Tidal Disruption of “Snow Clouds” by Unassociated Stars

Abstract

It has been suggested that star-forming galaxies may host a substantial, dark reservoir of gas in the form of planetary-mass molecular clouds that are so cold that H _2 can condense. Here we investigate the process of tidal disruption of such “snow clouds” by close passage of field stars. We construct a suite of simulations using the hydrodynamic formalism introduced by Carter and Luminet, and use it to explore the properties of the resulting tidal debris. The debris streams are tiny structures that are highly over-pressured relative to the ambient interstellar medium (ISM). They are also unusual in their composition—initially consisting of cold, gaseous He together with H _2 “snowballs” that may be as much as a meter in size. Each stream expands and cools and is subsequently shocked as it plows through the ISM; the snowballs are gradually eroded by the shocked gas. Snowballs streaming through the shocked ISM create microstructured plasma that is somewhat reminiscent of the “scattering screens” revealed by radio-wave scintillation studies. However, the tidal disruption rate is too low to account for the observed number of scattering screens if, as we assume here, the stars and clouds have no prior physical association so that disruptions occur as a result of chance encounters between stars and clouds.

Keywords

• Interstellar medium
• Molecular clouds
• Interstellar clouds
• Plasma clouds
• Hydrodynamics
• Interstellar scattering