Spectral analysis of three hot subdwarf stars: EC 11481-2303, Feige 110, and PG 0909+276: A critical oscillator-strength evaluation for iron-group elements

TL;DR

This study refines stellar atmosphere models for hot subdwarfs by evaluating and improving atomic data, especially oscillator strengths, through high-resolution ultraviolet spectroscopy, leading to more accurate temperature, gravity, and abundance measurements.

Contribution

It provides a critical evaluation of oscillator strengths for iron-group elements and refines stellar parameters for three hot subdwarfs, enhancing spectral analysis accuracy.

Findings

Confirmed effective temperatures and surface gravities within error margins.

Identified reliable reference lines for iron-group element abundance analysis.

Improved atomic data for spectral modeling of hot stars.

Abstract

For the precise spectral analysis of hot stars, advanced stellar-atmosphere models that consider deviations from the local thermodynamic equilibrium are mandatory. This requires accurate atomic data to calculate all transition rates and occupation numbers for atomic levels in the considered model atoms, not only for a few prominent lines exhibited in an observation. The critical evaluation of atomic data is a challenge because it requires precise laboratory measurements. Ultraviolet spectroscopy of hot stars with high resolving power provide such "laboratory" spectra. We compare observed, isolated lines of the iron group (here calcium to nickel) with our synthetic line profiles to judge the accuracy of the respective oscillator strengths. This will verify them or yield individual correction values to improve the spectral analysis, i.e., the determination of, e.g., effective temperature and abundances. To minimize the error propagation from uncertainties in effective temperature, surface gravity (g), and abundance determination, we start with a precise reanalysis of three hot subdwarf stars, namely EC 11481-2303, Feige 110, and PG 0909+276. Then, we measure the abundances of the iron-group elements individually. Based on identified, isolated lines of these elements, we compare observation and models to measure their deviation in strength (equivalent width). For EC 11481-2303 and Feige 110, we confirmed the previously determined effective temperatures and log g values within their error limits. For all three stars, we fine-tuned all metal abundances to achieve the best reproduction of the observation. For more than 450 isolated absorption lines of the iron group, we compared modeled and observed line strengths. We selected strong, reliable isolated absorption lines, which we recommend to use as reference lines for abundance determinations in related objects.