Strong interactions between g- and p-modes in the hybrid γ Doradus-δ Scuti CoRoT star ID 105733033^⋆,⋆⋆

¹ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
² Université de Toulouse, UPS-OMP, IRAP, 65000 Tarbes, France
e-mail: philippe.mathias@irap.omp.eu
³ CNRS, IRAP, 57 Avenue d’Azereix, BP 826, 65008 Tarbes, France
⁴ University of Vienna, Institute for Astronomy, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁵ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 22 June 2011
Accepted: 26 January 2012

Abstract

Context. The presence of stellar p- and g-modes allows us to test stellar structure models in great detail from the core to the envelope. As the driving mechanisms are not yet fully understood, the first important step is to provide clear evidence of these pulsation modes.

Aims. Recent space missions have confirmed that the γ Doradus and the δ Scuti observational instability strips overlap and consequently that many stars may be hybrids. CoRoT ID 105733033 is an excellent example of these hybrid pulsators as it shows g- and p-modes with almost similar amplitudes in two clearly distinct frequency domains. We present a detailed frequency analysis of the CoRoT star ID 105733033, which is obtained with a classical Fourier analysis.

Methods. After removing residual instrumental effects from the CoRoT light curve of N2 level, frequencies with an amplitude as small as 0.1 mmag were determined with Period04 and SigSpec up to 50 d^-1, although if deemed necessary lower amplitudes and higher frequencies were also investigated. The frequency spectrum of CoRoT ID 105733033 clearly consists of two distinct ranges, which are typical of γ Doradus and δ Scuti pulsation. Focus was placed on the identification of linear combinations and frequencies due to the coupling between γ Doradus and δ Scuti modes.

Results. We detect 198 γ Doradus type frequencies in the range  [0.25;4]  d^-1, of which 180 are not combination frequencies, and 24 of them are separated by a constant period-interval ΔP = 0.03074 d. According to the asymptotic theory, these 24 frequencies correspond to a series of g-modes of the same ℓ-degree and different radial orders n. We also detect 246 δ Scuti type frequencies in the range  [10.1;63.4]  d^-1. The dominant frequency F = 12.6759 d^-1 was identified as the fundamental radial mode. Our most noteworthy result is that all the main γ Doradus frequencies f_i are also detected in the δ Scuti domain as F ± f_i with four times smaller amplitudes. Once these frequencies were removed, only 59 can be considered as individual δ Scuti frequencies.

Conclusions. A coupling between g- and p-modes is proposed to be a tool for detecting g-modes in the Sun, but this coupling has never yet been observed. Our present study may be valuable input to theoretical studies, addressing the mutual influence of g- and p-mode cavities and the deviation from classical theory. Furthermore, we identify a sequence of g-modes belonging to the same ℓ but with consecutive orders n.