http://arxiv.org/abs/1503.04788
The existence of sub-stellar 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 such fluids is studied using the virial theorem and linear stability analysis. Then the non-linear dynamics is studied by using simulations in order 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 together 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 to correctly satisfy the ideal gas 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 sub-stellar H2 clumps in cold molecular clouds due to the combination of phase transition and gravity. Fluids in a phase transition are gravitationally unstable, independent of the strength of the gravitational potential. 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.
A. Fuglistaler and D. Pfenniger
Tue, 17 Mar 15
79/79
Comments: 24 pages, 44 figures. Submitted to A&A, recommended for publication
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