## 3. The ionized regionFor a given electron density and number of ionizing photons a simple ionization equilibrium calculation at K results in a value of the mass of ionized gas given by where s Since in this paper we are interested in the neutral shells of planetary nebulae, we require that the shell remains in part neutral over a significant fraction of its lifetime. This constrains the ratio to rather large values. Fig. 3 shows the values of as a function of time for =10, =2 (lower panel) and =3 (upper panel). Fig. 3 and Eq. 5 demonstrate that the condition that the shell remain partially neutral for yrs is where for and for the higher stellar mass cases.
Fig. 3 shows another important aspect of the PN evolution,
namely that the fractional mass of ionized gas, which depends on the
ratio , first increases with time, following
the steep increase of , then decreases as
decreases, and finally increases again at even
later times due to density decline. The advance of the ionization
front has important consequences on the properties (position and
temperature) of the dissociation front, as rapid advection of
molecular material into the PDR takes place. The recession adds atomic
gas to the PDR surface and results in non-equilibrium formation of
H We characterize the emission of the ionized region with the radiated Br intensity, which, for an ionization-bounded shell, is given by where cm. Note that I(Br) does not depend on shell parameters such as density and filling factor, but provides a direct measurement of the stellar Lyman continuum radiation field. © European Southern Observatory (ESO) 1998 Online publication: August 17, 1998 |