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We address two outstanding issues in the sequential accretion scenario forgas giant planet formation, the retention of dust grains in the presence of gasdrag and that of cores despite type I migration. The efficiency of theseprocesses is determined by the disk structure. Theoretical models suggest thatplanets form in protostellar disk regions with an inactive neutral ``deadzone'' near the mid plane, sandwiched together by partially ionized surfacelayers where magnetorotational instability is active. Due to a transition inthe abundance of dust grains, the active layer's thickness decreases abruptlynear the ice line. Over a range of modest accretion rates ($\sim10^{-9}-10^{-8} M_\odot$ yr$^{-1}$), the change in the angular momentumtransfer rate leads to local surface density and pressure distribution maximanear the ice line. The azimuthal velocity becomes super-Keplerian and thegrains accumulate in this transition zone. This barrier locally retainsprotoplanetary cores and enhances the heavy element surface density to thecritical value needed to initiate efficient gas accretion. It leads to apreferred location and epoch of gas giant formation. We simulate and reproducethe observed frequency and mass-period distribution of gas giants around solartype stars without having to greatly reduce the type I migration strength. Themass function of the short-period planets can be utilized to calibrate theefficiency of type I migration and to extrapolate the fraction of stars withhabitable terrestrial planets.

Toward a Deterministic Model of Planetary Formation V. Accumulation Near the Ice Line