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

Aims. Galactic chemical evolution (GCE) is important for understanding the composition of the present-day interstellar medium (ISM) and of our solar system. In this paper, we aim to track the GCE by using the Si-29/Si-30 ratios in evolved stars and tentatively relate this to presolar grain composition.Methods. We used the APEX telescope to detect thermal SiO isotopologue emission toward four oxygen-rich M-type stars. Together with the data retrieved from the Herschel science archive and from the literature, we were able to obtain the Si-29/Si-30 ratios for a total of 15 evolved stars inferred from their optically thin (SiO)-Si-29 and (SiO)-Si-30 emission. These stars cover a range of masses and ages, and because they do not significantly alter Si-29/Si-30 during their lifetimes, they provide excellent probes of the ISM metallicity (or Si-29/Si-30 ratio) as a function of time.Results. The Si-29/Si-30 ratios inferred from the thermal SiO emission tend to be lower 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 carbon star IRC+ 10216 and two red supergiants. There is a tentative correlation between the Si-29/Si-30 ratios and the mass-loss rates of evolved stars, where we take the mass-loss rate as a proxy for the initial stellar mass or current stellar age. This is consistent with the different abundance ratios found in presolar grains. Before the formation of the Sun, the presolar grains indicate that the bulk of presolar grains already had Si-29/Si-30 ratios of about 1.5, which is also the ratio we found for the objects younger than the Sun, such as VY CMa and IRC+ 10216. However, we found that older objects (up to possibly 10 Gyr old) in our sample trace a previous, lower Si-29/Si-30 value of about 1. Material with this isotopic ratio is present in two subclasses of presolar grains, providing independent evidence of the lower ratio. Therefore, the Si-29/Si-30 ratio derived from the SiO emission of evolved stars is a useful diagnostic tool for the study of the GCE and presolar grains.