Obscured asymptotic giant branch stars in the Magellanic Clouds - II. Near-infrared and mid-infrared counterparts

We have carried out an infrared search for obscured asymptotic giant branch (AGB) stars in the Magellanic Clouds. Fields were observed in the vicinity of IRAS sources with colours and flux densities consistent with such a classification. The survey uncovered a number of obscured AGB stars as well as some supergiants with infrared excess. We present photometry of the sources and discuss the colour diagrams and bolometric luminosities. One of the supergiants is close to the maximum luminosity allowed for red supergiants, implying a progenitor mass around 50 M_solar. Its late spectral type (M7.5) is surprising for such a massive star. Most of the AGB stars are luminous, often close to the classical limit of M_bol=-7.1. To determine whether the stars are oxygen-rich, we have acquired narrow-band mid-infrared photometry with the ESO TIMMI camera for several sources. All but one are found to show the silicate feature and therefore to have oxygen-rich dust; the colours of the remaining source are consistent with either an oxygen-rich or a carbon-rich nature. A method to distinguish carbon and oxygen stars based on H-K versus K-[12] colours is presented. We discuss several methods of calculating the mass-loss rate: for the AGB stars the mass-loss rates vary between approximately 5x10^-4 and 5x10^-6 M_solar yr^-1, depending on the assumed dust-to-gas mass ratio. We present a new way to calculate mass-loss rates from the OH maser emission. We find no evidence for a correlation of the mass-loss rates with luminosity in these obscured stars. Nor do the mass-loss rates for the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) stars differ in any clear systematic way from each other. Expansion velocities appear to be slightly lower in the LMC than in the Galaxy. Period determinations are discussed for two sources: the periods are comparable to those of the longer-period Galactic OH/IR stars. All of the luminous stars for which periods are available have significantly higher luminosities than predicted from the period-luminosity relations.