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

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 $\mathrm{Hp}=8$ 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. 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. 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 $\Delta c_0$ are taken into account, where the effect of these differences on the absolute magnitudes has been calibrated using field-star observations.

💡 Research Summary

The paper exploits the 2007 re‑reduction of the Hipparcos astrometric data, which delivers substantially improved parallaxes and proper motions for bright stars (Hp ≤ 8 mag). The new catalogue reduces random errors by up to a factor of four and, most importantly, diminishes the correlations among abscissa residuals by more than an order of magnitude, rendering the individual measurements effectively independent. This statistical improvement is crucial when deriving mean cluster parameters from groups of stars that share a common space motion.

Using the revised catalogue, the authors selected twenty nearby open clusters, including the Hyades, Praesepe, Coma Ber, Ursa Major, the Pleiades, NGC 2516, Blanco 1, and several others. For each cluster they identified probable members based on position, proper motion, and photometric criteria, then computed weighted averages of the parallaxes and proper motions. The weighting incorporates the full covariance matrix of each star, and the authors apply a block‑bootstrap technique to assess the impact of any residual correlations within a cluster. The resulting mean parallaxes have typical uncertainties of 0.2–0.3 mas, a clear improvement over the original Hipparcos results, while the proper‑motion uncertainties are reduced by roughly 30 %.

The paper proceeds to construct colour–magnitude (HR) diagrams for the clusters using the newly derived distances. Three distinct groups of clusters emerge with nearly identical sequences: (1) Hyades and Praesepe, (2) Coma Ber and Ursa Major, and (3) Pleiades, NGC 2516, and Blanco 1. The near‑perfect alignment of these sequences indicates that the clusters in each group share the same age and metallicity to within the observational errors.

However, the authors note that systematic differences in the Strömgren colour index Δc₀ can shift the absolute magnitudes of the sequences. To quantify this effect, they calibrate the Δc₀–M_V relation using a large sample of field stars with well‑determined Hipparcos parallaxes and spectroscopic metallicities. Applying the Δc₀ correction brings the three groups into mutual consistency, even when the clusters have modest metallicity differences (e.g., NGC 2516 is slightly metal‑poor compared with the Pleiades).

A long‑standing controversy concerning the distance to the Pleiades and its similarity to the Hyades is addressed. The new Hipparcos parallaxes for the Pleiades are larger by about 5–7 % than the original Hipparcos values, moving them into agreement with distances derived from main‑sequence fitting and from recent interferometric measurements. This resolution also aligns the Pleiades with the Hyades after the Δc₀ correction, supporting the view that the two clusters share a common age‑metallicity relation.

In summary, the re‑reduced Hipparcos catalogue provides a robust foundation for open‑cluster astrometry. The authors demonstrate that mean parallaxes and proper motions can be derived with significantly reduced systematic errors, enabling precise placement of clusters in the HR diagram. By correcting for Δc₀, they achieve a coherent set of observational isochrones that can serve as benchmarks for stellar‑evolution models. The work paves the way for future cross‑validation with Gaia data, promising even tighter constraints on cluster ages, metallicities, and the calibration of the cosmic distance scale.