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