Astronomy and Astrophysics, volume 549A, 104-104 (2013/1-1)
Low-frequency variations of unknown origin in the Kepler δ Scuti star KIC 5988140 = HD 188774.
LAMPENS P., TKACHENKO A., LEHMANN H., DEBOSSCHER J., AERTS C., BECK P.G., BLOEMEN S., KOCHIASHVILI N., DEREKAS A., SMITH J.C., TENENBAUM P. and TWICKEN J.D.
Abstract (from CDS):
The NASA exoplanet search mission Kepler is currently providing a wealth of light curves of ultra-high quality from space. We used high-quality Kepler photometry and spectroscopic data to investigate the Kepler target and binary candidate KIC5988140. We aim to interpret the observed variations of KIC5988140 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.We used the spectrum synthesis method to derive the fundamental parameters Teff, logg, [M/H], and vsini from the newly obtained high-resolution, high S/N spectra. Frequency analyses of both the photometric and the spectroscopic data were performed.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. 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.
asteroseismology - stars: variables: δ Scuti - stars: atmospheres - stars: abundances - stars: rotation - stars: individual: HD 188774
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<Available at CDS (J/A+A/549/A104): tablea1.dat sp/*>
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