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M. Ikoma (T. I. Tech. Japan)
In the context of the core accretion model, the massive envelope of a giant planet is formed by rapid accretion of circumstellar-disk gas accompanied by contraction of a pre-existing, less massive atmosphere. The contraction of the atmosphere happens once the mass of a solid core reaches a critical mass. Large critical core masses obtained so far (~10 Earth masses) require strict conditions (large initial surface densities of solid materials and/or significant depletion of dust grains in the atmosphere) for the formation of a giant planet within the lifetime of the disk gas. All of the previous numerical simulations assumed that the composition of the atmosphere had been solar: the major elements were hydrogen and helium. However, the atmosphere must have been very rich in heavy elements (for example, oxygen and carbon), at least, before the critical core mass was attained. The heavy elements could be supplied by incoming planetesimals through their evaporation and impact-induced degassing. The enrichment is expected to have been large, since the mean accretion rate of planetesimals was much higher than that of disk gas before the critical core mass. In this study, I have calculated structure of atmospheres that are mixtures of disk-gas and volatile materials of icy planetesimals, whose compositions are assumed to be that of comet Halley, and investigated the critical core mass for various mixing ratios. I have also estimated the gas-accretion times beyond the critical core mass. The critical core mass has been found to decrease dramatically with increasing abundance of heavy elements. Even a core of less than a few Earth masses can capture disk gas to form a giant planet within the lifetime of the disk gas, if more than 50 % of the atmospheric mass is occupied by the volatile materials of planetesimals. The results of this study make the necessary conditions for the giant planet formation less strict.
The author(s) of this abstract have provided an email address for comments about the abstract: mikoma@geo.titech.ac.jp
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.