Presolar Silicon Carbide Grains of Types Y and Z: Their Molybdenum Isotopic Compositions and Stellar Origins

Abstract

We report Mo isotopic compositions of 37 presolar SiC grains of types Y (19) and Z (18), rare types commonly argued to have formed in lower-than-solar metallicity asymptotic giant branch (AGB) stars. Direct comparison of the Y and Z grain data with data for mainstream grains from AGB stars of close-to-solar metallicity demonstrates that the three types of grains have indistinguishable Mo isotopic compositions. We show that the Mo isotope data can be used to constrain the maximum stellar temperatures (T MAX ) during thermal pulses in AGB stars. Comparison of FRUITY Torino AGB nucleosynthesis model calculations with the grain data for Mo isotopes points to an origin from low-mass (~1.5-3 M ! ) rather than intermediate-mass (>3-9 M ! ) AGB stars. Because of the low efficiency of 22 Ne(α,n) 25 Mg at the low T MAX values attained in low-mass AGB stars, model calculations cannot explain the large 30 Si excesses of Z grains as arising from neutron capture, so these excesses remain a puzzle at the moment.

Isotopic Signatures of Supernova Nucleosynthesis in Presolar Silicon Carbide Grains of Type AB with Supersolar 14N/15N Ratios

“…The minor type Y and Z grains (a few % of all SiC grains, depending on grain size), which fall to the 30 Si-rich side of the Si mainstream line, were also proposed to originate from low-mass AGB stars, but with metallicities lower than solar (Hoppe et al 1997;Amari et al 2001b). This low-metallicity scenario, however, was recently questioned (Liu et al 2019). The type C (∼0.1% of all SiC grains) and X grains (∼1% of all SiC grains) are believed to originate from core-collapse supernovae (CCSNe; Amari et al 1992;Hoppe et al 1996b;Gyngard et al 2010).…”

s-Processing in Asymptotic Giant Branch Stars in the Light of Revised Neutron-Capture Cross Sections

“…In Vescovi et al [18], the same model was shown to be able to explain the most anomalous Mo isotope ratios of Y and Z grains, which are thought to have originated in lower-than-solar metallicity AGB stars and have Mo isotopic compositions indistinguishable from MS grains (see Liu et al [57] for more details). In this regard, however, recent analyses are revealing that the three groups of grains have also similar Sr and Ba isotopic compositions, thus questioning the low-metallicity stellar origin of Y and Z grains [58]. From the comparison with grain data, models computed with the REF data set seem to give a better match while models adopting the ASTRAL v0 set only provide a partial overlap.…”

Mass and metallicity distribution of parent AGB stars of presolar SiC

“…Based on their Si and Ti isotopic compositions, Y and Z grains were previously inferred to have originated from low-mass AGB stars of 0.5 Z and 0.3 Z , respectively (Zinner et al 2007). However, Liu et al (2019) showed that these low-metallicity models adopted by Zinner et al (2007) cannot consistently explain the Mo isotopic compositions of Y and Z grains and that, more likely, Y and Z grains came from low-mass AGB stars of 0.7 Z . A small fraction of presolar SiC, including X and possibly AB grains, also came from ancient Type II supernovae (Zinner 2014;Liu et al 2017a;Hoppe et al 2019).…”