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Nearby Open Clusters and HR Diagram Calibration

Published online by Cambridge University Press:  14 August 2015

N. Robichon ,
F. Arenou ,
C. Turon ,
Y. Lebreton  and
J.-C. Mermilliod
N. Robichon
Affiliation:
DASGAL - CNRS URA 335 Observatoire de Paris, 5, place J. Janssen, F92195 Meudon cedex
F. Arenou
Affiliation:
DASGAL - CNRS URA 335 Observatoire de Paris, 5, place J. Janssen, F92195 Meudon cedex
C. Turon
Affiliation:
DASGAL - CNRS URA 335 Observatoire de Paris, 5, place J. Janssen, F92195 Meudon cedex
Y. Lebreton
Affiliation:
DASGAL - CNRS URA 335 Observatoire de Paris, 5, place J. Janssen, F92195 Meudon cedex
J.-C. Mermilliod
Affiliation:
Institut d’astronomie, Université de Lausanne CH1290 Chavannes-des-Bois

Extract

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13 clusters have been analysed (Coma Ber, the Pleiades, IC 2602, IC 2391, Praesepe, Alpha Per, NGC 6475, Blanco 1, Stock 2, NGC 6633, IC 4756, NGC 2516 and NGC 3532). Cluster members have been selected using Hipparcos proper motions and parallaxes. The mean cluster parallaxes have been determined using Hipparcos intermediate data (abscissae on the Reference Great Circles) taking into account star to star correlations. Both selection of members and computation of cluster mean parallaxes are described in Mermilliod et al., 1997 and Robichon et al., 1997.

For each cluster, a selection of fainter stars in the ‘Base des Amas’ (Mermilliod, 1988), based on photometric data as well as radialvelocities and proper motions when available, has been done. From this selection and the computed mean parallaxes, the location of the 13 cluster sequences in the observational HR diagram is derived very accurately, at least for the closest clusters for which good photometry and small errors on the mean parallaxes are available. Relative positions of cluster sequences cannot only be explained by differences between cluster metallicities. For example, the Pleiades sequence is 0.5 magnitude bellow the Praesepe sequence whereas the adopted metallicities (obtained spectroscopically i.e. whithout any assumption on the helium content) are respectively -0.034 and +0.10.

Type
II. Joint Discussions
Information
Highlights of Astronomy , Volume 11 , Issue 1: Highlights of Astronomy. As presented at the XXIIIrd General Assembly of the IAU (Part A), 1997 , 1998 , pp. 579
Copyright
Copyright © Kluwer 1998

References

Lebreton, Y. (1997), in preparationGoogle Scholar
Mermilliod, J.-C., Turon, C., Robichon, N., Arenou, F. and Lebreton, Y. (1997), Hipparcos Venice’97, ESA SP-402, p. 643Google Scholar
Mermilliod, J.-C. (1988), Bull. Inf. Centre Données Stcllaires, Vol. no. 35, pp. 77–91 Google Scholar
Robichon, N., Arenou, F., Turon, C., Mermilliod, J.-C. and Lebreton, Y. (1997), Hipparcos Venice’97, ESA SP-402, p. 567 Google Scholar