Are OH/IR Stars the Youngest post-AGB stars? A NICMOS Imaging Survey

Details Are OH/IR Stars the Youngest post-AGB stars? A NICMOS Imaging Survey Are OH/IR Stars the Youngest post-AGB stars? A NICMOS Imaging Survey

Jul 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003hst..prop.9801s&link_type=abstract

HST Proposal ID #9801

Mathematics

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Hst Proposal Id #9801 Cool Stars

Scientific paper

Essentially all well-characterized preplanetary nebulae {PPNe}- objects in transition between the AGB and planetary nebula evolutionary phases - are bipolar, whereas the mass-loss envelopes of AGB stars are strikingly spherical. In order to understand the processes leading to bipolar mass-ejection, we need to know at what stage of stellar evolution does bipolarity in the mass-loss first manifest itself. We have recently hypothesized that most OH/IR stars {evolved mass- losing stars with OH maser emission} are very young PPNe. We are conducting a multiwavelength survey program of imaging and spectroscopic observations of such objects, using a large, morphologically unbiased sample selected using IRAS 12-to-25 micron colors. Our ongoing HST/SNAP imaging survey of the optically bright half of this sample with WFPC2 and ACS is highly succesful: 19/32 objects observed are extended with bipolar/multipolar shapes {remaining objects are unresolved}. Slightly more than 50% of our sample are optically too faint or undetected but have strong near-IR counterparts - we therefore propose a NICMOS SNAPshot imaging survey of these optically-faint OH/IR stars. These observations are crucial for determining how and when the bipolar geometry asserts itself. The results from our NICMOS survey {together with the WFPC2/ACS survey} will allow us to draw general conclusions about the onset of bipolar mass-ejection during late stellar evolution. Our complementary program of interferometric mapping of the OH maser emission in our sources is yielding kinematic information with spatial resolution comparable to that in the HST images. The HST/radio data will provide crucial input for theories of post-AGB stellar evolution. In addition, these data will also indicate whether the multiple concentric rings, "searchlight beams'', and truncated equatorial disks recently discovered with HST in a few PPNe, are common or rare phenomena.

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