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Astron. Astrophys. 321, 145-150 (1997)
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:
- In other parts of the spectrum?
- 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)?
- In photospheric or "atmospheric" activity, or to some alteration
in the actual structure of the more superficial layers of the star?
- 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 Cep variables, as
could be the case for the three shortest orbital period known in such
stars (i.e. Vir, Cep, ...
, and 16 Lac). This could be an explanation of
and 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 Cep variables have the smallest amplitudes.
An exclusion between pulsation and "elliptical" (i.e. geometric)
variations can explain the decrease of , as non
spherical perturbations of the star should damp its radial modes (a
limit case being that of 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 ( 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 and . 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 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.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998
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