KIC 6951642: a Kepler δ Scuti-𝜸 Dor star affected by rotation and stellar activity in a (possible) single-lined binary system

Introduction: KIC 6951642 has been reported as a candidate hybrid pulsator of type γ Dor – δ Scuti from the first-quarters observations of the Kepler mission. The occurrence of a broad range of very regularly-spaced frequencies in the Fourier spectra of the available light curves and the long-term modulation of its surface brightness furthermore suggest the presence of, respectively, rotational modulation (due to spots) and stellar activity. 

Methods: We performed an iterative frequency-search of each Fourier spectrum, and searched for regular patterns in them. We applied spectrum synthesis to determine the atmospheric stellar parameters. Furthermore, since KIC 6951642 was announced to be a long-period binary system, we also fitted the radial velocities of our spectra from the high-resolution, high-efficiency spectrograph HERMES (Raskin et al. 2011) with some time delays derived from the light curves to improve the (first) orbit determination. 

Results: From high-resolution spectroscopy, we found that KIC 6951642 is a fast-rotating, F0-type, (possible) single-lined binary star with a period of ∼4.8 yr. We detected frequencies of type δ Scuti with the strongest mode located at 13.96 d⁻¹, as well as two multiplets with a regular mean frequency spacing of 0.481 d^-1 (natural large frequency spacing), and two multiplets with the rotational splitting as large as 0.719 d^-1. In the low-frequency (g-mode) regime, we recovered the frequency of 0.721 d⁻¹, as well as the frequency of 0.0087 d⁻¹. We attribute the first one to stellar rotation and the second one to stellar activity with a (possibly magnetic) cycle length of 3.2 yr. We detected g-modes of γ Dor-type with the mean period spacings of 207.6 s in the frequency range of (1.4-1.6) d⁻¹ .