UV and FUV spectroscopy of the hybrid PG 1159-type central star NGC 7094

TL;DR

This study investigates the iron deficiency in PG 1159 stars, focusing on NGC 7094, and finds no nickel overabundance, suggesting iron may have been transformed into heavier elements, though these remain unidentified due to limited atomic data.

Contribution

First spectroscopic search for nickel overabundance in NGC 7094, providing insights into iron transformation processes in PG 1159 stars.

Findings

No nickel overabundance detected in NGC 7094

Iron deficiency confirmed in the star

Heavier elements remain unidentified due to atomic data limitations

Abstract

Previous studies aiming at the iron-abundance determination in three PG 1159 stars (K 1-16, PG 1159-035, NGC 7094) and a [WC]-PG 1159 transition star (Abell 78) have revealed that no object shows any iron line in the UV spectrum. The stars are iron-deficient by at least 1 dex, typically. A possible explanation is that iron nuclei were transformed by neutron captures into heavier elements (s-process), however, the extent of the iron-destruction would be much stronger than predicted by AGB star models. But if n-captures are the right explanation, then we should observe an enrichment of trans-iron elements. In this paper we report on our search for a possible nickel overabundance in one of the four Fe deficient PG 1159 stars, namely the central star NGC 7094. We are unable to identify any nickel line in HST and FUSE spectra and conclude that Ni is not overabundant. It is conceivable that iron was transformed into even heavier elements, but their identification suffers from the lack of atomic data.