Fluorine in extremely hot post-AGB stars: Evidence for nucleosynthesis^*

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany e-mail: werner@astro.uni-tuebingen.de
² Dr.-Remeis-Sternwarte, Universität Erlangen-Nürnberg, Sternwartstraße 7, 96049 Bamberg, Germany
³ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

Received: 26 October 2004
Accepted: 30 November 2004

Abstract

We have discovered lines of highly ionized fluorine ( and ) in the far-UV spectra of extremely hot ( = 85 000–150 000 K) post-AGB stars. Our sample comprises H-rich central stars of planetary nebulae as well as H-deficient PG1159 stars. We performed non-LTE calculations and find strong F overabundances (up to 10^-4 by mass, i.e., 250 times solar) in a number of PG1159 stars, while F is essentially solar in the H-rich stars. Since PG1159 stars are believed to exhibit intershell matter of the preceding AGB phase on their surface, their chemical analyses allow for a direct insight into nucleosynthesis processes during the AGB phase. The high F abundances in PG1159 stars confirm the conclusion from abundance determinations in giants, that F is synthesized in AGB stars and that the F enrichment in the intershell must be very high.