Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992hst..prop.3513h&link_type=abstract
HST Proposal ID #3513
Other
Hst Proposal Id #3513
Scientific paper
Post-AGB stars in galactic globular clusters are Rosetta stones for our understanding of late phase of stellar evolution of low mass stars. They are also important objects to study the early phases of the chemical evolution of our galaxy because they are amongst very few cluster stars for which the helium abundance can be determined directly. ROB 162 (in NGC 6397) and K 648, the central star of a PN in M 15, are two post-AGB stars in very metal poor clusters. Quantitative optical spectroscopy indicated that these two stars have a very different evolutionary history with respect to dredge-up events of nuclear processed matter. K 648 is strongly enriched in carbon indicating strong dredge-up from interior layers of the progenitor star. ROB 162, on the other hand, does not show any indication for dredge-up from optical spectroscopy. It is proposed to determine C, N and O abundances from UV-spectroscopy with the FOS using appropriate NLTE model atmospheres developed by our group. These cannot be derived from optical spectra and will put important constraints on the dredge-up history of the two stars. In the case of K 648, it gives the unique possibility to study different layers of the progenitor star, since we can compare the photospheric abundance to that of the Planetary Nebula. In the case of ROB 162, these abundances can prove or disprove the absence of dredge-up. If proven, a direct determination the primordial helium abundance results.
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