On Monolithic Supermassive Stars
  Supermassive stars have been proposed as the progenitors of the massive
($\sim 10^{9}\,\rm{M}_{\odot}$) quasars observed at $z\sim7$. Prospects for
directly detecting supermassive stars with next-generation facilities depend
critically on their intrinsic lifetimes, as well as their formation rates. We
use the 1D stellar evolution code Kepler to explore the theoretical limiting
case of zero-metallicity, non-rotating stars, formed monolithically with
initial masses between $10\,\rm{kM}_{\odot}$ and $190\,\rm{kM}_{\odot}$. We
find that stars born with masses between $\sim60\,\rm{kM}_{\odot}$ and
$\sim150\,\rm{kM}_{\odot}$ collapse at the end of the main sequence, burning
stably for $\sim1.5\,\rm{Myr}$. More massive stars collapse directly through
the general relativistic instability after only a thermal timescale of
$\sim3\,\rm{kyr}$--$4\,\rm{kyr}$. The expected difficulty in producing such
massive, thermally-relaxed objects, together with recent results for currently
preferred rapidly-accreting formation models, suggests that such ``truly
direct'' or ``dark'' collapses may not be typical for supermassive objects in
the early Universe. We close by discussing the evolution of supermassive stars
in the broader context of massive primordial stellar evolution and the
possibility of supermassive stellar explosions.
