The evolutionary paths taken by very massive stars , M \gtrsim 60 \mathrm { M } _ { \odot } , remain substantially uncertain : they begin their lives as main sequence O stars , but , depending on their masses , rotation rates , and metallicities , can then pass through a wide range of evolutionary states , yielding an equally broad set of possible surface compositions and spectral classifications .
Here we present a new grid of calculations for the evolution of such stars , covering a broad range in mass ( M/M _ { \odot } = 60 - 150 ) , rotation rate ( v/v _ { crit } = 0 - 0.6 ) , metallicity ( [ \mathrm { Fe } / \mathrm { H } ] = -4 - 0 ) , and \alpha -element enhancement ( [ \alpha / \mathrm { Fe } ] = 0 - 0.4 ) .
We show that rotating stars undergo rotationally-induced dredge-up of nucleosynthetic products , mostly He and N , to their surfaces while still on the main sequence .
Non-rotating metal-rich stars also reveal the products of nucleosynthesis on their surfaces because even modest amounts of mass loss expose their ‘ ‘ fossil '' convective cores : regions that are no longer convective , but which were part of the convective core at an early stage in the star ’ s evolution .
Thus surface enhancement of He and N is expected for rotating stars at all metallicities , and for non-rotating stars if they are relatively metal rich .
We calculate a stellar atmosphere for a representative model from our grid , properly accounting for He- and N-enhancement , and show that the resulting spectrum provides a good match to observed WNL stars , strongly suggesting that the physical mechanisms we have identified are the ultimate cause of the WNL phase of massive stellar evolution .
We conclude that this phase has important implications for the ionising spectra of early galaxies .