The Chemical Evolution of Protostellar and Protoplanetary Matter

Abstract

The different processes that can affect the chemical composition of matter as it evolves from quiescent molecular clouds into protostellar and protoplanetary regions are discussed. Millimeter observations show that the chemical state of dense interstellar clouds prior to star formation is highly inhomogeneous: cold, dark clouds such as TMC-1 and L 134N show large chemical gradients on scales of a few tenths of a parsec, which are not well understood. Chemical models based on gas-phase ion-molecule reactions are moderately successful in reproducing the observed molecular abundances, but their predictive power is limited by unknown rates for several crucial reactions. Also, observational data for several important molecules are lacking. As a result, the abundances of the dominant oxygen- and nitrogen-bearing molecules prior to star formation are poorly determined. Some atoms and molecules are found to be depleted significantly onto grains, but the mechanisms for returning species to the gas phase in cold clouds are still uncertain. In star-forming regions, the high temperatures induced by radiation from newly formed stars can evaporate volatile grain mantles, and can open up additional gas-phase reaction channels. Powerful shocks associated with the outflows can also return more refractory material such as silicon and sulfur to the gas phase. These processes are operative in the best studied high-mass star-formation region Orion/KL, where interferometric observations reveal a complex chemistry with variations on scales of <2000 AU. For low-mass stars, observations of the chemistry in circumstellar disks on scales of 500 to 10,000 AU are only just becoming available. Initial studies of the young stellar object IRAS 16293-2422 also show large chemical gradients on scales of less than 1000 AU, but the chemical abundances appear less affected by the star-formation process than in Orion/KL. Systematic studies of the chemistry in star-forming regions are still lacking. On smaller scales of 50 to 500 AU corresponding to the outer solar nebula, observations of only the nearest and brightest objects are now possible, and detailed models of the chemistry have yet to be made. On scales appropriate to the solar nebula, 5 to 50 AU, material is thought to be physically and chemically modified in a number of ways: accretional shock heating, gas-drag heating on grains, and lightning are possible sources of energy. In the inner part of the nebula, temperatures are high enough for gas phase reactions to alter the molecular composition significantly; however, the amount of radial mixing between the inner and outer parts of the nebula may be limited. The extent to which outer solar nebula material retained its interstellar composition is therefore a highly complex issue, dependent on the body and the molecular/isotopic signature being considered. Comets almost certainly contain the most pristine matter, and a detailed comparison between the observed abundances in comet Halley and those found in interstellar clouds is made.

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