Volume 549, January 2013
|Number of page(s)||9|
|Section||Stellar structure and evolution|
|Published online||04 January 2013|
Koninklijke Sterrenwacht van België, Ringlaan 3,
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
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 Teff, 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.
Key words: asteroseismology / stars: variables:δScuti / stars: atmospheres / stars: abundances / stars: rotation / stars: individual: HD 188774
Based on data gathered with NASA’s Discovery mission Kepler and with the Hermes spectrograph, installed at the Mercator Telescope, operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, and with the 2-m Alfred-Jensch telescope of the Thüringer Landessternwarte Tautenburg.
Reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/549/A104
© ESO, 2013
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