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Astron. Astrophys. 341, L71-L74 (1999)

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1. Introduction

With reliable trigonometric distances available, open clusters become a very powerful tool for testing theories of stellar structure and evolution: the homogeneity in age and chemical composition of the member stars provide the empirical isochrones which are to be emulated by theoretical stellar evolution models in both the position of, and the population density along, the track. These models provide the only tool for estimating cluster ages, and their reliability is therefore of crucial importance in astrophysics. Since the publication of the Hipparcos data the issue of open cluster parallax determinations has been confusing rather than aiding astronomy: while the Hyades (Perryman et al., 1998) and Praesepe clusters were found close to the expected parallax values, the Pleiades parallax put it considerably (10 to 15 percent) closer to the Sun than expected (van Leeuwen and Hansen-Ruiz, 1997, Mermilliod et al., 1997 (Merm97)). Differences found were more than could be accommodated in currently available stellar evolution models or through abundance variations. This led to speculations that there could be local errors at a level of 1 mas (milli-arcsecond) in the Hipparcos parallaxes (Pinsonneault et al., 1998), though Merm97 had reported earlier that other young open clusters show the same sub-luminous characteristics as the Pleiades. It has been known for a long time (Lindegren, 1992) that some correlations (up to a level of 0.10 to 0.15, van Leeuwen, 1999) exist in the Hipparcos parallaxes for stars with separations of less than [FORMULA], but these can only explain small scale error correlations at a level of 0.1 mas.

The current study was set up to:

  • verify the methods used to determine cluster distances from the Hipparcos data,

  • treat systematically all clusters within an estimated parallax of at least 4 mas by exactly the same determination methods,

  • examine the composite HR diagram that results from these determinations,

  • examine the likelihood of local "discrepancies" in the Hipparcos parallaxes as a result of correlations between astrometric parameters (described in van Leeuwen, 1999).

Sect. 2 summarizes briefly the combined abscissae method used for deriving cluster parallaxes, and is followed by a summary of the astrometric parameter determinations for the 9 selected clusters. In Sect. 4 the composite HR diagram for the clusters is presented, followed by a brief discussion of the results.

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© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998