Low-frequency variations of unknown origin in the Kepler δ Scuti star KIC 5988140 = HD 188774^⋆,⋆⋆

¹ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
e-mail: Patricia.Lampens@oma.be
² Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
⁴ Department of Astrophysics, IMAPP, Radboud University Nijmegen, 6500 GL Nijmegen, The Netherlands
⁵ Abastumani Astrophysical Observatory, Ilia State University, 0301 Abastumani, Georgia
⁶ Konkoly Observatory of the Hungarian Academy of Sciences, PO Box 67, 1525 Budapest, Hungary
⁷ SETI Institute/NASA Ames Research Center, Moffett Field, CA 94035, USA

Received: 2 May 2012
Accepted: 17 October 2012

Abstract

Context. The NASA exoplanet search mission Kepler is currently providing a wealth of light curves of ultra-high quality from space.

Aims. We used high-quality Kepler photometry and spectroscopic data to investigate the Kepler target and binary candidate KIC 5988140. We aim to interpret the observed variations of KIC 5988140 considering three possible scenarios: binarity, co-existence of δ Sct- and γ Dor-type oscillations, and rotational modulation caused by an asymmetric surface intensity distribution.

Methods. We used the spectrum synthesis method to derive the fundamental parameters T_eff, log g, [M/H], and v sin i  from the newly obtained high-resolution, high S/N spectra. Frequency analyses of both the photometric and the spectroscopic data were performed.

Results. The star has a spectral type of A7.5 IV-III and a metallicity slightly lower than that of the Sun. Both Fourier analyses reveal the same two dominant frequencies F₁ = 2F₂ = 0.688 and F₂ = 0.344 d^-1. We also detected in the photometry the signal of nine more, significant frequencies located in the typical range of δ Sct pulsation. The light and radial velocity curves follow a similar, stable double-wave pattern which are not exactly in anti-phase but show a relative phase shift of about 0.1 period between the moment of minimum velocity and that of maximum light.

Conclusions. Such findings are incompatible with the star being a binary system. We next show that, for all possible (limit) configurations of a spotted surface, the predicted light-to-velocity amplitude ratio is almost two orders larger than the observed value, which pleads against rotational modulation. The same argument also invalidates the explanation in terms of pulsations of type γ Dor (i.e. hybrid pulsations). We confirm the occurrence of various independent δ Sct-type pressure modes in the Kepler light curve. With respect to the low-frequency content, however, we argue that the physical cause of the remaining light and radial velocity variations of this late A-type star remains unexplained by any of the presently considered scenarios.