A Spitzer study of the mass-loss and infrared variability properties of Asymptotic Giant Branch stars in the Magellanic Clouds

The Asymptotic Giant Branch (AGB) is one of the most interesting, and least understood, phases of stellar evolution. The fate of approximately solar mass stars as they exhaust their nuclear fuel for the final time, these stars are also one of the universe's primary sources for many heavy elements, such as carbon and oxygen. We have assembled a sample of ∼30,000 AGB stars in the Large Magellanic Cloud (LMC) with multi-wavelength data ranging from the ultraviolet to the mid-infrared, and multi-epoch data spanning 5 years. This dataset allows us to probe the variability and mass-loss properties of AGB stars at population scales, a valuable contribution to studies of stellar evolution and the mass budget of the interstellar medium (ISM).;We combine variability information from the MAssive Compact Halo Objects (MACHO; Alcock et al. 1997) microlensing survey with infrared photometry from the Spitzer Space Telescope legacy program Surveying the Agents of Galaxy Evolution (SAGE; Meixner et al. 2006) to determine the infrared period-magnitude relationships for three evolutionary classes of AGB stars at 8 different wavelengths. We find that the most evolved AGB stars are pulsating in the fundamental and first overtone, while less evolved stars are concentrated in higher-overtone modes. We show that the slope of the period-magnitude relationship becomes steeper for more evolved stars, at all wavelengths.;Using a grid of radiative transfer models of circumstellar dust shells (GRAMS; Sargent et al. 2011; Srinivasan et al. 2011) and photometry in 12 bands ranging from the ultraviolet to the infrared, we calculated individual bolometric luminosities and dust mass-loss rates for each AGB star in the LMC. This allowed us to calculate the total dust injection to the interstellar medium from these stars via direct summation. We find that the total mass injection rate (gas and dust) from AGB stars into the ISM of the LMC is ∼5x10 --3 M⊙ yr--1, and that carbon-rich AGB stars and oxygen-rich AGB stars contribute equal amounts of total mass. The carbon-rich stars dominate the dust injection rate by a factor of two and half, however. We show that all AGB stars pulsating in the first overtone or fundamental mode are producing measurable amounts of dust.;Using the warm mission of the Spitzer Space Telescope, we have taken 4 epochs of observations of the densest stellar regions of both Magellanic Clouds at 3.6 & 4.5 mum to investigate the infrared variability of sources not visible at the optical wavelengths explored by ground-based surveys. By comparing the brightness of every source in each pair of epochs, we identify 3660 variable sources in the LMC and 1283 such sources in the SMC. Comparing our dataset to other surveys such as OGLE and MACHO, 2741 of our variables have no currently known counterpart in these studies.