Stellar evolution near the main sequence: on some systematic differences between cluster sequences and model calculations.
Summary. Detailed comparisons in the colour-magnitude diagram are made between computed isochrones and observed sequences of some well known old open clusters, namely NGC 188, M 67, NGC 3680, NGC 752, NGC 2360 and Praesepe. As basic data, we have considered different evolutionary models, calibrations of Teff confirmed by recent models and the numerous results on the abundances of the clusters; the effects of the spread due to binaries are also examined in an appendix. The comparisons show that, despite the well known overall agreement between models and observations, there exist some clear and systematic differences concerning mainly the shape of the sequences in the rapid phase of core hydrogen exhaustion (or phase of overall contraction). The hook predicted by the evolutionary models is absent; this leads to a gap occurring vertically in the colour-magnitude diagram with a top lying much farther from the zero-age sequence than predicted. As these anomalies concern differential measurements, they are essentially free from the uncertainties of stellar atmospheric factors and interstellar reddening; moreover, they are unexplainable in terms of helium and metal contents. Concerning the origin of this situation, several arguments are given which tend to support that the observed differences result from effects related to the convective mixing of stellar cores. Key words: stellar evolution - open clusters
Astronomy and Astrophysics
- Pub Date:
- May 1974