then be seen to be associated with a large scale height of the molecular atmosphere.