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R. Sahai (Caltech/ JPL), C. Sanchez Contreras (Caltech)
Pre-Planetary nebulae (PPNs) - objects in transition between the Asymptotic Giant Branch (AGB) and planetary nebula (PN) evolutionary phases - hold the key to our understanding of these very late stages of stellar evolution for intermediate mass stars. Observationally, it is well-established that PNs show a dazzling variety of elliptical and bipolar morphologies, whereas AGB stars are surrounded by roughly spherical gas-dust envelopes resulting from dense, slow stellar winds. In the very short (103 yr) transition period between the AGB and PN phases (i.e. the PPN phase), some physical process (or processes) becomes operational which impose bipolarity upon most circumstellar outflows. Although most PPNs have central stars with spectral types between B and K, a few objects (e.g. OH231.8+4.2, V Hydrae) have bona-fide AGB central stars, yet they show strong evidence of bipolarity and/or high-speed outflows in their circumstellar envelopes, indicating that the processes leading to bipolarity can become operational even during the AGB phase.
We report here the results of an ongoing imaging survey of a large list (~100) of candidate young PPNs. We have constructed a morphologically unbiased sample of such objects from catalogs of OH/IR stars, which are evolved, visually faint, mass-losing stars with dense circumstellar envelopes, showing (generally double-peaked) OH maser emission. The IRAS spectral energy distributions (SEDs) of a large fraction of these objects indicate a lack of hot dust (12 to 25\micron~flux ratio F12/F25<1), implying a substantial recent decrease in their mass-loss rates which marks the beginning of their post-AGB evolution. We have discovered several bipolar objects in our survey. Low-resolution spectroscopy to identify the spectral types of the central stars has been carried out from the ground for a smaller number of objects from our list. Some of these show H\alpha in emission, most likely due to a fast post-AGB wind -- for these, we have used echelle spectroscopy to study the kinematics of this wind.
The author(s) of this abstract have provided an email address for comments about the abstract: sahai@jpl.nasa.gov
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.