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The evolutionary paths taken by massive stars with $M \gtrsim 60 \,\mathrm{M}_\odot$ remain substantially uncertain. They begin their lives asmain sequence (MS) O-stars. Depending on their masses, rotation rates, andmetallicities, they can then encounter a wide range of evolutionary states withan equally broad set of possible surface compositions and spectralclassifications. We present a new grid of calculations for the evolution ofsuch stars that covers a broad range in mass, M/M$_\odot = 60$ to $150$,rotation rate, $v \, / \, v_{\rm crit} = 0$ to $0.6$, metallicity,$[\mathrm{Fe}/\mathrm{H}] = -4$ to $0$, and $\alpha$-element enhancement,$[\alpha/\mathrm{Fe}] = 0$ to $0.4$. We show that rotating stars undergorotationally-induced dredge-up of nucleosynthetic products, mostly He and N, totheir surfaces while still on the MS. Non-rotating metal-rich stars also revealthe products of nucleosynthesis on their surfaces because even modest amountsof mass loss expose their "fossil" convective cores: regions that are no longerconvective, but which were part of the convective core at an early stage in thestar's evolution. Thus surface enhancement of He and N is expected for rotatingstars at all metallicities, and for non-rotating stars if they are relativelymetal-rich. We calculate a stellar atmosphere for a representative model fromour grid, properly accounting for He- and N-enhancement, and show that theresulting spectrum provides a good match to observed WNL stars, stronglysuggesting that the physical mechanisms we have identified are the ultimatecause of the WNL phase.

Helium and Nitrogen Enrichment in Massive Main Sequence Stars: Mechanisms and Implications for the Origin of WNL Stars