Obscured Asymptotic Giant Branch Stars in the Magellanic Clouds

The most drastic change in the life of an intermediate mass star occurs when it approaches the tip of the Asymptotic Giant Branch (AGB). Large amplitude pulsation of the stellar photosphere and favourable conditions for dust formation cause these stars to develop heavy mass loss, leading to the star's death. The dusty circumstellar envelope (CSE) obscures the optical light from the star and re-emits at longer wavelengths, making it a very bright infrared (IR) object. The physical mechanism of the mass loss and its temporal behaviour are not understood. AGB stars can be best studied in either of the Magellanic Clouds, as these stars are all at nearly the same, well known distance to us, and suffer relatively little interstellar extinction. The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) have metallicities a factor ~ 2 and 5 lower than the Milky Way, hence the metallicity dependence of the evolution and mass loss of AGB stars can be studied. A significant number of obscured AGB stars have been found in the Magellanic Clouds only very recently (Loup et al. 1997; Zijlstra et al. 1996; van Loon et al. 1997, 1998a; Groenewegen and Blommaert 1998). I first briefly describe our searches for AGB stars as counterparts of IRAS point sources in the Magellanic Clouds, using near-IR photometers and arrays. IR spectrophotometry and spectroscopy from the ground and from space (IRAS and ISO) are used to classify the stars as oxygen or carbon rich AGB stars. Both oxygen and carbon stars can be found at all luminosities from 6,000 to 40,000 Lo. Luminous carbon stars are the result of a reduced envelope mass due to mass loss, switching off Hot Bottom Burning. Near-IR monitoring has resulted in known periods and amplitudes for the obscured AGB stars in the Large Magellanic Cloud. The period-luminosity diagram of these Long Period Variables (LPVs) indicates the occurrence of thermal pulses. I show that the reddest stars, with the optically thickest CSEs, are not the most luminous stars, because of the differences in dust formation radius and (possibly) outflow velocities. OH masers yield outflow velocities, but they have been severely underestimated for sources in the Large Magellanic Cloud. I show our SiO and H2O maser detections (also: van Loon et al. 1996, 1998b) in some of these sources, and argue that there is no evidence for metallicity dependence of the outflow velocities. The available data are consistent with equal mass-loss rates irrespective of metallicity, with the dust-to-gas ratio directly proportional to metallicity. I then place the (obscured) AGB stars in the context of the hosting galaxies. Stellar evolution, mass loss, and metallicity differences all affect the period-luminosity diagram of LPVs, and I discuss the implications for their use as distance indicators. I discuss the use of obscured AGB stars as tracers of the star formation history, metallicity, and kinematics throughout the host galaxy. Finally I address the impact of AGB mass loss on the chemical enrichment of the interstellar medium.