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9. Cluster membership of the Cepheid variables

The positions of the Cepheid variables V Cen and S Norm are marked in the tex2html_wrap_inline3380, tex2html_wrap_inline3382 and tex2html_wrap_inline3384, tex2html_wrap_inline3386 diagrams of NGC 5662 (Fig. 15 (click here)) and NGC 6087 (Fig. 16 (click here)) respectively. For these, we used the UBV data listed by Schaltenbrand & Tammann (1971). The extremes of variability are also indicated.

Studies by Turner (1982) and more recently by Clariá\ et al.  (1991) indicate that the Cepheid variable V Cen located in the coronal region of NGC 5662 is a cluster member. Similarly, studies by Feast (1957) and more recently by Turner (1986) and Mermilliod et al.  (1987) confirm that the Cepheid variable S Norm, located near the optical centre of NGC 6087, is a cluster member. As the present determination of distances to these clusters is accurate (since it is based on the CCD photometry of the unevolved part of the cluster MS), it may be worth while to discuss cluster membership of these variables in the light of our new data. Our distance determinations for NGC 5662 and NGC 6087 provide the values of tex2html_wrap_inline3390 and -4.1 for V Cen and S Norm respectively. These luminosities agree very well with the corresponding values of -3.4 and -4.1 predicted from the period-luminosity relation of classical Cepheids given by Madore & Freedman (1991). The location of these Cepheids in the CM diagrams fits very well with the stellar evolutionary isochrone appropriate for the average age of their parent star clusters (see Figs. 15 (click here) and 16 (click here)). The values of E(B-V) derived by Dean et al.  (1978) are 0.29 and 0.18 mag for V Cen and S Norm respectively. These values agree fairly well with the corresponding values of E(B-V) derived by us for NGC 5662 and NGC 6087 (see Sect. 6). All these various pieces of evidence strongly suggest that the Cepheid variables V Cen and S Norm are indeed member of the open star clusters NGC 5662 and NGC 6087 respectively.


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