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Astron. Astrophys. 346, 487-490 (1999)

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4. Discussion

The analysis of the 34 short-period variable stars in [FORMULA] Cen stressed the importance of studying the Fourier parameters. The sample of high-amplitude [FORMULA] 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 [FORMULA] ratios are usually around 0.1, the amplitude of the [FORMULA] term is very small and observational errors can mask the asymmetry of the light curve.

In spite of that, the [FORMULA] 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 [FORMULA] 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 [FORMULA] parameter as a function of the period appears in a clear way. However, a careful analysis should take more details in consideration:

  1. Attention should be paid to the scatter in the distribution of the [FORMULA] parameters around 0.050 d in Fig. 2; in that region the mean error is [FORMULA]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.

  2. 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.

  3. The very low [FORMULA] value (1.28[FORMULA]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 [FORMULA] values are higher (2.52[FORMULA]0.05 and 2.64[FORMULA]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 [FORMULA] and [FORMULA] 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).

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999
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