Dust coagulation in infalling protostellar envelopes. I. Compact grains

Dust plays a key role in the optical, thermodynamic, and gasdynamical behav ior of collapsing molecular cores. Because of relative velocities of the in dividual dust grains, coagulation and shattering can modify the grain size distribution and-because of corresponding changes in the medium's opacity-s ignificantly influence the evolution during early phases of star formation. In order to study relevant timescales and possible consequences for interm ediate-mass stars, we examine the dust evolution in spherical protostellar envelopes that evolve from cloud clumps of masses 3, 5, and 10 M.. At first the collapse proceeds in the well-known nonhomologous manner until a centr al hydrostatic core is formed. During the non-steady state accretion the ac cretion luminosity of the central core reaches high values (approximate to 10(4) L.). Thus, differential radiative acceleration provides an important contribution to the relative velocities of the grains. In turn, the mass ac cretion rate, which determines the central core's accretion luminosity (and ultimately the final mass of the central object), depends strongly on the opacity distribution in the enshrouding envelope. We find that coagulation and shattering can lead to significant modifications of the dust size distr ibution and the opacity during early collapse phases. The visible and ultra violet extinction is most strongly affected.