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Quenching model

This model is based on chemical equilibrium calculations of gaseous molecular composition in a gas of the solar-system elemental composition with taking account of `quenching' (Lewis and Prinn, 1980), which is explained below.

In this model, it is assumed that the solar nebula was initially hot ( tex2html_wrap_inline1196 K). The gaseous composition at this temperature is mainly H tex2html_wrap_inline1146 O, CO, and N tex2html_wrap_inline1146 , plus H tex2html_wrap_inline1146 and He, of which the latter two are irrelevant to the condensation. Note that the nebular gas is of oxidized composition at high temperatures. In the course of the nebular cooling the gaseous composition is fixed at a certain temperature called a quenching temperature tex2html_wrap_inline1204 , and is maintained at temperatures lower than tex2html_wrap_inline1204 . This is because the time scale for achieving chemical equilibrium increases very rapidly as the temperature gets lower, and becomes much longer than the nebular dynamical time scale (Lewis and Prinn, 1980). ``Freezing" of the gaseous composition below the quenching temperature tex2html_wrap_inline1204 is the point of this model. Condensation of ices occurs in the quenched gas. The condensation process itself is treated in an equilibrium manner in which kinetics of condensation such as nucleation and grain growth, and resultant supercooling (Yamamoto and Hasegawa, 1977; Draine and Salpeter, 1977; Kozasa and Hasegawa, 1987) are not taken into account.

Figure 5 illustrates the quenching for gaseous carbon compounds, showing CO/CH tex2html_wrap_inline1154 abundance ratio as a function of temperature T and total pressure P, which is nearly equal to the partial pressure of H tex2html_wrap_inline1146 , the main component of the nebular gas.

It is seen that, if chemical equilibrium was achieved at all temperatures, CO is a dominant gaseous carbon species at high T and low P, and CH tex2html_wrap_inline1154 is a dominant species at low T and high P. Actually, however, the gaseous composition is ``frozen" at tex2html_wrap_inline1204 in the cooling, and as a result CO dominates CH tex2html_wrap_inline1154 even at low temperatures under the solar nebula conditions that Fegley and Prinn (1989) supposed. On the other hand, CH tex2html_wrap_inline1154 dominates CO at low temperatures under the conditions of their Jovian subnebula, since the pressure of their Jovian subnebula is much higher than that of the solar nebula. A similar situation holds for nitrogen compounds. Namely, tex2html_wrap_inline1234 in the solar nebula, and tex2html_wrap_inline1236 in the Jovian subnebula. Note that the oxidized CO, N tex2html_wrap_inline1146 -rich gaseous composition is realized in the solar nebula, and the reduced CH tex2html_wrap_inline1154 , NH tex2html_wrap_inline1150 -rich composition is realized in the Jovian subnebula.

In this model, cometary ice is regarded as a mixture of ices condensed in the solar nebula and the Jovian subnebula. The mixing mechanisms that Prinn and Fegley (1989) suggest are (i) sweep-up of the gas of the reduced composition in the Jovian subnebulae by proto-cometary objects of the oxidized composition formed in the solar nebula, or vice versa, and (ii) partial mixing of the subnebula gas with the solar nebula gas. It is not clear whether or not these mechanisms are efficient enough to be able to produce the estimated total mass of comets.

We point out an important consequence deduced from the quenching model, which is distinguished from that deduced from the interstellar-ice residue model described later. That is a condensation temperature of H tex2html_wrap_inline1146 O ice. In this model H tex2html_wrap_inline1146 O ice condenses at tex2html_wrap_inline1248 K under the solar nebula conditions of the pressure of tex2html_wrap_inline1250 to tex2html_wrap_inline1252 bar (Lewis and Prinn, 1980), and at higher temperatures under the Jovian subnebula conditions because of higher pressure.


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Next: Interstellar-ice residue model Up: Theories on the Origin Previous: Theories on the Origin

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Mon Sep 16 16:23:29 JST 1996