NEUTRAL OXYGEN IN PLANETARY-NEBULAE - PROBING RADIATIVE-TRANSFER AND NEBULAR STRUCTURE

For the first time, [O I] lambda-5577 and [O I] lambda-6364 have together been mapped across planetary nebulae, making possible direct measurement of transition zone temperatures. In NGC 2440, the neutral oxygen temperature is 10,400 +/- 600 K, compared with 10,200 +/- 200 K for the [N II] temperature and 14,100 +/- 300 K for the [O III] temperature. The [O I] temperature in NGC 3918 is 13,000 +/- 900 K, significantly different from that in NGC 2440. Models computed with the nebula code CLOUDY successfully reproduce the [O I] and [O III] temperatures observed in NGC 2440, indicating that the implemented blend of on-the-spot and outward-only approximations to the transfer of diffuse radiation is a reasonable representation of reality. Even though radiation hardening enhances the heating at large optical depths within a planetary nebula, an increase in the cooling efficiency can prevent a concomitant rise in temperature. It is predicted that hot transition zones, as observed in NGC 3918, should be restricted to nebulae with cooler central stars and metal abundances (particularly N) at or below solar.