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Galactic chemical evolution (GCE) is important for understanding thecomposition of the present-day interstellar medium (ISM) and of our solarsystem. In this paper, we aim to track the GCE by using the 29Si/30Si ratios inevolved stars and tentatively relate this to presolar grain composition. Weused the APEX telescope to detect thermal SiO isotopologue emission toward fouroxygen-rich M-type stars. Together with the data retrieved from the Herschelscience archive and from the literature, we were able to obtain the 29Si/30Siratios for a total of 15 evolved stars inferred from their optically thin 29SiOand 30SiO emission. These stars cover a range of masses and ages, and becausethey do not significantly alter 29Si/30Si during their lifetimes, they provideexcellent probes of the ISM metallicity (or 29Si/30Si ratio) as a function oftime. The 29Si/30Si ratios inferred from the thermal SiO emission tend to belower toward low-mass oxygen-rich stars (e.g., down to about unity for W Hya),and close to an interstellar or solar value of 1.5 for the higher-mass carbonstar IRC+10216 and two red supergiants. There is a tentative correlationbetween the 29Si/30Si ratios and the mass-loss rates of evolved stars, where wetake the mass-loss rate as a proxy for the initial stellar mass or currentstellar age. This is consistent with the different abundance ratios found inpresolar grains. We found that older objects (up to possibly 10 Gyr old) in oursample trace a previous, lower 29Si/30Si value of about 1. Material with thisisotopic ratio is present in two subclasses of presolar grains, providingindependent evidence of the lower ratio. Therefore, the 29Si/30Si ratio derivedfrom the SiO emission of evolved stars is a useful diagnostic tool for thestudy of the GCE and presolar grains.

Silicon isotopic abundance toward evolved stars and its application for presolar grains