SCYON Abstract

Received on March 7 2002

The blue to red supergiant ratio in young clusters at various metallicities

Authors	P. Eggenberger, G. Meynet, A. Maeder
Affiliation	Observatoire de Genève
Accepted by	Astronomy & Astrophysics
Contact	eggenbe7@etu.unige.ch
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Abstract

We present new determinations of the blue to red supergiant ratio (B/R) in young open clusters at various metallicities.
For this purpose, we examine the HR diagrams of 45 clusters in the Galaxy and of 4 clusters in the Magellanic Clouds. The identification of supergiants is based on spectroscopic measurements (with photometric counts to check the results). The new counts confirm the increase of the B/R ratio when the metallicity increases with the following normalized relation :
(B/R) / (B/R)_O ~ 0.05 x exp(3 Z / Z_O), where Z_O = 0.02 and (B/R)_O is the value of B/R at Z_O which depends on the definition of B and R and on the age interval considered (e.g. for spectroscopic counts including clusters with log age between 6.8 and 7.5, (B/R)_O ~ 3 when B includes O, B and A supergiants).