Discovery Of Binarity, Spectroscopic Frequency Analysis, And Mode Identification Of The Delta Scuti Star 4 CVn

More than 40 years of ground-based photometric observations of the delta 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 2.1m 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, f(1) = 7.3764 d(-1) and f(2) = 5.8496 d(-1); and determine the prograde or retrograde nature of 7 of the modes. The projected rotational velocity of the star, v(eq) sin i similar or equal to 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 4CVn, a complete description of rotation and the use of blended lines have to be included in mode-identification techniques.