Cosmic snow clouds: self-gravitating gas spheres manifesting hydrogen
  condensation
  We present hydrostatic equilibrium models of spherical, self-gravitating
clouds of helium and molecular hydrogen, focusing on the cold, high-density
regime where solid- or liquid-hydrogen can form. The resulting structures have
masses from 0.1 Msun down to several x 1.e-8 Msun, and span a broad range of
radii: 1.e-4 < R(AU) < 1.e7. Our models are fully convective, but all have a
two-zone character with the majority of the mass in a small, condensate-free
core, surrounded by a colder envelope where phase equilibrium obtains.
Convection in the envelope is unusual in that it is driven by a
mean-molecular-weight inversion, rather than by an entropy gradient. In fact
the entropy gradient is itself inverted, leading to the surprising result that
envelope convection transports heat inwards. In turn that permits the outer
layers to maintain steady state temperatures below the cosmic microwave
background. Amongst our hydrostatic equilibria we identify thermal equilibria
appropriate to the Galaxy, in which radiative cooling from H2 is balanced by
cosmic-ray heating. These equilibria are all thermally unstable, albeit with
very long thermal timescales in some cases. The specific luminosities of all
our models are very low, and they therefore describe a type of baryonic dark
matter. Consequently such clouds are thermally fragile: when placed in a harsh
radiation field they will be unable to cool effectively and disruption will
ensue as heat input drives a secular expansion. Disrupting clouds should leave
trails of gas and H2 dust in their wake, which might make them easier to
detect. Our models may be relevant to the cometary globules in the Helix
Nebula, and the G2 cloud orbiting Sgr A*.
