Period evolution of Very Long Period Variables: The Semiregulars

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Period evolution of Very Long Period Variables
The Semiregulars L. SABIN , A. A.
ZIJLSTRA University of Manchester, School of
Physics ? Astronomy P.O Box 88 Manchester M60
1QD, United Kingdom Email laurence.sabin_at_manchest
er.ac.uk aaz_at_iapetus.phy.umist.ac.uk
Introduction Like the Miras , the Semiregulars
are red giant or supergiant pulsating stars, but
oscillating with less regularity as indicated by
their name. They are divided in different
classes SRa, SRb, SRc, SRd (super giants), from
the most to the less regular shape of their light
curve. The study of the period changes in these
variable stars will not only reveal the action
(or not) of physical processes such as thermal
pulses, magnetism, convection . . . etc via the
observation of stable or unstable behavior in the
long run, but also we could be able to notice the
differences or similarities with the Mira-type
stars. The effect on the mass loss which is
characteristic of the AGB stars may therefore be
investigated. The study will concern stars with
periods greater than 450 days as their
long-period evolution is more likely to permit us
to see perturbations in their behavior.
The Sample





The
Semiregulars with period longer than 450 days are
not in high number using the database of the
AAVSO and the AFOEV we found 26 stars
corresponding to our criterion. The study of the
light curves of these stars involving the use of
the wavelet analysis (developed by Foster- 1996)
allows the correct visualization of periodic
variations in only 11 of them. This is due, in
part, to the fact that the stars present close
modes of pulsation ( believed to be the 1st and
2nd overtone) and there is a phenomenon of
aliasing preventing us to have a correct analysis.
Comparative study
The case of ST Psc is not common as the star
presents a nearly straight increase of 90 days
over 33 years. Other SRs (S Aur, S Per for
example) show that kind of rise or fall but the
evolution is characterised by oscillating
motions. They will therefore be put in the
category of meandering stars like the V Del-type
Miras.
Meandering behaviour V Cam-type trend
The period evolution We used a linear fitting
to define the change (by dlnP/dt the slope over
the mean period). In order to compare these
variations to the ones of the Miras we define the
same method of calculation and the same
significant threshold for the period change 2?
(1s dlnP/dt error 0.0002, in days) (Templeton,
Mattei and Willson 2005). It appears that the SRs
are very unstable and more than the miras.
We can see that the Semiregulars are more
unstable than the Miras. It may be due to the
fact that the SRs (may) pulsate according the
first overtone (and are most often accompanied
with a second one) while the Miras are believed
to pulsate in the fundamental mode. Both groups
present almost the same trends continuous
changes, (possible) sudden drops and more often
meandering behaviours. The particular study of
the period evolution for both types of stars
shows that most of the SRs and nearly half of the
O-rich and C-rich Miras studied show an
increasing trend in their behaviour the period
is rising. So if the process responsible for this
phenomenon is a thermal pulse, we can say that
the stars are experiencing the consequences of
one which has occurred some times ago. The stars
showing decreasing period changes, may present
the short moment after the appearance of the
pulse when the temperature is still high, and the
luminosity drops. The small variations
(meandering stars) could be due to other
phenomena. The Very Long Period Variables can be
seen, regarding their period instability, as the
witnesses of the physical changes occurring in
AGB stars . We must nevertheless also quote the
particularly unstable behaviour of 52 Mira-type
stars with a period between 200 and 450 days.
They have been taken out of a panel of 547 Mira
stars (Templeton et al. 2005).
The sign of a sudden change? RW Cyg presents
nearly the same period evolution as the Mira R
Cen, but we must underline the fact that the star
has been observed over only 30 years regarding to
88 for R Cen. So we have to be careful while
dealing with this object. Moreover, while for R
Cen the decrease in period is accompanied by a
decrease in amplitude, this is not the case for
RW Cyg.
The different classes of evolutionary trend. The
high level of perturbation observed while
studying the Semiregulars period evolution,
prevents us to establish a very good
classification of the different observable
trends. Nevertheless, even with our small sample,
we can find nearly the same categories as found
for the Miras.
Interpretation We can see that the period
evolution of the Semiregular stars can be
described nearly in the same way as for the
Miras. The meandering and continuous trends are
the most represented, part from the time we
consider as truthful the analysis for such a
biased type of signal. The denomination of the
stars (SRa., SRb...etc) does not seem to play a
role in the type of period evolution. The
correlation between these changes and thermal
pulses is not so obvious among the stars studied
by Lebzelter and Hron (2003), SW Gem and TW Peg
do not present Technetium the other stars have
not been studied. Moreover the presence of close
modes of pulsation prevents us to have a correct
analysis of the light curve.
Continuous decrease or increase
References Lebzelter T. and Hron J., 2003, A?A
411,533-542. Templeton M.R., Mattei
,J.A.,Willson L.A., 2005, AJ, 130776-788 Zijlstra
A.A and Bedding T.R, 2002,JAAVSO,31,2
We acknowledge the AAVSO and the AFOEV for the
data as well as all the observers who provide
them.