Formation of massive protostars in atomic cooling haloes
  We present the highest-resolution three-dimensional simulation to date of the
collapse of an atomic cooling halo in the early Universe. We use the
moving-mesh code arepo with the primordial chemistry module introduced in Greif
(2014), which evolves the chemical and thermal rate equations for over more
than 20 orders of magnitude in density. Molecular hydrogen cooling is
suppressed by a strong Lyman-Werner background, which facilitates the
near-isothermal collapse of the gas at a temperature of about $10^4\,$K. Once
the central gas cloud becomes optically thick to continuum emission, it settles
into a Keplerian disc around the primary protostar. The initial mass of the
protostar is about $0.1\,{\rm M}_\odot$, which is an order of magnitude higher
than in minihaloes that cool via molecular hydrogen. The high accretion rate
and efficient cooling of the gas catalyse the fragmentation of the disc into a
small protostellar system with 5-10 members. After about 12 yr, strong
gravitational interactions disrupt the disc and temporarily eject the primary
protostar from the centre of the cloud. By the end of the simulation, a
secondary clump has collapsed at a distance of $\simeq 150\,$au from the
primary clump. If this clump undergoes a similar evolution as the first, the
central gas cloud may evolve into a wide binary system. High accretion rates of
both the primary and secondary clumps suggest that fragmentation is not a
significant barrier for forming at least one massive black hole seed.
