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Leeuwen, F. van; Fantino, Elena

doi:10.1023/a:1026228018073

Cited by 15 publications

(21 citation statements)

References 8 publications

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“…The first is that this type of activity as observed in the Pleiades is restricted to stars of spectral type later than about K2 to K3 (van Leeuwen & Alphenaar 1982;van Leeuwen et al 1987). Observing similar activity among the more massive solar-type stars indicates an age for those stars considerably lower than the Pleiades age.…”

Section: The Pleiadesmentioning

confidence: 96%

Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue

Leeuwen¹

2009

A&A

276

114

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Context. A new reduction of the astrometric data as produced by the Hipparcos mission has been published, claiming that the accuracies for nearly all stars brighter than magnitude Hp = 8 are improved, by up to a factor 4, compared to the original catalogue. As correlations between the underlying abscissa residuals have also been reduced by more than an order of magnitude to an insignificant level, our ability to determine reliable parallaxes and proper motions for open clusters should be improved. Aims. The new Hipparcos astrometric catalogue is used to derive mean parallax and proper motion estimates for 20 open clusters. The HR-diagrams of the nearest clusters are compared and combined to provide future input to sets of observational isochrones. Methods. Three different methods are applied, according to the proximity of the cluster, to compensate, where needed, for projection effects, spread in position along the line of sight, and the internal velocity dispersion of the cluster stars. Results. The new parallaxes have accuracies between 2 and 2.5 times higher than what had been derived from the original Hipparcos catalogue. At least two to three groups of clusters, mostly of similar ages, are observed to each occupy their own specific space in the HR diagram. A significant discrepancy in distance moduli from those obtained with isochrone-based main-sequence fitting remains, in particular for one of these groups, containing the Pleiades, NGC 2516, and Blanco 1. The difference in absolute magnitudes between this group and one containing the Hyades and Praesepe clusters appears to be correlated with systematic differences in the Strömgren Δc 0 index between those groups. The same dependency has been known to exist for a long time, and is again confirmed by the Hipparcos data, in variations in absolute magnitudes for field stars of the same effective temperature. Conclusions. The positions of the cluster HR diagrams are consistent within different groups of clusters shown for example by the near-perfect alignment of the sequences for the Hyades and Praesepe, for Coma Ber and UMa, and for the Pleiades, NGC 2516, and Blanco 1. The groups are mutually consistent when systematic differences in Δc 0 are taken into account, where the effect of these differences on the absolute magnitudes has been calibrated using field-star observations.

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Section: The Pleiadesmentioning

confidence: 96%

Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue

Leeuwen¹

2009

A&A

276

114

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“…In many ancient civilizations it played a crucial role in defining the seasons, and in today's astrophysics it has led the way to a number of crucial discoveries: the colour-magnitude relation by Hertzsprung (1915), for example, was based on his study of the Pleiades and Hyades stars; the discovery in 1980 of very fast rotation of G and K dwarfs shortly before becoming proper main sequence stars was first made in the Pleiades cluster (van Leeuwen & Alphenaar 1982;Stauffer et al 1984;van Leeuwen et al 1987). And now there is the Hipparcos parallax determination for the cluster, which seems to be at odds with the expected value by quite a margin (van Leeuwen 1999a; Robichon et al 1999).…”

Section: Introductionmentioning

confidence: 99%

“…It just so happened to fit nicely on a standard photographic plate size for an astrographic refractor, which made it a very suitable early candidate for proper motion studies (see, for example, Hertzsprung 1947;Vasilevskis et al 1979). On the other hand, it is not too dense to generally cause problems in observations (unlike the Trapezium Cluster in Orion or most globular clusters), though it is situated in an area of dust clouds, showing as reflection nebulae and very significant variations in reddening for individual stars (van Leeuwen et al 1986).…”

Section: Introductionmentioning

confidence: 99%

“…In as far as the Hipparcos data are concerned the only way of tackling it is by going back to the original data and looking at all the individual steps and assumptions made along the way of the data reductions. That is a very big job, which has already taken several years (van Leeuwen & Fantino 2003). For all determinations that implement models, it is worthwhile to re-consider the accuracies and intrinsic (cosmic) noise levels associated with the calibrations on which they are calibrated.…”

Section: Introductionmentioning

confidence: 99%

“…The uncertainty in the absorption needs to be fully taken into account. For example, the Pleiades are affected by differential reddening, and the most seriously affected star (HII 1084, situated near the Merope nebula) shows quite clearly deviating reddening directions in the two-colour diagrams of the Walraven photometry (van Leeuwen et al 1986), indicating local, non-average dust properties.…”

Section: Introductionmentioning

confidence: 99%

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The Pleiades question, the definition of the zero-age main sequence, and implications

Leeuwen

2004

Proc. IAU

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Abstract. Determinations and measurements of the parallax of the Pleiades, obtained with ground-based studies and with Hipparcos data, are reviewed. A number of uncertainties in both sets of data are found. Although a further correction for abscissa correlations brings the Hipparcos determination closer to the ground-based value, the difference still seems too large. The new Hipparcos parallax determinations seem to reinforce a possible age related effect. A new reduction of the Hipparcos data is in progress. It reduces significantly the contribution of the attitude noise, providing higher accuracies and lower correlation levels for the brighter stars.

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The Solar Neighbourhood

Grijs

2011

An Introduction to Distance Measurement in Astronomy

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Cited by 15 publications

References 8 publications

Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue

Parallaxes and proper motions for 20 open clusters as based on the new Hipparcos catalogue

The Pleiades question, the definition of the zero-age main sequence, and implications

The Solar Neighbourhood

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