A spectroscopic test of the rotational modulation origin of periodic \emph{Kepler} photometric variability of A-type stars

TL;DR

This study investigates whether the periodic brightness variations in A-type stars observed by Kepler are caused by rotational modulation due to surface features, using spectroscopic data to test the magnetic origin hypothesis.

Contribution

The paper provides the first spectroscopic survey testing the rotational modulation hypothesis for Kepler A-type stars, revealing that a significant fraction may have surface inhomogeneities.

Findings

Over 10% of A-type stars may have surface features causing variability.

Spectroscopic data show some stars have inconsistent rotational velocities, suggesting other causes.

The incidence of surface inhomogeneities is likely less than 30%.

Abstract

High-precision space-based photometry obtained by the \emph{Kepler} and \emph{TESS} missions has revealed evidence of rotational modulation associated with main sequence (MS) A and late-B type stars. Generally, such variability in these objects is attributed to inhomogeneous surface structures (e.g. chemical spots), which are typically linked to strong magnetic fields ($B\gtrsim100\,{\rm G}$) visible at the surface. It has been reported that $\approx44$~per~cent of all A-type stars observed during the \emph{Kepler} mission exhibit rotationally modulated light curves. This is surprising considering that $\lesssim10$~per~cent of all MS A-type stars are known to be strongly magnetic (i.e. they are Ap/Bp stars). We present a spectroscopic monitoring survey of 44 A and late-B type stars reported to exhibit rotational modulation in their \emph{Kepler} light curves. The primary goal of this survey is to test the hypothesis that the variability is rotational modulation by comparing each star's rotational broadening ($v\sin{i}$) with the equatorial velocities ($v_{\rm eq}$) inferred from the photometric periods. We searched for chemical peculiarities and binary companions in order to provide insight into the origin of the apparent rotational modulation. We find that 14 stars in our sample have $v\sin{i}>v_{\rm eq}$ and/or have low-mass companions that may contribute to or be responsible for the observed variability. Our results suggest that more than $10$~per~cent of all MS A and late-B type stars may exhibit inhomogeneous surface structures; however, the incidence rate is likely $\lesssim30$~per~cent.