The onset of molecular hydrogen emission from proto-planetary nebulae

This paper presents results obtained from high-resolution spectroscopy of the 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-planetaries that have been detected as sources of H-2 emission at 2.1218 mu m. The 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 latitudes, consistent with previous results for planetary nebulae, and have central stars with intermediate or early spectral types. We were unable to detect 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 stars. These results suggest that the event that triggers the production of emission from shocked H-2 occurs at an intermediate evolutionary epoch in the postasymptotic giant branch evolution of transition objects, after the generation of bipolar structure and before the nebular envelopes are ionized. From constraints imposed by the galactic distribution of H-2-emitting planetaries, we estimate distances to the four pre-planetary nebulae that display H-2 emission.