Modelling the light variability of the Ap star epsilon Ursae Majoris

TL;DR

This study models the light variability of the Ap star epsilon Ursae Majoris by simulating surface chemical inhomogeneities and their effects on flux redistribution, achieving good agreement with observations across multiple photometric systems.

Contribution

The paper presents a detailed simulation of epsilon UMa's light variability using Doppler Imaging surface maps and stellar atmosphere models, clarifying the roles of Fe and Cr in flux redistribution.

Findings

Fe and Cr significantly influence light variability amplitude.

The refined rotational period is 5.088631 days.

Most observed variability can be explained by surface chemical inhomogeneities.

Abstract

We simulate the light variability of the Ap star epsUMa using the observed surface distributions of Fe, Cr, Ca, Mn, Mg, Sr and Ti obtained with the help of Doppler Imaging technique. Using all photometric data available we specified light variations of epsUMa modulated by its rotation from far UV to IR. We employed the LLmodels stellar model atmosphere code to predict the light variability in different photometric systems. The rotational period of epsUMa is refined to 5d088631(18). It is shown that the observed light variability can be explained as a result of the redistribution of radiative flux from the UV spectral region to the visual caused by the inhomogeneous surface distribution of chemical elements. Among seven mapped elements, only Fe and Cr significantly contribute to the amplitude of the observed light variability. In general, we find a very good agreement between theory and observations. We confirm the important role of Fe and Cr to the magnitude of the well-known depression around 5200 \AA\ through the analysis of the peculiar $a$-parameter. Finally, we show that the abundance spots of considered elements cannot explain the observed variability in near UV and $\beta$ index which are likely due to some other causes. The inhomogeneous surface distribution of chemical elements can explain most of the observed light variability of the A-type CP star epsUMa.