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

Abstract

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.

Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields

“…The Ba excess we occasionally observe may instead be attributable to a young age, A119, page 16 of 20 as found in field dwarfs of the solar neighbourhood or members of open clusters (e.g., Bensby et al 2007;D'Orazi et al 2009;da Silva et al 2012). In this respect, it may be regarded as significant that high abundances are found in the young, massive star HD 50890 and in the likely members of NGC 6633 with an estimated age in the range 450-575 Myrs according to Smiljanic et al (2009) andvan Leeuwen (2009). It is interesting to note that HD 170031 has a Ba abundance lower by about a factor 2 than these three stars, which strengthens the case for a non-membership (Sect.…”

“…Spectral types and magnitudes in the V band from SIMBAD database. Hipparcos parallaxes from van Leeuwen (2007). The instrumental broadening is the full width at half-maximum of lines measured in calibration lamps at ∼6700 Å.…”

Ultracool dwarfs in open clusters and star-forming regions

“…• Per is 80-90 Myr-old [27] and located at a distance of 170 pc from the Sun [28]. The cluster is located at low galactic latitude (b ∼ 7 • ) but shows a significant proper motion, of the order of 35 mas/yr.…”

“…• Praesepe is an old (590 Myr; [30]) nearby (182 pc; [28]) cluster with a large mean proper motion of ∼38 mas/yr. As for Per and the Pleiades, we exploited the GCS DR9 and identified over one thousand photometric and astrometric member candidates in 36 square degrees [13].…”

The stellar and substellar mass function in central region of the old open cluster Praesepe from deep LBT observations

“…It is located at a distance of 190 +6.0 −5.8 pc [12] and has an age of 590 +150 −120 Myr [7]. The determinations of the metallicity of Praesepe yield some discrepancies: from [Fe/H] = 0.038 ± 0.039 [8], and +0.13 ± 0.10, [3] In this proceeding, we present the results of a program to study, in detail, the MF of the open cluster Praesepe down to the even lower mass regimes (0.04 M ) based on the LBT blue and red cameras.…”