An asteroseismic study of the β Cephei star β Canis Majoris

Aims. We present the results of a detailed analysis of 452 ground-based, high-resolution high S/N spectroscopic measurements spread over 4.5 years for beta Canis Majoris with the aim of determining the pulsational characteristics of this star, and then using them to derive seismic constraints on the stellar parameters. Methods. We determined pulsation frequencies in the Si III 4553 angstrom line with Fourier methods. We identified the m-value of the modes by taking the photometric identifications of the degrees l into account. To this end we used the moment method together with the amplitude and phase variations across the line profile. The frequencies of the identified modes were used for a seismic interpretation of the structure of the star. Results. We confirm the presence of the three pulsation frequencies already detected in previous photometric datasets: f(1) = 3.9793 c d(-1) (46.057 mu Hz), f(2) = 3.9995 cd(-1) (46.291 mu Hz), and f(3) = 4.1832 cd(-1) (48.417 mu Hz). For the two modes with the highest amplitudes, we unambiguously identify (l(1), m(1)) = (2, 2) and (l(2), m(2)) = (0, 0). We cannot conclude anything for the third mode identification, except that m(3) > 0. We also deduce an equatorial rotational velocity of 31 +/- 5 kms(-1) for the star. We show that the mode f1 must be close to an avoided crossing. Constraints on the mass (13.5 +/- 0.5 M-circle dot), age (12.4 +/- 0.7 Myr), and core overshoot (0.20 +/- 0.05 H-P) of beta CMa are obtained from seismic modelling using f(1) and f(2).