The metallicity dependence of maser emission and mass loss from red supergiants and asymptotic giant branch stars

Abstract

Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) are some of the largest contributors of dust and material back to the Universe. While a substantial
amount of RSG dust will be destroyed by the subsequent supernova, the dust from AGB stars will be injected into the Interstellar Medium. Understanding the contribution of these stars and how it is affected by changes in stellar parameters is critically important to determining stellar lifetimes, constraining stellar models, and understanding the evolutionary paths of core-collapse progenitors and their subsequent supernova. This study has discovered four new circumstellar masers in the Large Magellanic Cloud (LMC), and increased the number of reliable wind speeds found outside of our galaxy from 5 to 13. Results have been used to develop a relation for the wind speed as a function of luminosity and metallicity. A further analysis of the spectral energy
distributions of these and Galactic OH/IR stars has led to the development of a robust empirical mass-loss prescription that uses luminosity, pulsation period and gas-to-dust ratio, a measure of metallicity. These results suggest that mass loss in this phase is (nearly) independent of metallicity between a half and twice solar metallicity. A radio survey of evolved stars in the Small Magellanic Cloud (SMC) has found that none of the most luminous sources expected to show OH maser emission, do so. While sources may lie just below both the detection threshold and/or the required OH column density, a number of sources should still exhibit OH maser emission. It is possible that the OH masing phase of lower metallicity massive AGB stars and RSGs is cut short. New observations in the radio, optical and infra-red have been used to constrain the location and mass-loss rate of the prominent dust producing LMC RSGs, IRAS 05280-6910 and IRAS 05346-6949. These results suggest that these sources may exhibit a dusty torus geometry, but confirmation will require further high angular resolution studies. The results of this work have made progress in understanding the wind driving and mass loss mechanism, and have set the stage for much larger upcoming surveys with the SKA and its pathfinders ASKAP and MeerKAT.