Interstellar Space - the Astrochemist's Laboratory

Citation

Allen, Mark Andrew (1976) Interstellar Space - the Astrochemist's Laboratory. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/B5RS-Y551. https://resolver.caltech.edu/CaltechETD:etd-12042003-154350

Abstract

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In the first part of this thesis, a mechanism for the formation molecules on small (radius [...] ) interstellar grains is proposed. It is suggested that the energy liberated when a chemical bond is formed between two atoms on a grain is transferred to the lattice vibrations of the grain, heating up the grain. The "hot" grain, during the time that it is radiatively recooling, may then liberate its adsorbed volatiles. The timescales for molecular desorption from a grain are calculated for OH and CO for four different grain compositions.

A simplified H2 formation model is then presented that utilizes this surface reaction mechanism. The resulting value for the formation rate constant R is [...] for dark clouds at 10[degrees]K. The nascent H2 molecules are ejected in excited states in qualitative agreement with Copernicus observations. With the synthesis of H2 the dominant process, a time-dependent treatment of the chemical evolution of a dark cloud with little or no ionizing radiation [...] shows that the clouds require more than 10[^7] years to achieve chemical equilibrium. It is also suggested that the observed residual atomic hydrogen in several dark clouds indicates that the clouds are l0[^6]- l0[^7] years old.

This approach is further developed into an ab initio chemical model for dense interstellar clouds that incorporates 598 grain surface reactions, with small grains again providing the key reaction area. Gas-phase molecules are depleted through collisions with grains. The abundances of 372 chemical species are calculated as a function of time and are found to be of sufficient magnitude to explain most observations. Peak abundances are achieved on timescales on the order of l0[^5]-l0[^6] years, depending on cloud density and kinetic ternperature. The reaction rates for ion-molecule chemistry are approximately the same, therefore indicating that surface and gas-phase chemistry may be coupled in certain regions. The composition of grain mantles is shown to be a function of grain radius. In certain grain size ranges, large molecules containing two or more heavy atoms are more predominant than lighter "ices"H_2O, NH_3, and CH_4. It is possible that absorption due to these large molecules in the mantles may contribute to the observed 3[...] band in astronomical spectra.

The second part of this thesis is an account of a radio astronomy observational program to detect new transitions of both previously observed and yet undetected interstellar molecules. The negative results yield order of magnitude upper limits to the column densities of the lower transition states of the various molecules. One special project was the search for the [...] - doublet transitions of the [...] state of OD. The resulting upper limit for the OD/OH column density ratio towards the galactic center is 1/400 and is discussed with reference to theories about deuterium enrichment in interstellar molecules.

Thesis Files