Volume 638, June 2020
|Number of page(s)||19|
|Section||Stellar structure and evolution|
|Published online||11 June 2020|
KIC 8975515: A fast-rotating (γ Dor – δ Sct) hybrid star with Rossby modes and a slower δ Sct companion in a long-period orbit
Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
2 Research Institute for Applied Physics and Astronomy, University of Tabriz, PO Box 51664, Tabriz, Iran
3 Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
4 Department of theoretical Physics and Astrophysics, Physics Faculty, University of Tabriz, PO Box 51664, Tabriz, Iran
Accepted: 16 April 2020
Aims. KIC 8975515 is a double-lined spectroscopic binary system with hybrid nature. Regular monitoring with the échelle spectrograph HERMES show that both components have matching atmospheric properties (Teff ∼ 7400 K), and that one of them is a fast rotator (v sin i = 162 versus 32 km s−1). Our aim is to study the Kepler light curve in order to determine the frequencies of the pulsations, to search for regular spacing patterns in the Fourier spectrum, if any, and to discuss their origin in the context of binarity and fast rotation.
Methods. In this paper, we study the properties of the stellar pulsations based on a careful analysis of the Fourier spectrum. This was done by performing repeated frequency-search analyses with successive prewhitenings of all the significant frequencies detected in the spectrum. Moreover, we searched for regular period spacing among the g modes as well as frequency splitting among the g and p modes in frequency.
Results. In the low-frequency regime, one series of prograde g modes and four series of retrograde r modes, were detected. The r modes are well-distributed with respect to the harmonics of the rotational frequency of the fast-rotating star frot = 1.647 d−1. The dominant g mode is f2 = 2.37 d−1. The strongest p mode is f1 = 13.97 d−1. We identified two multiplets of regularly split p modes with the mean values of 0.42 d−1 and 1.65 d−1. In addition, we also identified a few incidences of mode coupling (between f1 and a g or r mode).
Conclusions. We detected five series of retrograde r and prograde g modes as well as two multiplets of p modes with rotational frequency splitttings related to both components of KIC 8975515. We identified the fast-rotating component as a hybrid pulsator with r modes, and the slowly-rotating component as a δ Sct pulsator.
Key words: techniques: photometric / techniques: spectroscopic / binaries: spectroscopic / stars: variables: δ Scuti / asteroseismology / stars: rotation
© ESO 2020
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