Formation Conditions of Presolar TiC Core-Graphite Mantle Spherules in the Murchison Meteorite

Formation of TiC and graphite grains in the gas outflows from carbon-rich asymptotic giant branch (AGB) stars is investigated to reveal the formation conditions of presolar TiC core-graphite mantle spherules extracted from the Murchison meteorite. We employ the nonequilibrium condensation theory involving chemical reactions in nucleation and grain growth to derive the formation conditions not only from the condensation sequence for realizing the core-mantle structure but also from the sizes of observed TiC cores and graphite mantles. The results of calculations show that the sizes of TiC cores constrain the mass-loss rate Ṁ and the gas outflow velocity v at the formation sites to satisfy the relation 2.5×10−3<v2L½4/Ṁ5<0.185, where L4 is the stellar luminosity in units of 104 L☉, v is in km s-1, and Ṁ5 is in 10−5 M☉ yr−1. The constraint on the condensation sequence limits the C/O abundance ratio ϵ to less than 1.26-1.48 for Ṁ=10−6 to 10−4 M☉ yr−1. The more stringent constraint on Ṁ and v at the formation sites expressed by 2.5×10−3<v2L½4/Ṁ5<1.35×10−2 with ϵ<1.26-1.48 is imposed by the conditions necessary for reproducing the range of the observed sizes of TiC cores and graphite mantles simultaneously. The derived total gas pressure in the range of 2×10−3<P<0.1 dyn cm−2 and gas outflow velocity in the range 0.015<v<0.4 km s−1 at the formation sites of TiC cores seem to be consistent with the picture of steady state dust-driven winds around carbon-rich AGB stars. The discrepancy between the observed and the predicted size ratios of graphite mantles to TiC cores would be closely related to the detailed formation process of graphite mantles on TiC cores.