Energy Loss and Sticking Mechanisms in Particle Aggregation in Planetesimal Formation

Recent detection of protostellar disks indicates that the necessary environment for planetary-system formation is A crucial step in the development of planetary systems is the aggregation of small solid particles to form planetesimals commonly found around young stellar objects (Sargent in gaseous protoplanetary disks such as the primordial solar and Beckwith 1987, 1991). According to current theories, nebula. Among small (centimeter-sized) aggregates for which the formation of terrestrial planets progresses through at self-gravity is negligible, a sticking mechanism is needed to least four stages: (1) the condensation of heavy elements hold the aggregate together, even when the relative velocities into volatile and refractory grains (Völk et al. 1980, Nakaare very low. A similar cohesive process may also determine gawa et al. 1986); (2) the formation of planetesimals the size distribution of particles in planetary rings. In order to through cohesive collisions of the grains (Kerridge and provide the crucial data, we carry out experiments to investigate Vedder 1972, Hartmann 1978, Weidenschilling 1984, 1987, the contact sticking that occurs for surfaces coated with differWeidenschilling and Cuzzi 1993) or through gravitational ent types of frosts, deposited at various (low) temperatures and instability at larger sizes (Safronov 1969, Goldreich and pressures relevant to solar nebula conditions. Our preliminary Ward 1973); (3) the coagulation of planetesimals into promeasurements show that several types of frost-coated surfaces stick together when brought into contact at very low temperatoplanets (Safronov 1969, Hayashi et al. 1977, Greenberg tures (p100 K), but the sticking forces depend on the deposition et al. 1978, Wetherill 1980, 1989, 1990, Hornung et al. 1985, conditions. For ice particles covered with H2O and CO2 frost: Lissauer and Stewart 1993, Aarseth et al. 1993); and (4) (1) the energy loss in collisions depends strongly on the impact the clearing of residual planetesimals (Duncan and Quinn speed and surface structure, and (2) particle ‘‘sticking’’ can 1993). The formation of protogiant planets may proceed occur if the impact speed is sufficiently low. Static sticking through the initial emergence of solid cores, analogous to experiments using methanol (CH3OH) frost demonstrate that the prototerrestrial planets, through a similar sequence of methanol is also an effective ‘‘sticky’’ frost. We apply these events, followed by the accretion of gas from the solar results to planetesimal formation and suggest that a layer of nebula (Podolak et al. 1993). Because of the evolution of surface frost provides both the energy loss and the contact the primordial solar nebula, these four evolutionary stages sticking required for the formation of large aggregates.  1996 may occur concurrently rather than sequentially. As matter Academic Press, Inc.