[vsnet-chat 3650] re AF Cygni

[crawl@zoom.co.uk]

>> Finally, I've got a 80% complete analysis from a long run of
>> AF Cyg data from VSOLJ which clearly shows that the _tertiary_ period was
>>quite stable at the current 920 day ish value between JD 2424000 and
>>2437000, but cored about a slightly shorter value twixt from the last
date up to
>>JD 2444000 whence it returned to near 920 days.
>
>Is there any possibility of the stellar rotation period for
>such a stable periodicity?
>
>Regards,
>Taichi Kato

Sorry, I haven't explained myself properly: the tertiary period has fairly
rapidly changed from 920 days to another shorter value at one point, as per
the above, and then at a later above mentioned date fairly rapidly returned
to 920 days [918 days].

It is unlikely for an lpv to spin up and spin down in this manner, I think,
whilst the changeover points are too rapid, and the general situation too
completely non-sinusoidal or non-parabolic or whatever to suggest any sort
of apsidal varation of an orbital nature.

Meanwhile, as Stan Walker has noted in a earlier chat, Miras are known that
happily stick at stable core periodicities for long intervals of time, then
switch to others about only 1 or 2 percent different, the switch over
interval often being relatively abrupt.

This leaves me to feel that the tertiary period of AF Cygni has a more
intrinsic, as opposed to extrinsic, nature.

Actually, this sort of period change can also be seen with other pulsators
like delta Scutids etc, if the interpretation of the O-Cs can be trusted.

This does not preclude some tertiary periods being extrinsic: Kiss et al
recently showed this in a pre-print for an SR, RY UMa I _think_, where the
modulation of the lightcurve was on the several thousands of days level.
They believed it to be an orbital effect, but it should be noted that these
stars are thought to have rotation periods of that order too.


[On the purely conjectural level I sometimes wonder about things like
non-radial pulsations or convection cycles or whatever in LPVs that somehow
change sign/direction thus splitting the object into two periodicity
regimes at different times, with one being "parallel" and one being
"anti-parallel", and maybe the changeovers from one to the other are driven
by some sort of magnetic cycle, like the suns, etc, &s.  But as my brain
can't really handle the maths needed for such conjectures, and as there's
no source of any relevant data of long enough time baseline to test even if
I could handle the maths, I then stop worrying about it for a long while ;^)

One problem is that is is rare to find any repeatability in the lightcurves
elements.  Even century long data will only show a couple of these events
for any star that shows them.  The timescale is just too long, and it could
all just be coincidence.

However, the classic exception to the rule is T Herculis, whose saw-toothed
O-C has intrigued me for a long time.  It really does appear to oscillate
between two core period regimes.]

Cheers

John

JG, UK

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