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 β 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.<BR /> Methods: .We determined pulsation frequencies in the Si III 4553 Å 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.<BR /> Results: .We confirm the presence of the three pulsation frequencies already detected in previous photometric datasets: f[SUB]1[/SUB] = 3.9793~c d[SUP]-1[/SUP] (46.057~μHz), f[SUB]2[/SUB] = 3.9995~c d[SUP]-1[/SUP] (46.291~μHz), and f[SUB]3[/SUB] = 4.1832~c d[SUP]-1[/SUP] (48.417~μHz). 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[SUB]3[/SUB] > 0. We also deduce an equatorial rotational velocity of 31 ± 5~km s[SUP]-1[/SUP] for the star. We show that the mode f[SUB]1[/SUB] must be close to an avoided crossing. Constraints on the mass (13.5 ± 0.5~μHz), age (12.4 ± 0.7 Myr), and core overshoot (0.20 ± 0.05 H_P) of β CMa are obtained from seismic modelling using f[SUB]1[/SUB] and f_2.<BR />