Understanding the rotational variability of K2 targets

HgMn star KIC 250152017 and blue horizontal branch star KIC 249660366^★

¹ Department of Theoretical Physics and Astrophysics, Masaryk University, Kotlářská 2, 611,37 Brno, Czech Republic
e-mail: krticka@physics.muni.cz
² International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
³ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁴ Astronomical Institute, Academy of Science of CR, Fričova 298, 251 65 Ondřejov, Czech Republic

Received: 13 March 2020
Accepted: 14 May 2020

Abstract

Context. 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.

Aims. We aim to understand the origin of the light variability of K2 targets that show signatures of rotational modulation.

Methods. We used phase-resolved medium-resolution X-shooter 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.

Results. 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.