Astron. Astrophys. 324, 91-96 (1997)

4. Conclusion

Our new UBV photometry and systematic radial-velocity survey of 24 giant stars in the field of Mel 71 has allowed us to determine the membership of red giants in Mel 71 and to compute the mean cluster velocity (+50.14 0.14 km s^-1). Eight spectroscopic binaries have been discovered and the binary frequency is unusually high: 50% (8/16). Four orbits have been determined.

Although the overall agreement of the theoretical isochrone fit on the observed colour-magnitude diagram of Mel 71 seems satisfactory given the accuracy of the photometric data, a detailed comparison reveals that the position of the clump is not where it is expected to be, i.e. where the models predict the maximum star density. This conclusion does not depend on the exact position and overall fit of the isochrone, also in the main-sequence region, because the comparison is made differentially between the giants on the ascending and asymptotic giant branches.

With the present models, it is difficult to obtain a better fit. It would be important to obtain well-calibrated and accurate CCD observations of several clusters of similar ages in order to compare observations and theory on the basis of several well-populated colour-magnitude diagrams. The membership information and binary detection resulting from this long-term program will be crucial input to the analysis of the observed red giant locus.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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