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Session 18 - The Interstellar Medium.
Oral session, Monday, June 08
We present a chemical and thermal model of the neutral envelop of planetary nebula NGC 7027. In our model, the neutral envelop is composed of a thin dense shell of constant density and an outer stellar wind region with the usual inverse-square law density profile. The chemical and thermal structure is calculated by using a PDR code assuming a \chi=10^5 incident FUV radiation field on the inner surface. The rate coefficient for H_2 formation on grains is assumed to be 3\times 10^-18 cm^3 s^-1 since the dust-gas mass ratio in NGC 7027 is about 10% of the standard interstellar value. We assume a minimum temperature of 200 K in the dense shell to account for the effect of shock heating associated with the interaction of the two stellar winds. We calculate the intensities of the molecular far infrared rotational lines by using a revised version of escape probability formalism. The theoretical intensities of CO lines (from J=29-28 to J=14-13), CH^+ lines, OH lines and CH lines are shown to be in good agreement with ISO observations.
Program listing for Monday