Evolution of Massive Stars under New Mass-Loss Rates for RSG: Is the mystery of the missing blue gap solved ?

Abstract

In this paper we present new models of massive stars based on recent advancements in the theory of diffusive mixing and a new empirical formulation of the mass-loss rates of red supergiant stars. We compute two sets of stellar models of massive stars with initial chemical composition [Z=0.008, Y=0.25] and [Z=0.020, Y=0.28]. Mass loss by stellar wind is also taken into account according to the empirical relationship by de Jager et al. (1988). Despite the new mixing prescription, these models share the same problems of older models in literature as far as the interpretation of the observational distribution of stars across the HRD is concerned. Examining possible causes of the failure, we find that the adopted rate of mass loss for the red supergiant stages under-estimates the observational values by a large factor. Revising the whole problem, first we adopt the recent formulation by Feast (1991), and secondly we take also into account the possibility that the dust to gas ratio varies with the stellar luminosity. Stellar models are then calculated with the new prescription for the mass-loss rates. The models now possess very extended loops in the HRD and are able to match the distribution of stars across the HRD from the earliest to the latest spectral types both in the Milky Way, LMC and SMC. Finally, because the surface H-abundance is close to the limit adopted to start the Wolf-Rayet phase (WNL type), we suggest that a new channel is possible for the formation of low luminosity WNL stars, i.e. by progenitors whose mass can be as low as 20 \msol, that have evolved horizontally across the HRD following the blue-red-blue scheme and suffering large mass loss during the red supergiant stages.