Helium and Nitrogen Enrichment in Massive Main Sequence Stars:
  Mechanisms and Implications for the Origin of WNL Stars
  The evolutionary paths taken by massive stars with $M \gtrsim 60 \,
\mathrm{M}_\odot$ remain substantially uncertain. They begin their lives as
main sequence (MS) O-stars. Depending on their masses, rotation rates, and
metallicities, they can then encounter a wide range of evolutionary states with
an equally broad set of possible surface compositions and spectral
classifications. We present a new grid of calculations for the evolution of
such 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 undergo
rotationally-induced dredge-up of nucleosynthetic products, mostly He and N, to
their surfaces while still on the MS. 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.
