Diffusion of Oxygen Isotopes in Thermally Evolving Planetesimals and Size Ranges of Presolar Silicate Grains

Presolar grains are small particles found in meteorites through their isotopic compositions which are considerably different from those of materials in the Solar System. If some isotopes in presolar grains diffused out beyond their grain sizes when they were embedded in parent bodies of meteorites, their isotopic compositions could be washed out, and hence the grains cannot be identified as presolar grains any more. We explore this possibility for the first time by self-consistently simulating the thermal evolution of planetesimals and the diffusion length of $^{18}$O in presolar silicate grains. Our results show that presolar silicate grains smaller than $\sim$ 0.03 $\mu m$ cannot keep their original isotopic compositions even if the host planetesimals experienced maximum temperature as low as 600 $^{\circ}$C. Since this temperature corresponds to the one experienced by petrologic type 3 chondrites, the isotopic diffusion can constrain the size of presolar silicate grains discovered in such chondrites to be larger than $\sim$ 0.03 $\mu m$. We also find that the diffusion lengths of $^{18}$O reach $\sim$ 0.3-2 $\mu m$ in planetesimals that were heated up to 700-800 $^{\circ}$C. This indicates that, if the original size of presolar grains spans a range from $\sim$ 0.001 $\mu m$ to $\sim$ 0.3 $\mu m$ like that in the interstellar medium, the isotopic records of the presolar grains may be almost completely lost in such highly thermalized parent bodies. We propose that isotopic diffusion could be a key process to control the size distribution and abundance of presolar grains in some types of chondrites.
Comment: 7 pages, 2 figures, accepted for publication in ApJ