Very Massive Stars - Evolution with Mass Loss - Part One - the Hydrogen and Helium Burning Phase

The evolution of mass-losing very massive stars in the 500 10000M _⊙ range has been investigated for two different initial compositions, (X, Z)=(0.8,0.0) and (X, Z)=(1.0,0.0). The evolutionary tracks are governed by two opposing factors which are the increase in the mean molecular weight in the convective core and the effect of mass loss. Conservative evolution of stars with massM≪10000M _⊙ is similar to that of massive stars (20 100M _⊙), always moving to lower effective temperatures. For low values of the standard mass loss parameterN (50≲N≲200) the two opposing factors are almost in balance and the star is forced to move in a series of loops. For higher mass loss rates the loops disappear. In the 10000M _⊙ case no loops are observed and the tracks always move to higher effective temperatures. For a given mass loss rate the transition between right and left moving tracks occurs at higher masses the lower is the mass loss rate.