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

We present new determinations of the blue to red supergiant ratio () 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 ratio when the metallicity increases with the following normalized relation: , where and is the value of at which depends on the definition of B and R and on the age interval considered (e.g. for spectroscopic counts including clusters with between 6.8 and 7.5, when B includes O, B and A supergiants). tars: evolution – stars: supergiants – galaxies: Magellanic Clouds