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An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars

¹ Astrophysics Group, School of Physical & Geographical Sciences, Keele University, Staffordshire ST5 5BG, UK e-mail: jacco@astro.keele.ac.uk
² European Southern Observatory, Karl-Schwarzschild Straße 2, 85748 Garching bei München, Germany
³ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ School of Physics and Astronomy, University of Manchester, Sackville Street, PO Box 88, Manchester M60 1QD, UK
⁵ Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France

Received: 16 December 2004
Accepted: 12 April 2005

Abstract

We present an empirical determination of the mass-loss rate as a function ofstellar luminosity and effective temperature, for oxygen-rich dust-enshroudedAsymptotic Giant Branch stars and red supergiants. To this aim we obtainedoptical spectra of a sample of dust-enshrouded red giants in the LargeMagellanic Cloud, which we complemented with spectroscopic and infraredphotometric data from the literature. Two of these turned out to be hotemission-line stars, of which one is a definite B[e] star. The mass-loss rateswere measured through modelling of the spectral energy distributions. We thusobtain the mass-loss rate formula, validfor dust-enshrouded red supergiants and oxygen-rich AGB stars. Despite the lowmetallicity of the LMC, both AGB stars and red supergiants are found at latespectral types. A comparison with galactic AGB stars and red supergiants showsexcellent agreement between the mass-loss rate as predicted by our formula andthat derived from the 60 μm flux density for dust-enshrouded objects, butnot for optically bright objects. We discuss the possible implications of thisfor the mass-loss mechanism.