The wind speeds, dust content, and mass-loss rates of evolved AGB and RSG stars at varying metallicity

Abstract

We present the results of our survey of 1612 MHz circumstellar OH maser\nemission from asymptotic giant branch (AGB) stars and red supergiants (RSGs) in\nthe Large Magellanic Cloud. We have discovered four new circumstellar maser\nsources in the LMC, and increased the number of reliable wind speeds from IR\nstars in the LMC from 5 to 13. Using our new wind speeds, as well as those from\nGalactic sources, we have derived an updated relation for dust driven winds:\n$v_{exp} \\propto Z L^{0.4}$. We compare the sub-solar metallicity LMC OH/IR\nstars with carefully selected samples of more metal-rich OH/IR stars, also at\nknown distances, in the Galactic Centre and Galactic Bulge. For 8 of the Bulge\nstars we derive pulsation periods for the first time, using near-IR photometry\nfrom the VVV survey. We have modeled our LMC OH/IR stars and developed an\nempirical method of deriving gas-to-dust ratios and mass loss rates by scaling\nthe models to the results from maser profiles. We have done this also for\nsamples in the Galactic Centre and Bulge and derived a new mass loss\nprescription that includes luminosity, pulsation period, and gas-to-dust ratio\n$\\dot{M} = 1.06^{+3.5}_{-0.8} \\rm{ \\cdot\n}10^{-5}\\,(L/10^4\\,\\rm{L}_\\odot)^{0.9\\pm0.1}(P/500\\,\\rm{d})^{0.75\\pm0.3}\n(r_{gd}/200)^{-0.03\\pm0.07}\\,\\rm{M_{\\odot}}\\, yr^{-1}$. The tightest\ncorrelation is found between mass loss rate and luminosity. We find that the\ngas-to-dust ratio has little effect on the mass loss of oxygen-rich AGB stars\nand RSGs within the Galaxy and the LMC. This suggests that mass loss of\noxygen-rich AGB stars and RSGs is (nearly) independent of metallicity between a\nhalf and twice solar.\n