Search For Iron, Nickel, and Fluorine in PG1159 Stars

TL;DR

This study investigates the elemental composition of PG1159 stars, focusing on iron, nickel, and fluorine, to understand their peculiar abundances and stellar evolution, but finds no nickel enhancement and confirms fluorine overabundance.

Contribution

It provides the first search for nickel lines in PG1159 stars and presents wind analysis results for F abundances in specific objects, highlighting unresolved iron deficiency.

Findings

No nickel overabundance detected in PG1159 stars.

Fluorine is overabundant in studied PG1159 stars.

Indications of very low oxygen abundance in NGC 7094.

Abstract

A possible origin of the iron-deficiency in PG1159 stars could be neutron captures on Fe nuclei. A nickel overabundance would corroborate this idea. Consequently we are looking for nickel lines in PG1159 stars. Prime targets are relatively cool objects, because Ni VI is the dominant ionisation stage and the spectral lines of this ion are accessible with UV observations. We do not find such lines in the coolest PG1159 star observed by FUSE (PG1707+427, Teff = 85,000 K) and conclude that the nickel abundance is not enhanced. Hence, the Fe-deficiency in PG1159 stars remains unexplained. In addition, we present results of a wind analysis of the hybrid-PG1159 star NGC 7094 and the [WC]-PG1159 transition-type object Abell 78 in order to derive F abundances from the F VI 1139.5 Angstrom line. In both cases, we find F overabundances, in agreement with results of photospheric analyses of many PG1159 stars. Surprisingly, we find indications for a very low O abundance in NGC 7094.