A snowflake's chance in heaven

Abstract

We consider the survival of solid H2 in the diffuse interstellar medium, with application to grains which are small enough to qualify as dust. Consideration of only the thermal aspects of this problem leads to the familiar conclusion that such grains sublimate rapidly. Here we show that charging plays a critical role in determining the sublimation rate, because an electric field helps to bind molecules to the grain surface. A key aspect of the charging process is that the conduction band of solid hydrogen lies above the vacuum free-electron energy level, so low-energy electrons cannot penetrate the solid. But they are attracted by the dielectric and by positive ions in the matrix, so they become trapped in vacuum states just above the surface. This charge-separated configuration suppresses recombination and permits overall neutrality, while supporting large electric fields at the surface. Charging ceases when the potential energy just outside the electron layer coincides with the conduction band minimum. By that stage the heat of sublimation has increased tenfold, effecting a huge reduction in the sublimation rate. Consequently hydrogen grains may be able to survive indefinitely in the diffuse ISM. There are good prospects for identifying H2 grains, if they exist, as fully localised surface electrons should exhibit discrete energy levels, with a corresponding spectral line signature.

12 pages, 4 figures, MNRAS in press

References in corpus (5)

• The Diffuse Interstellar Bands: A Major Problem in Astronomical Spectroscopy
• The Micro-Arcsecond Scintillation-Induced Variability (MASIV) Survey II: The First Four Epochs
• An Optical Survey for mm-Sized Interstellar Meteoroids
• Perspectives on Interstellar Dust Inside and Outside of the Heliosphere
• Interstellar Solid Hydrogen

Cited by in corpus (10)

• Constraints on the Occurrence of 'Oumuamua-Like Objects
• Noodle Model for Scintillation Arcs
• Assessing the Formation of Solid Hydrogen Objects in Starless Molecular Cloud Cores
• Solid H2 in the interstellar medium
• Cosmic snow clouds: self-gravitating gas spheres manifesting hydrogen condensation
• The infrared dielectric function of solid para-hydrogen
• Substellar fragmentation in self-gravitating fluids with a major phase transition
• Magnetized Filament Models for Diverging Plasma Lenses
• Absorption spectra of electrified hydrogen molecules
• Tidal disruption of "snow clouds" by unassociated stars