AAS 202nd Meeting, May 2003
Session 24 When Do Planets Form?
Topical Oral, Tuesday, May 27, 2003, 8:30-10:00am and 10:45am-12:30pm, 204

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[24.03] Accumulation of Giant Planet Atmospheres Around Cores of a Few Earth Masses

O. Hubickyj, P. Bodenheimer (UCO/Lick Observatory, UCSC), Lissauer (NASA - Ames Research Center)

The widely accepted core instability model states that gas giant planets form by accretion of a massive solid core from the planetesimals in the solar nebula followed by the capture of a massive envelope from the solar nebula gas. Our simulations based on this model have been successful in explaining many features of the giant planets. Recent interior models of Jupiter and Saturn suggest smaller solid core masses that had been predicted by previous formation models. We have computed simulations of the growth of Jupiter where we vary the grain opacity and the planetesimal surface density of the solar nebula. We also explore the implications of halting the solid accretion at selected core mass values during the protoplanet's growth, thus simulating the presence of a competing embryo.

Our results demonstrate that decreasing the grain opacity results in reducing the evolution time by more than half of that for models computed with full interstellar grain opacity values. In fact, it is the reduction of the grain opacity in the upper portion of the envelope with T < 500 K that governs the lowering of the formation time. Decreasing the surface density of planetesimals lowers the final core mass of the protoplanet but increases the formation timescale. Finally, a core mass cutoff results in a reduction of the time needed for a protoplanet to evolve to the stage of runaway gas accretion provided the cutoff mass is not too small.

Derived core masses and observed short lifetimes of protoplanetary disks strongly constrain conditions for forming gas giant planets. The key to satisfying the constraints appears to be grain opacity substantially less than the interstellar value, consistent with recent calculations of grain settling in giant planet atmospheres.

This research is supported by NASA's Origins of Solar Systems Program grant NAG5-9661.

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.