Currently, one of the major theoretical problems concerning planet formation in close binary systems is whether the strong perturbation from the companion star can increase the relative velocities (△ V) of planetesimals around the primary and thus hinder their growth. According to previous studies, while gas drag can reduce the △ V between bodies of the same size by forcing orbital alignment on planetesimals, it increases the △Vamong bodies of different sizes. In this paper, focusing on the Cephei binary system, we propose a mechanism that can overcome this difficulty. We show that in a dissipating gas disk (with a typical dissipation timescale of ～10~5-10~6 yr), all the planetesimals eventually converge toward the same forced orbits regardless of their size, leading to much lower impact velocities. This process of decreasing △V progressively increases net mass accretion and can even trigger runaway growth for large bodies (radius above 15 km). The effect of the size distribution of planetesimals is discussed and is found to be one of the dominant factors determining the outcome of collisional evolution. In any case, it can be concluded that by including gas dissipation in the early stages of disk evolution, the conditions for planetesimal accretion become much better, and the progression from planetesimals to planet embryos can be accomplished in close binary systems such as γ Cep.
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