Formation and survival of Population III stellar systems
  The initial mass function of the first, Population III (Pop III), stars plays
a vital role in shaping galaxy formation and evolution in the early Universe.
One key remaining issue is the final fate of secondary protostars formed in the
accretion disc, specifically whether they merge or survive. We perform a suite
of hydrodynamic simulations of the complex interplay between fragmentation,
protostellar accretion, and merging inside dark matter minihaloes. Instead of
the traditional sink particle method, we employ a stiff equation of state
approach, so that we can more robustly ascertain the viscous transport inside
the disc. The simulations show inside-out fragmentation because the gas
collapses faster in the central region. Fragments migrate on the viscous
timescale, over which angular momentum is lost, enabling them to move towards
the disc centre, where merging with the primary protostar can occur. This
process depends on the fragmentation scale, such that there is a maximum scale
of $(1 - 5) \times 10^4$ au, inside which fragments can migrate to the primary
protostar. Viscous transport is active until radiative feedback from the
primary protostar destroys the accretion disc. The final mass spectrum and
multiplicity thus crucially depends on the effect of viscosity in the disc. The
entire disc is subjected to efficient viscous transport in the primordial case
with viscous parameter $\alpha \le 1$. An important aspect of this question is
the survival probability of Pop III binary systems, possible gravitational wave
sources to be probed with the Advanced LIGO detectors.
