A Chemical Model of the Neutral Envelope of the Planetary Nebula NGC 7027

Abstract

In response to the recent molecular observations by ISO and millimeter-wavetelescopes, we havedeveloped a spherically symmetric, steady state chemical model forthe planetary nebula NGC 7027. Inour model, the neutral envelope consists of a geometrically thin,high-density shell of constant densityand an outer stellar wind region with an inverse-square law densitypro�le. The neutral envelope is subjectedto ultraviolet (UV) radiation both from the central star and fromthe external interstellar �eld.Under an assumed visual extinction of 4 mag for the neutral envelope,the gas in our model is halfmolecular and half atomic. Simple molecules such as CH, CH`, OH areabundantly formed at anassumed gas temperature of 800 K in the partially molecular shell.The presence of molecular ions suchas HCO` and CO` is well explained by photochemistry at high temperatures.The model CN/HCNratio is almost constant at about 30 throughout the wind region andis 1�10 in the dense shell, comparableto observed values in planetary nebulae. Our model suggests that moleculesare as efficiently formedwith timescales shorter than 100 yr as they are photodissociated inthe neutral envelope of NGC 7027and probably in other young planetary nebulae.