David Weintraub and Tracy Huard (Vanderbilt), Joel Kastner (MIT), and Ian Gatley (Rochester Institute of Technology) used the new high-resolution spectrometer Phoenix on the 2.1-m telescope at KPNO to search for emission in the 2.1218 Ám line of molecular hydrogen in five pre-planetary nebulae (pPNe). H2 emission was detected in two bipolar pPN, AFGL 6815S (Figure 1) and IRAS 17441-2411, both of which strongly resemble the well-studied bipoloar pPNe AFGL 2688 in recent high-resolution images. These new detections thus bring to four the number of bipolar pre-planetaries that have been detected in molecular hydrogen emission.
Caption: Figure 1: Top: Spectral image of AFGL 6815S in the geocentric rest frame. Telluric OH emission lines are seen as bright vertical lines; continuum of IRAS AFGL 6815S stretches horizontally across the image at RA Offset = 0. The bright emission patch just shortward of 2.122 Ám is due to shocked molecular hydrogen. Bottom: Spectrum of AFGL 6815S extracted from top image, obtained by summing flux in a 7.4" strip running parallel to the dispersion direction and centered on the nebular continuum. The spectrum has been transformed to local standard of rest velocities.
The appearance of bipolar structure in the late stages of evolution of many intermediate mass stars is commonly seen, but poorly understood. Observations of molecular emission, however, appear to provide clues to the origin of bipolarity. According to Gatley's Rule, a planetary nebula that has molecular hydrogen emission 2.1218 Ám will be structurally bipolar. On the basis of Gatley's Rule, one can determine the structure of a PN with a single spectral observation and without any direct spatial information, provided the spectrum shows H2 emission. The relationship between H2 emission and PN structure suggests that further studies may ultimately reveal more about the origin, timing, and mechanism of formation of bipolarity and the generation of H2 emission. A growing number of objects suspected to be in transition from red giant to PNe have been revealed to be bipolar via high-resolution optical or near-infrared imaging. These observations of bipolar structure in transition objects suggest that the onset of bipolarity occurs before the nebular envelopes are ionized. Notably, two of the best-studied transition objects, the bipolar pPNe AFGL 618 and AFGL 2688 are H2 sources. Phoenix observations may now allow a way to study the early morphological evolution of planetary nebulae from their spectra alone.
All four pre-planetary nebulae with molecular hydrogen have central stars with intermediate or early spectral types. Weintraub et al. contrast these objects to the bipolar, post-main sequence sources OH231.8+4.2, IRAS 07131-0147, and IRAS 09371+1212 in which they could not detect molecular hydrogen. Of the four bipolar sources in this sample for which H2 emission is detected at 2.1218 Ám, the three with known spectral types for their central stars are G2 or earlier. All three bipolar sources for which no H2 line emission was detected have M spectral types. Thus, the absence of H2 emission appears to be related to post-main sequence evolutionary age. Evidently the least evolved pPNe have not reached the stage where shocks develop and produce H2 emission. It follows that the central star of IRAS 17441-2411, which has yet to be classified, is likely to be of intermediate spectral type. The detection of H2 emission only from the bipolar pPNe with intermediate- and early-type central stars suggests that the event that triggers the formation of bipolarity precedes the event that produces the shocked H2 emission, and that both these developments take place before the photoionization event occurs and transforms a pPN into a PN.