The destruction and growth of dust grains in interstellar space - II. Destruction by grain surface reactions, grain-grain collisions and photodesorption

Abstract

Chemical reactions on the surfaces of ice grains are shown to be unimportant as destruction agents. However, graphite grains can be destroyed by reaction with chemisorbed hydrogen and oxygen atoms, for grain temperatures in excess of ∼ 65 K, leading to an explanation for the weakness of the 2175 Å extinction feature towards several stars embedded in H II regions. It is also shown that such reactions govern the conditions under which graphite grains can condense in stellar atmospheres. The classical Oort–van de Hulst destruction mechanism of grain–grain collisions during cloud collisions is shown to be ineffective, by means of a detailed consideration of the shock wave structure at the interface between the colliding clouds. Magnetic field influenced grain collisions in shock fronts, between grains from the same cloud, do not lead to a significant overall destruction rate for dust grains in the interstellar medium. It is argued that photodesorption is the dominant destruction mechanism for ice grains held together by weak van der Waals dispersion forces. The timescale for destruction of an ice grain of radius 10−5 cm by the interstellar ultraviolet radiation field is derived to be ∼ 5 × 104 yr, much shorter than for other destruction mechanisms.