Astron. Astrophys. 320, 799-810 (1997)

5. Conclusions

Our work shows that the zero point of the surface brightness - colour relations valid for cool giants and supergiants can now be obtained with high accuracy from the many precise angular diameters now available for such stars from Michelson interferometry. Our results indicate that giants and supergiants follow the same relations. The slopes of the various surface brightness - colour diagrams as determined from Cepheid variables are indistinguishable from the slopes defined by the stable giants and supergiants, and do not depend in a significant way on the pulsation period or luminosity of the Cepheids, in agreement with the fact that the slopes are also identical for giants and supergiants. The agreement of our surface brightness - colour zero point with the one derived from the measured angular diameter of the Cepheid Gem further supports the evidence that Cepheids and stable giants and supergiants do follow identical surface brightness - colour relations.

The results of the optical and near-infrared Barnes-Evans solutions on the Cepheid U Sgr suggest that there is no significant systematic difference between the distance and radius results as obtained from the three versions of the technique we employ, but there is a huge gain in accuracy going to the infrared. Whether there is a systematic difference between the optical and near-infrared solutions will have to be further investigated, applying the different techniques to many more variables. However, even if it turns out that there is no systematic offset between optical and near-infrared Barnes-Evans distances and radii of Cepheid variables, it is clear that the infrared is the region to work because of the very large increase in the accuracy of the results.

We believe that the application of our near-infrared calibrations of the Barnes-Evans technique has the potential to settle the zero point of the Cepheid period-luminosity relation to about 0.02 mag. The most precise distances may be expected to the Cepheid-rich Magellanic Cloud clusters which are clearly within the reach of the technique, and where the individual distances to cluster Cepheids can be averaged to yield very accurate distances of these clusters. From this kind of work, it should be possible in the near future to eliminate the weakest link in the application of Cepheids as our most trusted extragalactic distance indicator, which is the relatively uncertain local calibration of Cepheid distances.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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