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

1Shigeru Wakita, 2Takaya Nozawa, 3Yasuhiro Hasegawa
The Astrophysical Journal 836, 106 Link to Article [https://doi.org/10.3847/1538-4357/aa5b8c]
1Center for Computational Astrophysics, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
2Division of Theoretical Astronomy, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

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 could no longer be identified as presolar grains. We explore this possibility for the first time by self-consistently simulating the thermal evolution of planetesimals and the diffusion length of 18O in presolar silicate grains. Our results show that presolar silicate grains smaller than ~0.03 μm cannot keep their original isotopic compositions even if the host planetesimals experienced a maximum temperature as low as 600 °C. Since this temperature corresponds to that experienced by petrologic type 3 chondrites, isotopic diffusion can constrain the size of presolar silicate grains discovered in such chondrites to be larger than ~0.03 μm. We also find that the diffusion length of 18O reaches ~0.3–2 μm in planetesimals that were heated up to 700–800°C. This indicates that, if the original size of presolar grains spans a range from ~0.001 μm to ~0.3 μm like that in the interstellar medium, then 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.

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