On the ratio of mixing length to scale height in red dwarfs

Previous completely convective theoretical models of low-mass stars, M< or approx. =0.3 M/sub sun/, predict a luminosity-mass relation which is below the observed one. We explain this disagreement by using the latest molecular opacities and by the consequent need to assume an l/H/sub p/ ratio at T<9000 K in the range 0.07--0.17, rather than the more conventional values of 1.0--2.0. When such a low surface layer l/H/sub p/ ratio is assumed, we find significantly higher central temperatures (and hence luminosities) and quite large radiative cores, regardless of the deeper l/H/sub p/ value and equation-of-state uncertainties. The low l/H/sub p/ ratio is explained by the interaction of a magnetic field with convection. Several consequences of this result, including the interstellar abundance of /sup 3/He/sup +/ are discussed.