Mathematics – Probability
Scientific paper
Sep 1975
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975phdt.......173b&link_type=abstract
Ph.D. Thesis, University of Sussex, U.K.
Mathematics
Probability
7
Interstellar Dust Grains, Ice Mantles, Sputtering, Grain-Grain Collisions, Adsorption, Photodesorption, Gas-Phase Depletion, Grain Growth
Scientific paper
The processes governing the growth and destruction of dust grains in interstellar space are investigated with a view to establishing the conditions for the existence of ice mantles on grains. First the relative importance of the various destruction mechanisms is established. Several mechanisms previously thought to be efficient destruction agents are shown to be ineffective.
Previous sputtering yield estimates which were based on theoretical considerations are shown to be greatly in error for incident particle energies of less than one keV. Empirical formulae for the sputtering threshold energy and sputtering yield are derived from the extensive experimental data available. Sputtering of ice grains in HII regions, in the intercloud medium and in shock waves produced by cloud-cloud collisions, expanding HII regions and supernova remnants is shown to be unimportant as a large-scale destruction agent.
Detruction rates for grains bombarded by keV particles in stellar winds and by cosmic rays are estimated. It is shown that collision cascade sputtering dominates evaporative sputtering produced by thermal spikes. It is argued that the electron excitation energy loss of cosmic rays penetrating grains will not be transferred as heat to the lattice particles of the grain, so that no destructive effects are expected. It is shown that even if all the electron excitation energy loss was transferred to the lattice, the observed cosmic ray flux spectrum would not cause significant destruction of ice grains.
The Orrt-van de Hulst destruction mechanism of grain-grain collisions during cloud collisions is shown to be ineffective through a consideration of the shock wave structure at the interface between two colliding clouds. Buffer zones for the slowing of grains are shown to be created, leading to a drastic reduction in the derived grain destruction probability.
Chemical reactions at the surface of ice grains are shown to be unimportant as destruction agents. However, it is demonstrated that graphite grain surfaces can react efficiently with adsorbed hydrogen and oxygen atoms for temperatures in excess of 60-70 K, leading to an explanation for the weakness of the 2175A extinction feature towards several stars embedded in HII regions.
It is shown that photodesorption is the dominant destruction mechanism for ice grains, which are argued to be held together by only weak van der Waals dispersion forces (~0.1 eV per molecule). The timescale for destruction of an ice grain of radius 10(-5) cm by the interstellar radiation field shortward of 1900A is derived to be ~ 5x10(4) years, as against lifetimes of 10(9) years for all the other destruction mechanisms.
The processes influencing the growth of mantles are then considered. The nature of the adsorption potential felt by atoms and molecules on various grain materials is investigated. It is shown that atoms will be chemisorbed on the surface of graphite grains, while silicate grains give rise only to physical adsorption. Graphite grains are shown to play the dominant role in H2 formation in interstellar clouds. The ejection of H2 molecules with excess kinetic energy from the surfaces of graphite grains is shown to be a significant heating process for interstellar clouds.
It is shown that metallic elements in interstellar clouds will be trapped on the surfaces of graphite grains, leading to their gas-phase depletion, whereas non-metal elements will be ejected as saturated hydrides from the surfaces of graphite grains (and will be ejected as monohydrides from other grains), with no consequent depletion.
The conditions for mantle growth are established. The ejection of non-metal atoms from the surfaces of grains, as well as the process of photodesorption, prevent the growth of mantles in ordinary diffuse interstellar clouds. In dense clouds, where non-metal elements are in saturated molecules, only photodesorption prevents mantle growth and for high enough cloud densities and sufficient ultraviolet shielding mantle growth will take place. Upon exposure to the unshielded interstellar ultraviolet radiation field such mantles will rapidly be destroyed, and so ice mantles will be found only in dense obscured regions of interstellar space. Throughout most regions of space the interstellar grain mixture consists only of refractory particles.
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