An explanation of the long-term behaviour of the pulsation amplitudes of the β Cephei star 16 (EN) Lacertae.
The three short-period terms in the variation of this star, having frequencies f1= 5.9112, f2= 5.8551 and f3= 5.5033 d-1, are known to vary in strength on a long time-scale. Using all available blue-light and radial-velocity observations the authors find that (1) the f3 term is a doublet, consisting of sinusoidal components with frequencies f3,1= 5.5025742± 0.0000007 and f3,2= 5.5040529± 0.000012 d-1, blue-light amplitudes a3,1= 8.29±0.16 and a3,2= 4.81±0.16 mmag, and radial-velocity amplitudes K3,1= 2.73±0.17 and K3,2= 1.96±0.17 km s-1, and (2) the time-scales of the amplitude variation of the first two terms are close to each other and equal to about 50 yr. This time-scale is close to the reciprocal of the growth rates of several l ≤ 2 pulsation modes which are excited in models of EN Lac and can be identified with the f1 and f2 terms. Since amplitude modulation on a time-scale of the order of the reciprocal of the growth rate is predicted by the theory of non-linear interaction of pulsation modes, the authors hypothesize that the observed variation of the amplitudes of the f1 and f2 modes results from (1) the 1:1 resonance between them, or (2) a resonant coupling to other modes. The authors discuss these possibilities in some detail and point out that they could be distinguished observationally. As far as the f3 doublet is concerned, the simplest explanation is that it represents an accidental near-coincidence of two self-excited modes. Using results of Dziembowski and Jerzykiewicz (1996), the authors propose several possible identifications of the observed doublet components with the l = 1 or 2, low-radial-order modes. They find no correlation of the pulsation amplitudes with phase of the orbital period.