Abstract :

Context : The presence of A/F-type Kepler hybrid stars extending across the entirety of the δ Scuti - γ Doradus instability strips and beyond remains largely unexplained. In order to better understand these particular stars, we performed a multi-epoch spectroscopic study of a sample of 49 candidate A/F-type hybrid stars and one cool(er) hybrid object detected by the Kepler mission. We determined a lower limit of 27% for the multiplicity fraction. For six spectroscopic systems, we also reported long-term variations in the time delays (TDs). For four systems, the TD variations are fully coherent with those of the radial velocities (RVs ) and can be attributed to orbital motion. 
Aims : We aim to improve the orbital solutions for those spectroscopic systems with long orbital periods (order of 4-6 years) among the Kepler hybrid stars that we continued to observe. 
Methods: The orbits are computed based on a simultaneous modelling of the RVs obtained with high-resolution spectrographs and the photometric TDs derived from time-dependent frequency analyses of the Kepler light curves. 
Results : We refined the orbital solutions of four spectroscopic systems with A/F-type Kepler hybrid component stars: KIC 4480321, 5219533, 8975515, and KIC 9775454. Simultaneous modelling of both data types analysed together enabled us to improve the orbital solutions (all), obtain more robust and accurate information on the mass ratio (some for the first time), and identify the component with the short-period δ Sct-type pulsations (all). The information gained is maximized when one of the components, generally the one exhibiting the δ Sct-type pulsations, is a fast rotator. In several cases, we were also able to derive new constraints for the minimum component masses. From a search for regular frequency patterns in the high-frequency regime of the Fourier transforms of each system, we found no evidence of tidal splitting among the triple systems with close (inner) companions. However, some systems exhibit frequency spacings that can be explained by the mechanism of rotational splitting.

This study is based on spectra obtained with the HERMES échelle spectrograph installed at the Mercator Telescope, operated by the IvS, KULeuven, funded by the Flemish Community, and located at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofísica de Canarias.
 

Publication : Astronomy & Astrophysics, Volume 647, id.A139, 10 pp.
DOI : 10.1051/0004-6361/202039389 
Bibcode : 2021A&A...647A.139L 
Keywords : binaries: spectroscopic; stars: variables: δ Scuti; asteroseismology; techniques: radial velocities; techniques: photometric; Astrophysics - Solar and Stellar Astrophysics