Fundamental parameters of the Ap-stars GO And, 84 UMa, and $\kappa$ Psc

TL;DR

This study determines fundamental parameters and chemical abundances of three Ap stars using spectroscopic methods, compares results with interferometry, and finds good agreement in radii and luminosities but higher spectroscopic temperatures for hotter stars.

Contribution

It provides a detailed spectroscopic analysis of three Ap stars and compares these results with interferometric data, enhancing the understanding of Ap star parameters.

Findings

Chemical abundances show heavy element excesses up to 4 dex.

Surface chemical inhomogeneity has minimal impact on temperature estimates.

Spectroscopic temperatures are higher than interferometric ones for stars above 9000 K.

Abstract

The aim of this work is to determine fundamental parameters of three Ap stars, GO And (HD 4778), $\kappa$ Psc (HD 220825), and 84 UMa (HD 120198), using spectroscopic techniques. By analysing these stars, we complete the sample of Ap stars for which fundamental parameters have additionally been derived by means of interferometry. This enables a cross-comparison of results derived by direct and indirect methods. For all investigated stars, we determined fundamental parameters and derived chemical abundances that are typical for Ap stars. The abundances are mainly characterised by a gradual increase of heavy element atmospheric abundances from an order of magnitude for iron peak elements up to very significant excesses of 3-4 dex of the rare-earth elements relative to the solar values. The only exception is Ba, whose abundance is close to the solar abundance. There is also a significant He deficiency in the atmospheres of HD 120198 and HD 220825, whereas the He abundance in HD 4778 is close to the solar abundance. We do not find significant Fe and Cr stratification. Using these abundances, we constructed self-consistent atmospheric models for each star. The effect of the surface chemical inhomogeneity on the derived fundamental parameters did not exceed +/-100 K in effective temperature, which lies within the range of errors in similar self-consistent analyses of Ap stars. Finally, we compared spectroscopically derived effective temperatures, radii, and luminosity for 13 out of 14 Ap stars in a benchmark sample with the interferometric results. While radii and luminosity agree within the quoted errors of both determinations, the spectroscopic effective temperatures are higher than the interferometric temperatures for stars with temperatures $T_{eff} >$ 9000 K. The observed hydrogen line profiles favour the spectroscopically derived temperatures.