Why Interstellar Ice Dust Grains Should be Elongated

Abstract

Models of interstellar dust alignment assume that dust grains are elongated but none of these models explain why dust grains should be elongated. On the other hand, models of interstellar dust grain growth assume that dust grains are spherical and not elongated. We show that when dusty plasma effects and the dipole moment of water molecules are together taken into account, it is concluded that ice grains in interstellar space should be prolate ellipsoids and not spheres. Dusty plasma analysis shows that an ice grain is charged to a negative potential that has magnitude nearly equal to the electron temperature. Several different mechanisms causing deviation from sphericity are identified; these mechanisms involve the interaction of the dipole moment of water molecules with electric fields associated with ice grain charging. These mechanisms include focusing of water molecule trajectories, migration of water molecules in a quasi-liquid layer on the grain surface towards regions where the electric field is strongest, enhancement of this migration by bombardment of energetic protons that gain energy upon falling into the ice grain negative potential, and mutual repulsion by electric charges having the same sign. The aspect ratio is established shortly after the ice grain is formed and then is maintained as the grain grows.