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

Abstract

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 29 Si/ 30 Si 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 29 Si 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 29 Si/ 30 Si 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 29 Si/ 30 Si 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 29 Si/ 30 Si 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 29 Si/ 30 Si ratio derived from the SiO emission of evolved stars is a useful diagnostic tool for the study of the GCE and presolar grains.

On Variations of Pre-Supernova Model Properties

“…The solar 29 Si abundance is ≈ 1.4 times larger than the solar 30 Si abundance, in concordance with observations of some individual stars (Peng et al 2013) and pre-solar silicon-carbide grains (e.g., Hoppe et al 2010), yet in tension with common models of 1D core-collapse supernova, Type Ia supernovae, and AGB stars (e.g., Timmes & Clayton 1996;Lugaro et al 1999;Lewis et al 2013;Wasserburg et al 2015). It is curious that the 30 Si abundance, normalized to solar in in Figure 25, is larger than the 29 Si abundance in the M ZAMS = 20 and 30 M models but traditionally smaller in the M ZAMS = 15 and 25 M models.…”

Molecular line study of the S-type AGB star W Aquilae

“…For comparison, the solar 29 Si/ 30 Si ratio, as derived by Asplund et al (2009) is 1.52, which is also consistent with the ratio derived for the ISM (Wolff 1980;Penzias 1981). Peng et al (2013) investigate the 29 Si/ 30 Si ratio for 15 evolved stars and come to the conclusion that the older low-mass oxygen-rich stars in their sample have lower 29 Si/ 30 Si ratios, but the ratios are not influenced strongly by AGB evolution and merely reflect the interstellar environment in which the star has been born. The only S-type star in their sam-ple, χ Cyg, shows a 29 Si/ 30 Si ratio of 1.1, which is incidentally identical to our derived value for W Aql.…”

An ALMA view of CS and SiS around oxygen-rich AGB stars

“…For 28 Si/ 30 Si, also calculated from SiS, they find 16, less than half of our value of 40 ± 8. Peng et al (2013) observed the less abundant isotopologues of SiO and hence calculated the 29 Si/ 30 Si ratio for a sample of stars, including IK Tau. They found a ratio of 1.60 ± 0.30, in good agreement with our result of 1.7 ± 0.3.…”