The problem of the Pleiades distance - Constraints from Stromgren photometry of nearby field stars

The discrepancy between the Pleiades cluster distance based on Hipparcos pa rallaxes and main sequence fitting is investigated on the basis of Stromgre n photometry of F-type stars. Field stars with the same metallicity as the Pleiades have been selected from the m(1) index and a technique has been de veloped to locate the ZAMS of these field stars in color-magnitude diagrams based on the color/temperature indices b - y, v - y, and beta. Fitting the Pleiades to these ZAMS relations results in a distance modulus of 5.61 +/- 0.03 mag in contrast to the Hipparcos modulus of 5.36 +/-0.06 mag. Hence, w e cannot confirm the recent claim by Grenon (1999) that the distance proble m is solved by adopting a low metallicity of the Pleiades ([Fe/H](Pleiades) = 0.11) as determined from Geneva photometry. The metallicity sensitivity of the ZAMS determined by the field stars is investigated, and by combining this sensitivity in all three color/temperature indices b - y, v - y, and beta we get a independent test of the Pleiades distance modulus which suppo rt our value of 5.61 mag. Furthermore, the field star sample used for the c omparison is tested against theoretical isochrones of different ages to sho w that evolutionary effects in the field star sample are not biasing our di stance modulus estimate significantly. Possible explanations of the Pleiade s distance problem are discussed and it is suggested that the discrepancy i n the derived moduli may be linked to a non-spherical shape of the cluster.