Solid Hydrogen Coated Graphite Particles in the Interstellar Medium--I

Solid hydrogen coated graphite particles may be expelled from regions of star-formation into the general interstellar medium. The solid para-hydrogen mantles, which contain a small proportion of ortho-H ₂ could be stable against evaporation in the general interstellar radiation field. They are also stable against physical and chemical sputtering in H I regions. Extinction efficiencies are calculated for solid hydrogen particles and for graphite particles with solid hydrogen mantles. Graphite core-solid hydrogen grains are capable of producing excellent agreement with the interstellar extinction observations from 20 000–1100 Å. The graphite core radius may be in the range $$r_{0} = 0.04-0.06 \,\mu$$ and the solid hydrogen mantle radius in the range $$r = 0.15-0.25 \,\mu.$$ The albedo and phase function of these particles are consistent with the requirements imposed by the diffuse galactic light.