2.3.1. ``The Pleiades Problem''

The Pleiades cluster at 130 pc has been taken to be the first milestone of the distance work, since it has nearly solar abundance of heavy elements. This cluster is already too far to obtain a reliable parallax with the ground based observations, and its distance is estimated by tying it with nearby stars with solar metallicity employing main sequence fitting of FGK dwarfs (e.g., van Leeuwen 1983). The distance obtained this way agrees with an estimate via the Hyades, the nearest cluster to which geometric distance is available from the ground (Hanson 1980; van Altena et al. 1997), after a correction for large metallicity of the Hyades (VandenBerg & Bridges 1984). It was then a natural exercise to confirm these estimates with a parallax measured by the Hipparcos. The result showed that the Pleiades distance is shorter by 0.25 mag (12%) (van Leeuwen & Hansen-Ruiz 1997, Mermilliod et al. 1997)! This is summarised in TABLE 5.

Mermilliod et al.'s (1997) (see also de Zeeuw et al. 1997) have shown that such a disagreement is seen not only for the Pleiades but also for other open clusters to some degree. A noteworthy example is that the locus of the Praesepe ([Fe/H] = +0.095) agrees with that of the Coma Ber ([Fe/H] = -0.065) without metallicity corrections, while we anticipate the former to be 0.25 mag brighter due to higher metellicity.

This is a serious problem, since the disagreement means that either our understanding of FGK dwarfs, for which we have the best knowledge for stellar evolution, is incomplete, or the Hipparcos parallax contains systematic errors (Pinsonneault et al. 1998; Narayanan & Gould 1999). The origin is not understood yet.