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Astron. Astrophys. 341, 480-486 (1999)

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Asteroseismology of the [FORMULA] Cephei stars

II. 12 (DD) Lacertae

W.A. Dziembowski 1,2 and M. Jerzykiewicz 3

1 Warsaw University Observatory, al. Ujazdowskie 4, PL-00-478 Warsaw, Poland
2 Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, PL-00-716 Warsaw, Poland (e-mail: wd@astrouw.edu.pl)
3 Wrocaw University Observatory, ul. Kopernika 11, PL-51-622 Wrocaw, Poland (e-mail: mjerz@astro.uni.wroc.pl)

Received 11 August 1998 / Accepted 21 October 1998


Five pulsation modes are simultaneously excited in this well-known [FORMULA] Cephei star. Three of them, including the one with the largest light and radial-velocity amplitudes, form a triplet. The triplet is equidistant in frequency 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) nonlinear phase lock. Unfortunately, apart from the frequencies, the data are meager. Photometric indices yield the effective temperature and surface gravity of rather low accuracy. In addition, the existing determinations of the spherical 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 model is unlikely because it would require excessively strong dipolar field or a special field geometry. The rotational splitting is a possibility, but only for an [FORMULA], [FORMULA] mode in a model with specific values of the effective temperature and surface gravity. Finally, we note that the nonlinear phase lock may be the solution. However, verifying this depends on the progress of nonlinear calculations.

Key words: stars: individual: DD Lac – stars: interiors – stars: oscillations – stars: variables: general

Send offprint requests to: W.A. Dziembowski

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998