Shock‐Wave Heating Model for Chondrule Formation: Prevention of Isotopic Fractionation

Chondrules are considered to have much information on dust particles andprocesses in the solar nebula. It is naturally expected that protoplanetarydisks observed in present star forming regions have similar dust particles andprocesses, so study of chondrule formation may provide us great information onthe formation of the planetary systems. Evaporation during chondrule melting may have resulted in depletion ofvolatile elements in chondrules. However, no evidence for a large degree ofheavy-isotope enrichment has been reported in chondrules. In order to meet thisobserved constraint, the rapid heating rate at temperatures below the silicatesolidus is required to suppress the isotopic fractionation. We have developed a new shock-wave heating model taking into account theradiative transfer of the dust thermal continuum emission and the line emissionof gas molecules and calculated the thermal history of chondrules. We havefound that optically-thin shock waves for the thermal continuum emission fromdust particles can meet the rapid heating constraint, because the dust thermalemission does not keep the dust particles high temperature for a long time inthe pre-shock region and dust particles are abruptly heated by the gas dragheating in the post-shock region. We have also derived the upper limit ofoptical depth of the pre-shock region using the radiative diffusionapproximation, above which the rapid heating constraint is not satisfied. It isabout 1 - 10.Comment: 58 pages, including 5 tables and 15 figures, accepted for publication in The Astrophysical Journa