Collapse and Fragmentation Models of Oblate Molecular Cloud Cores. III. Formation of Small Protostellar Clusters

Abstract

Recent near-infrared imaging surveys of the giant molecular cloud L1641 have revealed the existence of small clusters consisting of on the order of 10-50 young stars. While these observations suggest that stars may actually form in a range of stellar densities, the multiplicity of pre-main-sequence stars, along with the emerging evidence for protostellar condensations, indicate that stars may ultimately form through fragmentation of collapsing molecular cloud cores. Previous collapse calculations have shown that prolate cloud models are highly susceptible to fragmentation into protostellar binaries. While these results apply to star formation in an isolated environment, small cluster formation may require starting the collapse from oblate, rather than prolate, shapes. Here we use a smoothed-particle hydrodynamics code to further investigate the gravitational collapse and fragmentation of centrally condensed (Gaussian), oblate molecular clouds with varied thermal energy (α) and axial ratios of 2:1, 4:1, and 8:1. All models start with uniform rotation and ratios of rotational to gravitational energy β ≈ 0.02. Because of the oblate geometry, the clouds first collapse down to their equatorial plane, forming a rotationally unbalanced, intermediate central core that may be spherical, oblate spheroidal, or disklike, depending on the value of α. The evolution proceeds with the central core collapsing radially inward while becoming progressively flatter. During this phase, a rotationally supported, thin pancake forms, which quickly becomes gravitationally unstable to the point of fragmentation. The results indicate that the 2:1 clouds all produced low-order protostellar systems consisting of 2-3 clumps for 0.18 ≤ α ≤ 0.48. Similarly, the 4:1 and 8:1 clouds formed low-order systems of 3-5 clumps for α ≥ 0.37 and α = 0.48, respectively. Formation of small clusters of ~10 protostars occurred only in the 4:1 clouds for α = 0.18 and in the 8:1 clouds for α ≤ 0.37. Thus, increasing the degree of cloud oblateness extends the range of α values for which fragmentation into a larger number of clumps is allowed. Since extreme axial ratios of 4:1 and 8:1 are consistent with the observations, the results may have implications for understanding the formation of the small clusters observed in L1641.