Post-AGB objects and planetary nebulae

Abstract

Planetary nebulae and post-AGB objects are luminous dust emitters in the infrared. Indeed it is by virtue of their strong dust emission that most post-AGB objects (proto-planetary nebulae) have usually been recognised as such, given that the strong optical line emission from photoionized gas that is typical of PNe is absent from the spectra of post-AGB objects, whose central stars occupy the intermediate temperature range between the cool AGB stars and the hot central stars of PNe. Figure 1 shows the characteristic double-peaked optical-IR spectrum of the post-AGB object HD 161796, from Hoogzaad et al. [12]. At optical wavelengths the spectrum of the F3Ib central star dominates, but at IR wavelengths a second peak, due to emission by dust ejected during the earlier AGB phase, is very prominent in its ISO spectrum. Hoogzaad et al. found that contributions from both amorphous and crystalline silicate particles, as well as crystalline water ice were needed to fit the IR spectrum of this oxygen-rich object. Figure 2, from Molster et al. [15], shows the ISO SWS+LWS infrared spectrum of a much more evolved object, the massive bipolar PN NGC 6302. As highlighted in the caption to Fig. 2, a wide variety of C-rich and O-rich particles contribute to its exceedingly rich spectrum