Gravitational fragmentation of extremely metal-poor circumstellar discs

We study the gravitational fragmentation of circumstellar discs accreting extremely metal-poor (Z <= 10(-3) Z(circle dot)) gas, performing a suite of 3D hydrodynamic simulations using the adaptive mesh refinement code ENZO. We systematically follow the long-term evolution for 2 x 10(3) yr after the first protostar's birth, for the cases of Z = 0, 10(-5), 10(-4), and 10(-3) Z(circle dot). We show that evolution of number of self-gravitating clumps qualitatively changes with Z. Vigorous fragmentation induced by dust cooling occurs in the metal-poor cases, temporarily providing similar to 10 self-gravitating clumps at Z = 10(-5) and 10(-4) Z(circle dot). However, we also show that the fragmentation is a very sporadic process; after an early episode of the fragmentation, the number of clumps continuously decreases as they merge away in these cases. The vigorous fragmentation tends to occur later with the higher Z, reflecting that the dust-induced fragmentation is most efficient at the lower density. At Z = 10(-3) Z(circle dot), as a result, the clump number stays smallest until the disc fragmentation starts in a late stage. We also show that the clump mass distribution depends on the metallicity. A single or binary clump substantially more massive than the others appear only at Z = 10(-3) Z(circle dot), whereas they are more evenly distributed in mass at the lower metallicities. We suggest that the disc fragmentation should provide the stellar multiple systems, but their properties drastically change with a tiny amount of metals.