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Identifying the chemistry of the dust around AGB stars in nearby galaxies

Asymptotic giant branch (AGB) stars are significant contributors to the chemical enrichment of the interstellar medium (ISM) of galaxies. It is therefore essential to constrain the AGB contribution to the dust budget in galaxies. Recent estimates of the total dust injection rate to the Large and Small Magellanic Clouds (LMC and SMC; Riebel et al. 2012, Boyer et al. 2012, Srinivasan et al. in prep) have used data from the Spitzer Space Telescope SAGE (Surveying the Agents of Galaxy Evolution; Meixner et al. 2006) and SAGE-SMC (Gordon et al. 2011) surveys. When sorted by dust chemistry, the data allow for a comparison of O-rich and carbonaceous dust-production rates. In the LMC, for instance, the rate of dust production from carbon stars is about two and a half times that from oxygen-rich AGBs. A reliable determination of the fractional contributions of the two types of dust would serve as input to models of chemical evolution. However, the Spitzer IRAC photometric bands do not sufficiently probe the characteristic mid-infrared spectral features that can distinguish O-rich AGBs from carbon stars - namely, the 9.7 $μ$m silicate feature and the 11.3 $μ$m silicon carbide feature. With the continuous spectral coverage in the 4-30 $μ$m range, SPICA has the potential to distinguish these two types of chemistries. In this contribution, synthetic photometry from the model grid of AGB stars, GRAMS (Sargent et al. 2011; Srinivasan et al. 2011) will be used to discuss the science possibilities that SPICA might offer this study.