The onset of molecular hydrogen emission from proto-planetary nebulae

This paper presents results obtained from high-resolution spectroscopy of t he 2.1218 mu m emission line of molecular hydrogen toward five bipolar pre- planetary nebulae. Our detections of IRAS 17441--2411 and AFGL 6815S bring to four, including AFGL 618 and AFGL 2688, the number of bipolar pre-planet aries that have been detected as sources of H-2 emission at 2.1218 mu m. Th e spatially resolved H-2 line profile of AFGL 6815S suggests that the bulk of the detected emission arises in an expanding molecular torus surrounding the central star. All of the H-2-emitting pre-planetary nebulae are found at low galactic lat itudes, consistent with previous results for planetary nebulae, and have ce ntral stars with intermediate or early spectral types. We were unable to de tect H-2 emission from the bipolar, post-main-sequence sources OH 231.8+4.2 , IRAS 07131-0147, and IRAS 09371+1212, all of which have M-type central st ars. These results suggest that the event that triggers the production of e mission from shocked H-2 occurs at an intermediate evolutionary epoch in th e postasymptotic giant branch evolution of transition objects, after the ge neration of bipolar structure and before the nebular envelopes are ionized. From constraints imposed by the galactic distribution of H-2-emitting plan etaries, we estimate distances to the four pre-planetary nebulae that displ ay H-2 emission.