EDP Sciences
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Volume 459, Number 2, November IV 2006
Page(s) 589 - 596
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20064980

A&A 459, 589-596 (2006)
DOI: 10.1051/0004-6361:20064980

An asteroseismic study of the $\beta$ Cephei star $\beta$ Canis Majoris

A. Mazumdar1, 2, M. Briquet1, M. Desmet1 and C. Aerts1, 3

1  Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 B, 3001 Leuven, Belgium
    e-mail: anwesh.mazumdar@yale.edu
2  Astronomy Department, Yale University, PO Box 208101, New Haven, CT 06520-8101, USA
3  Department of Astrophysics, University of Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands

(Received 7 February 2006 / Accepted 11 July 2006)

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 Å line with Fourier methods. We identified the m-value of the modes by taking the photometric identifications of the degrees $\ell$ 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: $f_1 = 3.9793~\mathrm{c\,d}^{-1}$ ( $46.057~\mu\mathrm{Hz}$), $f_2 = 3.9995~\mathrm{c\,d}^{-1}$ ( $46.291~\mu\mathrm{Hz}$), and $f_3 =
4.1832~\mathrm{c\,d}^{-1}$ ( $48.417~\mu\mathrm{Hz}$). For the two modes with the highest amplitudes, we unambiguously identify $(\ell_1,m_1) = (2,2)$ and $(\ell_2,m_2) = (0,0)$. We cannot conclude anything for the third mode identification, except that m3 > 0. We also deduce an equatorial rotational velocity of $31 \pm 5~\mathrm{km\,s}^{-1}$ for the star. We show that the mode f1 must be close to an avoided crossing. Constraints on the mass ( $13.5 \pm 0.5~\mu\mathrm{Hz}$), age ( $12.4 \pm 0.7$ Myr), and core overshoot ( $0.20 \pm 0.05\,H_P$) of $\beta$ CMa are obtained from seismic modelling using f1 and f2.

Key words: stars: early-type -- stars: individual: $\beta$ Canis Majoris -- techniques: spectroscopic -- stars: oscillations

© ESO 2006

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