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Issue A&A
Volume 386, Number 2, May I 2002
Page(s) 576 - 582
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20020262



A&A 386, 576-582 (2002)
DOI: 10.1051/0004-6361:20020262

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

P. Eggenberger, G. Meynet and A. Maeder

Geneva Observatory CH-1290 Sauverny, Switzerland
    e-mail: Georges.Meynet@obs.unige.ch, Andre.Maeder@obs.unige.ch

(Received 6 December 2001 / Accepted 18 February 2002)

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: $\frac{B/R}{(B/R)_{\odot}} \cong 0.05\cdot {\rm e}^{3 \frac{Z}{Z_{\odot}}}$, where $Z_{\odot}=0.02$ and $(B/R)_{\odot}$ is the value of B/R at $Z_{\odot}$ 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, $(B/R)_{\odot}\cong3$ when B includes O, B and A supergiants). tars: evolution - stars: supergiants - galaxies: Magellanic Clouds


Offprint request: P. Eggenberger, eggenbe7@etu.unige.ch

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