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Astron. Astrophys. 321, 145-150 (1997)

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9. Looking for explanations

Although the long term energy variations in the visible are rather small compared to the total stellar output (from about 3 to about 1% in decades), one can wonder where this energy variation can be redistributed:

  1. In other parts of the spectrum?
  2. In unknown modes or different time constant variations (for example shorter than 0.01 d time constants would hardly be noticed the way we actually do differential photometry)?
  3. In photospheric or "atmospheric" activity, or to some alteration in the actual structure of the more superficial layers of the star?
  4. In tidal coupling that would reduce the rotation speed of the primary? (the time scales and amount of energy transfered would not allow an easy detection in the frequency couplings).

    According to Waelkens and Rufener (1983), tidal interaction damps the pulsation phenomenon in [FORMULA] Cep variables, as could be the case for the three shortest orbital period known in such stars (i.e. [FORMULA] Vir, [FORMULA] Cep, ... , and 16 Lac). This could be an explanation of [FORMULA] and [FORMULA] behavior in 16 Lac. However as it is a slow rotator, its amplitudes are not likely to be damped completely by rotation: as pointed out by Jakate (1979) the fastest rotators among the [FORMULA] Cep variables have the smallest amplitudes.

An exclusion between pulsation and "elliptical" (i.e. geometric) variations can explain the decrease of [FORMULA], as non spherical perturbations of the star should damp its radial modes (a limit case being that of [FORMULA] Vir, which rotation probably finally got synchronized with its binary orbit). Accordingly, if energy is still available in 16 Lac to feed pulsation, it should appear better in non radial modes ([FORMULA] for example).

A good test of such an explanation would be to monitor carefully the 6.05 d period detected by Jerzykiewicz (eventually a semi-orbital period) and see if there is any correlation between its amplitude and that of [FORMULA] and [FORMULA]. One should remember here that any such "elliptical" variation would be mixed with the primary's light reflexion onto the secondary (there is about an 8 magnitudes difference between components !).

As pointed out in Paper I, the [FORMULA] amplitudes could still be phase-locked with the orbit, the maxima occuring about a fifth of the orbital period after the periastron. At that phase, if the primary is somewhat ellipsoidal, it should also present a maximum apparent surface.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998