Titanium isotopic compositions of well-characterized silicon carbide grains from Orgueil (CI): Implications for s-process nucleosynthesis

Abstract

We have measured the titanium isotopic compositions of 23 silicon carbide grains from the Orgueil (CI) carbonaceous chondrites for which isotopic compositions of silicon, carbon, and nitrogen and aluminum-magnesium systematics had been measured previously. Using the 16 mostprecise measurements, we estimate the relative contributions of stellar nucleosynthesis during the asymptotic giant branch (AGB) phase and the initial compositions of the parent stars to the compositions of the grains. To do this, we compare our data to the results of several published stellar models that employ different values for some important parameters. Our analysis confirms that s-process synthesis during the AGB phase only slightly modified the titanium compositions in the envelopes of the stars where mainstream silicon carbide grains formed, as it did for silicon. Our analysis suggests that the parent stars of the >1 μm silicon carbide grains that we measured were generally somewhat more massive than the Sun (2-3 Mʘ) and had metallicities similar to or slightly higher than solar. Here we differ slightly from results of previous studies, which indicated masses at the lower end of the range 1.5-3 Mʘ and metallicities near solar. We also conclude that models using a standard 13C pocket, which produces a good match for the main component of s-process elements in the solar system, overestimate the contribution of the 13C pocket to s-process nucleosynthesis of titanium found in silicon carbide grains. Although previous studies have suggested that the solar system has a significantly different titanium isotopic composition than the parent stars of silicon carbide grains, we find no compelling evidence that the Sun falls off of the array defined by those stars. We also conclude that the Sun does lie on the low-metallicity end of the silicon and titanium arrays defined by mainstream silicon carbide grains.