Abstract
During the post-AGB phase the dust shell detaches from the central star and dust temperatures varies between 100 and 200 K. In order to study the onset of photo-ionization in post-AGB stars, IRSPEC near-infrared spectroscopy of 40 IRAS sources from the Point Source Catalogue was conducted. In 27 of the post-AGB observed Br gamma is detected. In addition, molecular emission lines of H[2] 1-0 S(1) and 2-1 S(1) were detected in 11 objects. The observations show that all the sources but one, with molecular emission, exhibit photo-ionization. A detailed description and discussion of the results are presented.
