OH/IR stars in the Magellanic Clouds

Abstract

A sample of IRAS point sources in the Magellanic Clouds has been searched for 1612 MHz maser emission and for infrared emission at JHKL. Six point sources, all in the LMC, are found to be OH masers while 19 have strong infrared emission typical of evolved giant and supergiant stars with dense circumstellar shells. Near-infrared monitoring over 1700 days has yielded pulsation periods for nine of the sources. All sources are classified as AGB stars, core helium burning supergiants or preplanetary nebulae according to pulsation amplitude, luminosity, and redness. The stellar wind expansion velocities deduced from the OH line profiles are only ~0.6 times those expected for comparable OH/IR stars in the Galaxy, a result explicable in terms of the lower metal abundance in the LMC. Mass-loss rates for the LMC OH/IR stars are estimated to be a few times 10^-5^ to a few times 10^-4^ M_sun_ yr^-1^, corresponding to a ratio Mv_exp_c/L~0.5 to 2. Evidence is presented that low-metallicity (SMC) AGB stars have lower mass-loss rates than higher metallicity stars, provided carbon star formation is not involved. It is suggested that carbon star formation in low-metallicity systems keeps mass-loss rates high. There does not appear to be any evidence for the existence of AGB stars in the SMC and LMC which substantially exceed the classical AGB limit of M_bol_~ -7.1, although this possibility has recently been predicted by theoretical calculations.