A&A 374, 105-115 (2001)
DOI: 10.1051/0004-6361:20010665

The problem of the Pleiades distance

Constraints from Strömgren photometry of nearby field stars

D. Stello and P. E. Nissen

Institute of Physics and Astronomy (IFA), University of Aarhus, 8000 Aarhus, Denmark
    e-mail: pen@ifa.au.dk

(Received 3 April 2001 / Accepted 7 May 2001 )

Abstract
The discrepancy between the Pleiades cluster distance based on Hipparcos parallaxes and main sequence fitting is investigated on the basis of Strömgren photometry of F-type stars. Field stars with the same metallicity as the Pleiades have been selected from the m₁ index and a technique has been developed to locate the ZAMS of these field stars in color-magnitude diagrams based on the color/temperature indices b-y, v-y, and . Fitting the Pleiades to these ZAMS relations results in a distance modulus of mag in contrast to the Hipparcos modulus of mag. Hence, we cannot confirm the recent claim by Grenon (1999) that the distance problem 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 we get a independent test of the Pleiades distance modulus which support our value of 5.61 mag. Furthermore, the field star sample used for the comparison is tested against theoretical isochrones of different ages to show that evolutionary effects in the field star sample are not biasing our distance modulus estimate significantly. Possible explanations of the Pleiades distance problem are discussed and it is suggested that the discrepancy in the derived moduli may be linked to a non-spherical shape of the cluster.

Key words: open clusters and associations: individual: Pleiades -- stars: Hertzsprung-Russell (HR) and C -- M diagrams -- stars: distances -- stars: evolution -- stars: abundances

Offprint request: D. Stello, stello@ifa.au.dk

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