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 emission from asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud. We have discovered four new circumstellar maser sources in the LMC, and increased the number of reliable wind speeds from IR stars in the LMC from 5 to 13. Using our new wind speeds, as well as those from Galactic sources, we have derived an updated relation for dust driven winds: $v_{exp}\propto Z{L}^{0.4}$. We compare the sub-solar metallicity LMC OH/IR stars with carefully selected samples of more metal-rich OH/IR stars, also at known distances, in the Galactic Centre and Galactic Bulge. For 8 of the Bulge stars we derive pulsation periods for the first time, using near-IR photometry from the VVV survey. We have modeled our LMC OH/IR stars and developed an empirical method of deriving gas-to-dust ratios and mass loss rates by scaling the models to the results from maser profiles. We have done this also for samples in the Galactic Centre and Bulge and derived a new mass loss prescription that includes luminosity, pulsation period, and gas-to-dust ratio $\dot{M}=1.06_{-0.8}^{+3.5}\cdot 10^{-5}(L/10^4{\mathrm{L}}_{\odot }{)}^{0.9\pm 0.1}(P/500\mathrm{d}{)}^{0.75\pm 0.3}({\mathrm{r}}_{\mathrm{gd}}/200{)}^{-0.03\pm 0.07}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. The tightest correlation is found between mass loss rate and luminosity. We find that the gas-to-dust ratio has little effect on the mass loss of oxygen-rich AGB stars and RSGs within the Galaxy and the LMC. This suggests that mass loss of oxygen-rich AGB stars and RSGs is (nearly) independent of metallicity between a half and twice solar.

33 pages, 29 figures, accepted for publication in MNRAS

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