  Halos with masses in excess of the atomic limit are believed to be ideal
environments in which to form heavy black hole seeds with masses above 10^3
Msun. In cases where the H_2 fraction is suppressed this is expected to lead to
reduced fragmentation of the gas and the generation of a top heavy initial mass
function. In extreme cases this can result in the formation of massive black
hole seeds. Resolving the initial fragmentation scale and the resulting
protostellar masses has, until now, not been robustly tested. Cosmological
simulations were performed with the moving mesh code Arepo using a primordial
chemistry network until z = 11. Three haloes with masses in excess of the
atomic cooling mass were then selected for detailed examination via zoom-ins.
The highest resolution simulations resolve densities up to 10^-6 g cm^-3 (10^18
cm^-3) and capture a further 100 yr of fragmentation behaviour at the center of
the halo. Our simulations show intense fragmentation in the central region of
the halos, leading to a large number of near-solar mass protostars. Despite the
increased fragmentation the halos produce a protostellar mass spectrum that
peaks at higher masses relative to standard Population III star forming halos.
The most massive protostars have accretion rates of 10^-3-10^-1 Msun yr^-1
after the first 100 years of evolution, while the total mass of the central
region grows at 1 Msun yr^-1. Lower resolution zoom-ins show that the total
mass of the system continues to accrete at 1 Msun yr^-1 for at least 10^4 yr,
although how this mass is distributed amongst the rapidly growing number of
protostars is unclear. However, assuming that a fraction of stars can continue
to accrete rapidly the formation of a sub-population of stars with masses in
excess of 10^3 Msun is likely in these halos.
