Tidal disruption of "snow clouds" by unassociated stars

TL;DR

This paper investigates how close encounters with stars can tidally disrupt hypothetical cold, planetary-mass molecular clouds called 'snow clouds', producing tiny, over-pressured debris streams with potential radio-wave scattering implications.

Contribution

It introduces hydrodynamic simulations of star-induced tidal disruption of 'snow clouds', revealing their properties and potential observational signatures, a novel exploration of these cold molecular structures.

Findings

Debris streams are highly over-pressured and composed of cold gas and snowballs.

Snowballs are eroded as they pass through shocked interstellar medium.

Disruption rate is too low to explain observed scattering screens.

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 $\text{H}_{2}$ 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 $\text{H}_{2}$ "snowballs" that may be as much as a metre 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.