SCYON Abstract

Received on January 9 2001

The peculiar cluster HS 327 in the Large Magellanic Cloud:
can OH/IR stars and carbon stars be twins?

AuthorsJacco Th. van Loon1, A.A. Zijlstra2, L. Kaper3, G.F. Gilmore1, C. Loup4, J.A.D.L. Blommaert5
Affiliation1Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, United Kingdom,
2UMIST, P.O.Box 88, Manchester M60 1QD, United Kingdom,
3Astronomical Institute, University of Amsterdam, Kruislaan 403, NL-1098 SJ Amsterdam, The Netherlands,
4Institut d'Astrophysique de Paris, 98bis Boulevard Arago, F-75014 Paris, France,
5ISO Data Centre, Astrophysics Div., Science Dept. of ESA, Villafranca del Castillo, P.O.Box 50727, E-28080 Madrid, Spain
Accepted byAstronomy & Astrophysics
Contactjacco@ast.cam.ac.uk
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Abstract

The obscured OH/IR star IRAS05298-6957 in the LMC was recently noticed to be member of the small double cluster HS 327 that also contains a carbon star (van Loon et al., 1998, A&A 329, 169). Hence they are coeval and have (nearly) the same progenitor mass, which can only be understood if Hot Bottom Burning (HBB) has prevented IRAS05298-6957 from being a carbon star.

We present extensive visual and near-IR photometric data for > 104 stars in and around HS 327, and spectroscopic data for some of the brightest AGB stars amongst these. Colour-magnitude diagrams are used to estimate the age for the cluster and its members, and luminosities are derived for the stars for which spectra have been obtained. The age for IRAS05298-6957 and the carbon star is estimated to be ~200 Myr. This corresponds to a Main-Sequence progenitor mass ~4.0 MO --- the first direct measurement of the lower mass threshold for HBB. This agrees with stellar evolution models that, however, fail to reproduce the low luminosity of the carbon star.