12 Conclusions and prospects

The new Strömgren photometry of the two open clusters NGC 6134 and NGC 3680 have provided the following information:

1.

Large scale flat field errors were removed by taking shifted and rotated frames of NGC 6134. This improves the individual parameters for bright stars, e.g. the known $\delta$ Scuti stars in the cluster.

2.

We have made an improved calibration of the lower end of the main sequence for the well studied cluster NGC 3680.

3.

We have obtained reliable estimates of the interstellar reddening of the clusters. For NGC 6134 this has been used to determine which stars are clearly not members of the cluster.

4.

The approximate metallicity of the clusters have been found.

5.

In NGC 6134 several Am type stars are found.

6.

By using theoretical isochrones we show that models including overshooting are needed to fit the extended hook of the observed colour-magnitude diagram of NGC 6134.

Our results for the interstellar reddening, metallicity, age, and distance of each cluster agree reasonably well with recent published results. Still, reliable values for the metallicities of the clusters are missing. Attempts have been made by using DDO photometry of giant stars and Strömgren photometry of F type stars, which yield quite different results. The metallicity must be obtained by using spectroscopy which would also be valuable for determining the global properties of these clusters, i.e. age and distance.

The results presented here are perhaps most interesting for NGC 6134, as six $\delta$ Scuti stars have been found in this cluster by Frandsen et al. (1996). In principle we can extract new information contained in the observed pulsation frequencies and amplitudes by constructing theoretical models of the variable stars. The information needed is:

• Pulsation frequencies and amplitudes (multi site time-series photometry).
• The fundamental parameters of the cluster (Strömgren photometry and spectroscopy).
• Rotational velocities (spectroscopy).

By now we have two of the three needed pieces in the puzzle! Still, the metallicity is unreliable, and a better determination is needed, e.g. by using spectroscopy. We have been allocated observing time for doing spectroscopy of the brighter stars in the two clusters. These observations hopefully provide a complete set of parameters for the $\delta$ Scuti stars in NGC 6134 making this cluster one of the key clusters for tests of stellar evolution.

The photometry is available at the CDS database in Strasbourg and through anonymous ftp at aauobs.ifa.au.dk in the /pub/bruntt directory.

Acknowledgements

This research was supported by the Danish Natural Science Research Council through its Centre for Ground-based Observational Astronomy and by the Danish National Foundation through its establishment of the Theoretical Astrophysics Center. We thank Tim Bedding for reading our manuscript. We are grateful to Erik Heyn Olsen who provided us with an updated list of the observed standard stars.