Relaxation of protostellar accretion shocks using the smoothed particle
  hydrodynamics
  It is believed that protostellar accretion disks to be formed from nearly
ballistic infall of the molecular matters in rotating core collapse. Collisions
of these infalling matters lead to formation of strong supersonic shocks, which
if they cool rapidly, result in accumulation of that material in a thin
structure in the equatorial plane. Here, we investigate the relaxation time of
the protostellar accretion post-shock gas using the smoothed particle
hydrodynamics (SPH). For this purpose, a one-dimensional head-on collision of
two molecular sheets is considered, and the time evolution of the temperature
and density of the post-shock region simulated. The results show that in strong
supersonic shocks, the temperature of the post-shock gas quickly increases
proportional to square of the Mach number, and then gradually decreases
according to the cooling processes. Using a suitable cooling function shows
that in appropriate time-scale, the center of the collision, which is at the
equatorial plane of the core, is converted to a thin dense molecular disk,
together with atomic and ionized gases around it. This structure for accretion
disks may justify the suitable conditions for grain growth and formation of
proto-planetary entities.
