SCYON Abstract

Received on December 5 2000

The tricky galactic cluster NGC 2420

AuthorsP. G. Prada Moroni1, V. Castellani2, S. Degl'Innocenti2 and M. Marconi3
Affiliation1Dipartimento di Fisica, Università di Genova, and INFN, Sezione di Genova, Genova, Italy,
2 Dipartimento di Fisica, Università di Pisa, and INFN, Sezione di Pisa, Pisa, Italy ,
3Osservatorio Astronomico di Capodimonte, Napoli, Italy
LinksNGC 2420


We discuss the CM diagram of the galactic cluster NGC 2420 to the light of current theoretical predictions. By relying on the most recent updating of the physical input, one finds too luminous theoretical He burning stars together with the evidence for a misfitting of the lower portion of the MS. Moreover one finds two well known overshooting signatures, as given by i) the large extension of the ``hook'' preceding the overall contraction gap, and ii) the scarcity of stars just at the end of the gap. We show that the overluminosity of He burning stars appears as a constant prediction of models based on updated physics, whereas alternative assumptions about the Equation of State can account for the MS fitting. Moreover, due to the scarse statistical significance of the observational sample, one finds that overshooting signatures can be present also in canonical (without overshooting) predictions. We conclude that, unfortunately, NGC 2420 does not keep the promise to be of help in costraining the actual dimensions of convective cores in H burning MS stars, suggesting in the meantime that using clumping He burning stars as theoretical standard candle is at least a risky procedure. In this context the need for firmer constraints about the reddening of galactic clusters is shortly discussed.