Element Abundance Determination in Hot Evolved Stars

TL;DR

This study analyzes element abundances in hot post-AGB stars to understand nuclear processes and compares observations with stellar evolution models, highlighting both agreements and discrepancies.

Contribution

It provides detailed spectral analyses of PG1159 stars using advanced non-LTE models and compares observed abundances with theoretical predictions.

Findings

Good agreement for He, C, N, O, Ne, F, Si, Ar

Discrepancies for P, S, Fe suggest model shortcomings

UV spectra are essential for trace element identification

Abstract

The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted element abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars. Almost all of the chemical trace elements in these hot stars can only be identified in the UV spectral range. The Far Ultraviolet Spectroscopic Explorer and the Hubble Space Telescope played a crucial role for this research.