KIC 8975515: A fastrotating (γ Dor  δ Sct) hybrid star with Rossby modes and a slower δ Sct companion in a longperiod orbit
Abstract
Aims: KIC 8975515 is a doublelined spectroscopic binary system with hybrid nature. Regular monitoring with the échelle spectrograph HERMES show that both components have matching atmospheric properties (T_{eff} ∼ 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 frequencysearch 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 lowfrequency regime, one series of prograde g modes and four series of retrograde r modes, were detected. The r modes are welldistributed with respect to the harmonics of the rotational frequency of the fastrotating star f_{rot} = 1.647 d^{1}. The dominant g mode is f_{2} = 2.37 d^{1}. The strongest p mode is f_{1} = 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 f_{1} 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 fastrotating component as a hybrid pulsator with r modes, and the slowlyrotating component as a δ Sct pulsator.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 June 2020
 DOI:
 10.1051/00046361/201936555
 arXiv:
 arXiv:2004.08048
 Bibcode:
 2020A&A...638A..57S
 Keywords:

 techniques: photometric;
 techniques: spectroscopic;
 binaries: spectroscopic;
 stars: variables: δ Scuti;
 asteroseismology;
 stars: rotation;
 Astrophysics  Solar and Stellar Astrophysics;
 Astrophysics  Instrumentation and Methods for Astrophysics
 EPrint:
 Accepted to publish on A \&