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Astron. Astrophys. 340, 149-159 (1998)

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2. The target stars

In this paper, we report the analysis of the data obtained during the STEPHI VI campaign on another pair of stars of the Praesepe cluster (NGC 2632, M44). This cluster offers one of the best situations where to observe [FORMULA] Scuti stars: their global parameters are rather well determined and, besides, there are 14 stars of this type in the cluster, which is one of the largest sample of its kind. It is considered as a Population I cluster with a metal content near the solar value ([FORMULA], Cayrel de Strobel et al. 1992). Recent distance estimations by HIPPARCOS satellite give a value of [FORMULA] pc (Mermilliod et al. 1997).

The two [FORMULA] Scuti stars selected to be observed during January and February of 1995 were BQ Cnc (HD 73729, KW 292) and BW Cnc (HD 73798, KW 340). As a comparison star we chose HD 73574 (KW 203), an A5V star that belongs to the same cluster. Target and comparison stars were chosen properly to be located within the limits of our field of view ([FORMULA]). Table 1 shows the main observational parameters of these stars as taken from the SIMBAD database operated by CDS (Centre de Données Stellaires) in Strasbourg. KW nomenclature comes from the proper motion study of Praesepe members made by Klein-Wassink (1927). Narrow band photometry values from Crawford and Barnes (1969).


Table 1. Observational properties of the target stars in STEPHI VI

BQ Cnc is considered as a double-lined spectroscopic binary (SB2), (Abt & Biggs 1972). From the catalogue of individual radial velocities (Abt 1970), we have an averaged total amplitude of the velocity of 145 km s-1, while Bolte (1991), from the same measurements, reports a  K value of 125 km s-1. However, in later observations, Mason et al. (1993) studied our target stars by ICCD speckle techniques trying to detect them as binary systems, with negative results within the given resolution. By observing a binary system within a cluster, we enhance the power of the astero-seismological tools since the stellar information we can obtain is potentially larger. Detailed observations of the radial velocity curves due to the orbital motion could provide additional information on this system.

Randich & Schmitt (1995) by means of ROSAT observations did found that BQ Cnc (KW 292) was indeed an X-Ray source with a luminosity [FORMULA] = [FORMULA] erg s-1 and [FORMULA] = [FORMULA]. The fact that this star is an X-Ray source, gives another important fact to consider for further theoretical work.

Tsvetkov's (1993) work on the Praesepe cluster shows that for the two stars chosen in our program, the computed age by different authors is systematically larger than the age of the other components of the cluster. He points out that the presence of a companion can apparently "increase" the estimated age of the star. There may be other effects like rotation of the star, that could be considered into the theoretical models. Recently, Pamyatnykh et al. (1998) included rotation effects in stellar evolution and linear nonadiabatic oscillation for the seismic models of a [FORMULA] Scuti star XX Pyxidis. They give references to earlier work. Still much work has to be done from the theoretical point of view.

The comparison star KW 203 was chosen because it was the brightest object in the field around the target stars. After Mason et al. (1993), this star is considered as binary by speckle photometry and also by ordinary photometry (Couteau 1979). However, it is not catalogued as a spectroscopic binary. Anyway, for the stars studied in the survey by Couteau (1979) during several years, minimum orbital periods of tens of years were estimated for those stars. Narrow band photometry of KW 203 shows that this star follows quite well the "standard relations" valid for other clusters (Crawford and Barnes 1969). Light curves of KW 203 during the observations have also shown its constancy (see Fig. 1). In this way, the potential effect of the binarity in KW 203 as a comparison star can be neglected.

[FIGURE] Fig. 1. Examples of light curves of the target and comparison stars in three non-consecutive nights observed from each site. The curves have been shifted in time and magnitude in sake of clarity. Two points can be deduced from the graph: a) the constancy of the comparison star, specially clear in the night from SPM; b) the power of the differential photometry since, even in cases of large transparency fluctuations as the ones showed in the night from XL, the quality of the resulting differential light curves is really high.

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© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998