Discovery of binarity, spectroscopic frequency analysis, and mode identification of the δ Scuti star 4 CVn^{⋆,⋆⋆,⋆⋆⋆}

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: valentina.schmid@ster.kuleuven.be
² Max-Planck Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
³ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria
⁴ Department of Astronomy, University of Texas at Austin 2515 Speedway, Stop C1400 Austin, Texas 78712-1205, USA
⁵ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁶ Laboratoire AIM, CEA/DSM – CNRS – Université Denis Diderot – IRUF/SAp, 91191 Gif-sur-Yvette Cedex, France

Received: 18 February 2014
Accepted: 10 July 2014

Abstract

More than 40 years of ground-based photometric observations of the δ Sct star 4 CVn has revealed 18 independent oscillation frequencies, including radial as well as non-radial p-modes of low spherical degree ℓ ≤ 2. From 2008 to 2011, more than 2000 spectra were obtained at the 2.1 m Otto-Struve telescope at the McDonald Observatory. We present the analysis of the line-profile variations, based on the Fourier-parameter fit method, detected in the absorption lines of 4 CVn, which carry clear signatures of the pulsations. From a non-sinusoidal, periodic variation of the radial velocities, we discover that 4 CVn is an eccentric binary system with an orbital period P_orb = 124.44 ± 0.03 d and an eccentricity e = 0.311 ± 0.003. We detect 20 oscillation frequencies, 9 of which previously unseen in photometric data; attempt mode identification for the two dominant modes, f₁ = 7.3764 d^-1 and f₂ = 5.8496 d^-1; and determine the prograde or retrograde nature of 7 of the modes. The projected rotational velocity of the star, v_eqsini ≃ 106.7 km s^-1, translates to a rotation rate of v_eq/v_crit ≥ 33%. This relatively high rotation rate hampers unique mode identification, since higher order effects of rotation are not included in the current methodology. We conclude that, in order to achieve unambiguous mode identification for 4 CVn, a complete description of rotation and the use of blended lines have to be included in mode-identification techniques.