1 ISO Data Centre, VILSPA, Madrid, Spain
2 Max-Planck-Institut für Astrophysik, Garching, Germany
3 Astronomisch Instituut, Univ. of Amsterdam, The Netherlands
4 Sterrewacht Leiden, The Netherlands
5 CNRS, Institut d'Astrophysique Paris, France
6 UMIST, Manchester, United Kingdom
We used ISOCAM and ISOPHT to observe the spectral energy distribution (SED) between 3.6 and 60 microns of Asymptotic Giant Branch stars in the MCs detected by IRAS. PHT-S and CAM-CVF spectra are made which enable us to establish the carbon- or oxygen-rich nature of the stars. The 9.7 micron silicate feature gives a measure of the optical depth of the dust shell. Ground-based near-infrared data has been obtained for the full sample.
At the tip of the AGB, just before becoming Planetary Nebulae and ultimately
White Dwarfs, the stars become highly variable with large amplitude
variations. These conditions lead to a high mass loss (up to
)
which will dominate the further evolution of
the star. The
circumstellar shell created obscures the star and shifts its SED to the
infrared. We are analysing the SEDs using a radiative transfer model
which provides accurate determinations of the luminosities and mass loss
rates of the objects. This sample allows a careful study of the final stages
of the evolution of low- to intermediate mass stars, not hampered by
uncertainties in the distances. The effect of the high mass loss rates
on the evolution is studied.
Combining the data on the SMC, LMC and existing data on the Galaxy will answer the open question of the metallicity dependence of mass loss in late-type stars. This in turn is important in the ejection of matter by AGB stars to the interstellar medium over the age of the Galaxy.
The results so far confirm the AGB nature of the selected IRAS sources and
show that the sample consists of Carbon- and oxygen-rich stars with
luminosities ranging from 5000 Lsun up to the AGB-limit of 55,000 Lsun and
mass loss rates between
and
.
The position of the stars in the Period-Luminosity diagram (important in the
understanding of the evolution) will be shown.
Some peculiar cases like IRAS04496-6958, a carbon star with silicate dust
(the first one detected in the LMC) will be presented.