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Astron. Astrophys. 346, 487-490 (1999)
4. Discussion
The analysis of the 34 short-period variable stars in
Cen stressed the importance of
studying the Fourier parameters. The sample of high-amplitude
Sct and SX Phe stars is considerably
enlarged by these new variables especially toward shortest periods. In
general, many variable stars show a very small amplitude, below 0.10
mag. Such a small value is probably responsible for the high number of
sinusoidal light curves: since the
ratios are usually around 0.1, the amplitude of the
term is very small and observational
errors can mask the asymmetry of the light curve.
In spite of that, the parameters
are confined in a narrow strip for periods between 0.042 d and 0.07 d.
Toward longer periods, there is an overlapping with the values
obtained in the case of galactic stars. Toward shorter periods, the
tendency to decreasing values is also
verified. It should be noted that there is a strong difference with
the results obtained by analyzing the stars in the Carina dwarf
Spheroidal Galaxy (Poretti 1999), where the distribution is not as
clear as it is here.
As a general consideration, the progression of the
parameter as a function of the period
appears in a clear way. However, a careful analysis should take more
details in consideration:
-
Attention should be paid to the scatter in the distribution of the
parameters around 0.050 d in Fig. 2;
in that region the mean error is 0.20
rad. Hence, this intriguing feature is on the borderline to be
considered as a real change in the progression. By analogy to Cepheid
light curves (Pardo & Poretti 1997), such a change can be the
signature of a resonance between the fundamental mode and a higher
overtone.
-
The small bunch of points above the progression at 0.038 d suggests
a different light curve family. Since this group of stars shows a very
small amplitude, it is possible that they are nonradial pulsators, not
necessarily radial pulsators in a higher overtone.
-
The very low value
(1.28 0.31 rad) emphasizes the
anomalous light curve of OGLEGC 26. The fact that such a light
curve is observed in a Pop. II object is quite surprising, since
V1719 Cyg and V798 Cyg (whose light curves are similar) are very
probably Pop. I stars having a quite normal metallic content.
However, their values are higher
(2.52 0.05 and
2.64 0.06 rad, respectively) and hence
the light curves are a little different. In many cases, it seems that
the phenomenon at the origin of the anomalous brightness increase
should be carefully evaluated when dealing with pulsating star
models.
It is of paramount importance to obtain very accurate light curves
to give more confidence to these results. However, it should be noted
that the and
values (Table 1) supply a good
confirmation of the reliability of the least-squares fits: indeed,
their mean values (1.20 rad and 5.18 rad, respectively) are in
excellent agreement with the expected ones on the basis of the results
on the galactic variables (see Fig. 2 in Antonello et al. 1986, upper
and middle panels).
© European Southern Observatory (ESO) 1999
Online publication: May 21, 1999
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