Understanding the rotational variability of K2 targets. HgMn star KIC 250152017 and blue horizontal branch star KIC 249660366

TL;DR

This study investigates the rotational light variability of two stars, revealing that one is an inhomogeneous HgMn star and the other a He-weak horizontal branch star with variability caused by surface composition and binary reflection effects.

Contribution

It provides the first detailed spectroscopic analysis explaining the rotational variability origins of these two specific stars.

Findings

KIC 250152017's variability is due to surface inhomogeneities of manganese and iron.

KIC 249660366's variability is caused by reflection effects in a binary system.

First detailed understanding of variability in a HgMn star.

Abstract

Ultraprecise space photometry enables us to reveal light variability even in stars that were previously deemed constant. A large group of such stars show variations that may be rotationally modulated. This type of light variability is of special interest because it provides precise estimates of rotational rates. We aim to understand the origin of the light variability of K2 targets that show signatures of rotational modulation. We used phase-resolved medium-resolution XSHOOTER spectroscopy to understand the light variability of the stars KIC~250152017 and KIC~249660366, which are possibly rotationally modulated. We determined the atmospheric parameters at individual phases and tested the presence of the rotational modulation in the spectra. KIC 250152017 is a HgMn star, whose light variability is caused by the inhomogeneous surface distribution of manganese and iron. It is only the second HgMn star whose light variability is well understood. KIC 249660366 is a He-weak, high-velocity horizontal branch star with overabundances of silicon and argon. The light variability of this star is likely caused by a reflection effect in this post-common envelope binary.