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Astron. Astrophys. 333, 524-530 (1998)

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4. A successful search

We have observed two clusters, NGC 1817 and NGC 2215, at the IAC80 at Observatorio del Teide, Tenerife, 1996, using a Thomson CCD. The field of view is appr. [FORMULA]. The CM diagrams can be found in The Book. After detecting only a few pulsating stars in most other cases (including NGC 2215), NGC 1817 turned out to be of special interest. This is the first northern open cluster suitable for relative CCD photometry, in the sense that it contains a nice set of [FORMULA]  Scuti stars within a small field of view. Time series were obtained for this cluster on two nights, 5 hours each night. The guiding was good. Only a part of the cluster could be covered by the FOV of the CCD. A region was chosen avoiding the brightest stars (Fig. 5). Light curves were obtained for 176 stars in the field. Among the light curves several showed periodicities in the range typical for [FORMULA]  Scuti stars. These stars are labeled with their sequence number in the finding chart.

[FIGURE] Fig. 5. One CCD frame for the open cluster NGC 1817. The variable stars are labeled with a running number.

In the CM diagram (Fig. 6) the position of the variables has also been plotted. Most of them are located at the estimated position of the instability strip. The CM diagram is typical for a middle age cluster, [FORMULA], and the distance is given as 1.8 Kpc (Astronomical Almanac 1996).

[FIGURE] Fig. 6. Colour-Magnitude diagram for NGC 1817 (Mermilliod, 1994). Variables are plotted as diamonds. The position of the instability is plotted as well.

The light curves have been analysed and dominant frequencies determined for the variables. Table 1 summarises the information for all stars. The classification is preliminary, is based on the shape of the lightcurve only, and is not necessarily consistent with the location in the CM diagram.


Table 1. Detected frequencies in the proposed variables in the field of NGC 1817. The phase is relative to Feb. 13, 1996 at 0h UT. The classification comes from the light curve only.

To document the detection of the variables, we present three figures with lightcurves for the two nights observed. In the case of the star 163 we are looking at the light curve of a [FORMULA]  Scuti star with more than one period (Fig. 7). Subsequent Fourier analysis (Fig. 10) confirmed that there are two periods (see Table 1). Two additional examples are shown in Fig. 8 and Fig. 9, where the variation is less conspicuous, but still unambiguous. Again the periods are typical for [FORMULA]  Scuti stars, and the position in the CM diagram is consistent with this interpretation. One variable is definitely not a [FORMULA]  Scuti star, as the colour is much too red. It appears not to be member of the cluster.

[FIGURE] Fig. 7. Light curve for a [FORMULA]  Scuti star (# 163). This is a case easily recognized as a [FORMULA]  Scuti. It is evident that more than one mode is present.

[FIGURE] Fig. 8. Light curve for # 114.

[FIGURE] Fig. 9. Light curve for # 99.

[FIGURE] Fig. 10. Amplitude spectrum of the light curve of # 163. This is the most prominent variable.

More observations are needed, before a multisite campaign can be initiated. The procedures to be followed in the future are described in the next section, where we discuss the revised strategy for STACC after the observations done the last few years.

For the cluster NGC 2215 the search was less successful. Only 5 hours on one night were obtained, and no variable stars were identified. This could be a result of the small size of the data set, and the lack of [FORMULA]  Scuti stars with large (above a few mmag) amplitudes.

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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998