Destruction and Survival of Interstellar Material

From the point of view of nebular chemistry starless cores are interesting because they are regions which are cold enough, for long enough, that a significant amount of the heavy molecular material (e.g. CO) can be depleted onto the dust grains. These 'depletion cores' can have very high isotopic fractionation, particularly in deuterium, and, since this material will ultimately appear in the protostellar disk, may be important for the high D/H ratios seen in, albeit processed, solar system material. Nevertheless, as cold interstellar gas and ice-mantled dust grains fall inwards,

Figure 3.1. Schematic diagram of the protosolar nebula. The positions of the Kuiper belt and Oort cloud are shown, together with the formation zones of comets comprising these two families: Oort cloud (long period) comets are thought to form closer to the protostar than the Kuiper belt (short period) ones.

Figure 3.1. Schematic diagram of the protosolar nebula. The positions of the Kuiper belt and Oort cloud are shown, together with the formation zones of comets comprising these two families: Oort cloud (long period) comets are thought to form closer to the protostar than the Kuiper belt (short period) ones.

the condition of isothermality breaks down and interstellar material can begin to undergo physical and chemical change. For example, heating by thermal radiation, and the friction of gas-grain drag, can erode grain mantles as dust grains approach and enter the disk (e.g. Lunine et al. 1991). As material approaches the protoplanetary disk, many more endo-thermic chemical reactions come into play, driven by the increasing temperature. Radiation chemistry involving X-rays and UV from the accretion shock also play a role. Eventually, the accretion shock is encountered and shock processes come to dominate the chemistry of the material first entering the nebula (Lunine 1989; Neufeld & Hollenbach 1994). Specific regions of the disk favour survival of various interstellar materials (refractory metals, refractory and volatile organics, and ice) (see also Simonelli et al. 1997; Chick & Cassen 1997). The smallest shock speeds and preshock densities favour the survival of the most volatile material and occur in the outermost regions.

This is the first chemical processing distinct from interstellar chemistry that organic material experienced upon incorporation into the nebula. One important issue is to determine under what conditions largely unmodified interstellar matter could first arrive in the protosolar nebula. That is, in which regions of the nebula are ice mantles evaporated preshock, survive the shock, and simply recondense as ices of similar composition in the postshock flow.

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