ALBERT

Description: We provide detailed comparison between the adaptive mesh refinement (AMR) code enzo -2.4 and the smoothed particle hydrodynamics (SPH)/ N -body code gadget -3 in the context of isolated or cosmological direct baryonic collapse within dark matter (DM) haloes to form supermassive black holes. Gas flow is examined by following evolution of basic parameters of accretion flows. Both codes show an overall agreement in the general features of the collapse; however, many subtle differences exist. For isolated models, the codes increase their spatial and mass resolutions at different pace, which leads to substantially earlier collapse in SPH than in AMR cases due to higher gravitational resolution in gadget -3. In cosmological runs, the AMR develops a slightly higher baryonic resolution than SPH during halo growth via cold accretion permeated by mergers. Still, both codes agree in the build-up of DM and baryonic structures. However, with the onset of collapse, this difference in mass and spatial resolution is amplified, so evolution of SPH models begins to lag behind. Such a delay can have effect on formation/destruction rate of H 2 due to UV background, and on basic properties of host haloes. Finally, isolated non-cosmological models in spinning haloes, with spin parameter ~ 0.01–0.07, show delayed collapse for greater , but pace of this increase is faster for AMR. Within our simulation set-up, gadget -3 requires significantly larger computational resources than enzo -2.4 during collapse, and needs similar resources, during the pre-collapse, cosmological structure formation phase. Yet it benefits from substantially higher gravitational force and hydrodynamic resolutions, except at the end of collapse.

Description: We perform 3D smoothed particle hydrodynamics (SPH) simulations of gas accretion on to the seeds of binary stars to investigate their short-term evolution. Taking into account the dynamically evolving envelope with non-uniform distribution of gas density and angular momentum of accreting flow, our initial condition includes a seed binary and a surrounding gas envelope, modelling the phase of core collapse of gas cloud when the fragmentation has already occurred. We run multiple simulations with different values of initial mass ratio q 0 (the ratio of secondary over primary mass) and gas temperature. For our simulation setup, we find a critical value of q c = 0.25 which distinguishes the later evolution of mass ratio q as a function of time. If q 0 q c , the secondary seed grows faster and q increases monotonically towards unity. If q 0 q c , on the other hand, the primary seed grows faster and q is lower than q 0 at the end of the simulation. Based on our numerical results, we analytically calculate the long-term evolution of the seed binary including the growth of binary by gas accretion. We find that the seed binary with q 0 q c evolves towards an equal-mass binary star and that with q 0 q c evolves to a binary with an extreme value of q . Binary separation is a monotonically increasing function of time for any q 0 , suggesting that the binary growth by accretion does not lead to the formation of close binaries.