THE PROBLEM OF THE BLUE-TO-RED SUPERGIANT RATIO IN GALAXIES

By summarizing the available observations, we investigate and confirm the trend that the number ratio of blue-to-red supergiants appears to be an increasing function of the metallicity. Furthermore, we outline that virtually no assumption on stellar model physics explored so far is able to cope with this trend. We present new stellar models in the mass range 15 - 40 M., and find that the Ledoux criterion for convecti on plus semiconvection yields satisfying results at low Z (Z = 0.002), but readily fails at high Z (Z = 0.02). We discuss the dependence of the theoretical B/R-ratio, especially as function of the treatment of internal convective mixing, and point out the leakage of appropriate c onvection models. We conclude that a mixing efficiency in between the Schwarzschild and the Ledoux criterion might be most appropriate to ex plain the B/R-trend with Z, but argue that rotational mixing may have some contribution as well.