Substellar fragmentation in self-gravitating fluids with a major phase
  transition
  The existence of substellar cold H2 globules in planetary nebulae and the
mere existence of comets suggest that the physics of cold interstellar gas
might be much richer than usually envisioned.
  We study the case of a cold gaseous medium in ISM conditions which is subject
to a gas-liquid/solid phase transition.
  First the equilibrium of general non-ideal fluids is studied using the virial
theorem and linear stability analysis. Then the non-linear dynamics is studied
by using simulations to characterize the expected formation of solid bodies
analogous to comets. The simulations are run with a state of the art molecular
dynamics code (LAMMPS). The long-range gravitational forces can be taken into
account with short-range molecular forces with finite limited computational
resources by using super-molecules, provided the right scaling is followed.
  The concept of super-molecule is tested with simulations, allowing us to
correctly satisfy the Jeans instability criterion for one-phase fluids. The
simulations show that fluids presenting a phase transition are gravitationally
unstable as well, independent of the strength of the gravitational potential,
producing two distinct kinds of sub-stellar bodies, those dominated by gravity
("planetoids") and those dominated by molecular attractive force ("comets").
  Observations, formal analysis and computer simulations suggest the
possibility of the formation of substellar H2 clumps in cold molecular clouds
due to the combination of phase transition and gravity. Fluids presenting a
phase transition are gravitationally unstable, independent of the strength of
the gravitational potential. Arbitrarily small H2 clumps may form even at
relatively high temperatures up to 400 - 600K, according to virial analysis.
The combination of phase transition and gravity may be relevant for a wider
range of astrophysical situations, such as proto-planetary disks.
