Asteroseismology of the beta Cephei stars - II. 12 (DD) Lacertae

Five pulsation modes are simultaneously excited in this well-known beta Cep hei star. Three of them, including the one with the largest light and radia l-velocity amplitudes, form a triplet. The triplet is equidistant in freque ncy to within the errors of measurement, that is, 0.0003 d(-1) Explaining why the triplet should be so nearly equidistant turns out to be a real challenge to the theory. We investigate the following three options: (1) rotational splitting, (2) an oblique magnetic pulsator, and (3) nonlin ear phase lock. Unfortunately, apart from the frequencies, the data are mea ger. Photometric indices yield the effective temperature and surface gravit y of rather low accuracy. In addition, the existing determinations of the s pherical harmonic degree of even the strongest observed mode are discrepant . Consequently, the model parameters are not well constrained. We show that of the three above-mentioned options, the oblique pulsator mod el is unlikely because it would require excessively strong dipolar field or a special field geometry. The rotational splitting is a possibility, but o nly for an l = 2, p(0) mode in a model with specific values of the effectiv e temperature and surface gravity. Finally, we note that the nonlinear phas e lock may be the solution. However, verifying this depends on the progress of nonlinear calculations.