Lampens, P. ; Frémat, Y. ; Vermeylen, L. ; Sòdor, À. ; Skarka, M. ; De Cat, P. ; Bognàr, Zs. ; De Nutte, R. ; Dumortier, L. ; Escorza, A. ; Oomen, G. M. ; Van de Steene, G. ; Kamath, D. ; Laverick, M. ; Samadi, A. ; Triana, S. ; Lehmann, H.

published in Astronomy & Astrophysics, 610 issue A17, pp. 1-33

Abstract: Hundreds of candidate hybrid pulsators of intermediate type A--F were revealed by recent space missions. Hybrid pulsators allow us to study the full stellar interiors, where both low-order \textit{p}- and high-order \textit{g}-modes are simultaneously excited. The true hybrid stars must be identified since other processes, related to stellar multiplicity or rotation, might explain the presence of (some) low frequencies observed in their periodograms. We measured the radial velocities of 50 candidate $\delta$ Scuti - $\gamma$ Doradus hybrid stars from the \textit{Kepler} mission with the \textsc{Hermes} and \textsc{Ace} spectrographs over a time span of months to years. We aim to derive the fraction of binary and multiple systems and to provide an independent and homogeneous determination of the atmospheric properties and \vsini for all targets. The long(er)-term objective is to identify the (probable) physical cause of the low frequencies.We computed {one-dimensional} cross-correlation functions (CCFs) in order to find the best set of parameters in terms of the number of components, spectral type(s), and \vsini for each target. Radial velocities were measured using spectrum synthesis and a {two-dimensional} cross-correlation technique in the case of double- and triple-lined systems. Fundamental parameters were determined by fitting (composite) synthetic spectra to the normalised median spectra corrected for the appropriate Doppler shifts. We report on the analysis of 478 high-resolution \textsc{Hermes} and 41 \textsc{Ace} spectra of A/F-type candidate hybrid pulsators from the \textit{Kepler} field. We determined their radial velocities, projected rotational velocities, and atmospheric properties and classified our targets based on the shape of the CCFs and the temporal behaviour of the radial velocities. We derived orbital solutions for seven new systems. Three preliminary long-period orbital solutions are confirmed by a photometric time-delay analysis. Finally, we determined a global multiplicity fraction of 27\% in our sample of candidate hybrid stars.

Keyword(s): Techniques: spectroscopic -- Stars: binaries: spectroscopic -- Techniques: photometric -- Stars: oscillations {\it (including pulsations)} -- Stars: rotation -- Stars: variables: delta Scuti
Note: Article in press
DOI: 10.1051/0004-6361/201730694
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