The Distance of the Pleiades and Nearby Clusters
Abstract
We have determined the distance to nearby open clusters from the means of Hipparcos parallaxes recomputed from the intermediate data. The first result worth noticing is that Praesepe, Coma Ber, Alpha Persei and Blanco 1 define the same main sequence in spite of their differences in metallicity. The second and more surprising result is that the Pleiades, and probably IC 2391 and 2602, define a sequence about 0.5 mag fainter than the previous one. The mean parallax of the Pleiades is 8.60 mas, corresponding to 116 pc. There is no way to reconcile the trigonometric and photometric distances of the Pleiades. These results contradict the commonly accepted interpretation of the metallicity effects. The results are the same for the UBV, uvby and Geneva photometric systems. The parameter responsible for these differences has not yet been identified, although they could be accounted for by a significant difference in the helium abundance. However there is presently no indication of a much higher helium abundance in nearby young clusters or associations. The present result could be linked to the so-called Hyades anomaly.
- Publication:
-
Hipparcos - Venice 1997
- Pub Date:
- August 1997
- Bibcode:
- 1997ESASP.402..643M
- Keywords:
-
- OPEN CLUSTERS;
- HIPPARCOS;
- STARS: DISTANCE;
- STARS: CHEMICAL COMPOSITION;
- Mermilliod: Institut d'Astronomie;
- Université de Lausanne;
- Switzerland Others: DASGAL/URA CNRS 335;
- Observatoire de Paris;
- 92195 Meudon CEDEX