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Title:
Three Modes of Star Formation in the Early Universe
Authors:
Smith, Britton D.; Sigurdsson, S.; O'Shea, B. W.; Norman, M.
Affiliation:
AA(University of Colorado), AB(The Pennsylvania State University), AC(Los Alamos National Laboratory), AD(University of California at San Diego)
Publication:
American Astronomical Society, AAS Meeting #211, #89.01; Bulletin of the American Astronomical Society, Vol. 39, p.878
Publication Date:
12/2007
Origin:
AAS
Bibliographic Code:
2007AAS...211.8901S

Abstract

The nature of the first metal-enriched stars to form in the universe remains largely a mystery today. The exact masses of the very first, metal-free stars are still uncertain, but it is generally accepted that they were significantly more massive than the stars observed today. This suggests that there was a transition in star-formation modes that was most likely related to the metallicity of the star-forming environment. We study how the addition of heavy elements alters the dynamics of collapsing gas by performing a series of numerical simulations of primordial star-formation with various levels of pre-enrichment, using the adaptive mesh refinement, hydrodynamic + N-body code, Enzo. At high redshifts, the process of star-formation is heavily influenced by the cosmic microwave background (CMB), which creates a temperature floor for the gas. Our results show that cloud-collapse can follow three distinct paths, depending on the metallicity. For very low metallicities (log(Z/Zsolar) < -4), star-formation proceeds similar to the metal-free case, producing only massive, singular objects. For high metallicities (log(Z/Zsolar) > -3.5), efficient cooling from the metals cools the gas to the CMB temperature when the core density is still very low. The gas becomes very thermally stable, which suppresses further fragmentation. The resulting pre-stellar cloud-cores have mass-scales of a few hundred Msolar. For metallicities between these two limits, the gas cools efficiently, but never reaches the CMB temperature. Fragmentation is able to proceed to much higher densities than in the other two case, resulting in cloud-cores of only a few Msolar. We discuss the evolution of these three modes with redshift, as well as the consequences for star-formation in the early universe.
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