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

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 ($B\gtrsim 100\mathrm{G}$) visible at the surface. It has been reported that $\approx 44$~per~cent of all A-type stars observed during the \emph{Kepler} mission exhibit rotationally modulated light curves. This is surprising considering that $\lesssim 10$~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 ($v_{\mathrm{eq}}$) 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 $v\sin i>v_{\mathrm{eq}}$ 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 $\lesssim 30$~per~cent.

18 pages, 12 figures. Accepted for publication in MNRAS

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