The Transition from Population III to Population II Star Formation in the Early Universe

Open Access
Author:
Smith, Britton Devon
Graduate Program:
Astronomy and Astrophysics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 27, 2007
Committee Members:
  • Steinn Sigurdsson, Committee Chair
  • Niel Brandt, Committee Member
  • Jane Camilla Charlton, Committee Member
  • Deirdre Shoemaker, Committee Member
  • Richard Alan Wade, Committee Member
Keywords:
  • numerical simulation
  • cosmology
  • star formation
Abstract:
The first stars in the universe formed in a unique environment that was free of heavy elements. The lack of efficient radiative cooling in primordial gas meant that the collapse of the first star-forming cloud-cores proceeded very slowly and without fragmenting into multiple objects. This resulted in the first stars being very massive (30 M<sub>sun</sub> &#8804; M &#8804; 300 M<sub>sun</sub>), isolated objects. When these stars died, in violent supernovae, they produced the very first heavy elements, or metals, and dispersed them into the surrounding gas. The addition of these metals changed forever the way stars formed by allowing the gas to cool much more efficiently as it collapsed. It is thought that when the chemical abundance in the next generation star-forming environments reached a critical level, the process of star-formation began producing stars that resemble those observed in the local universe, with characteristic masses of ~1 M<sub>sun</sub>, instead of the behemoths of the early universe. Simulating this transition in star-formation mode requires the inclusion of the complex chemistry and radiative cooling of heavy elements, which has traditionally been too computationally expensive to be done properly in three-dimensional simulations. In this dissertation, I introduce a new method for treating the radiative cooling from metals in large-scale, three-dimensional hydrodynamic simulations that is fast, accurate, and complete in its coverage of the first 30 elements. I use this cooling method to examine the ability of metals to induce fragmentation in collapsing gas-clouds through lowering of the cooling time-scales and the creation of thermal instabilities. Comparing the cooling and dynamical time-scales within collapsing gas, I calculate that the critical metallicity, Z<sub>cr</sub>, required for fragmentation into multiple, low-mass objects is ~10<sup>-4.2</sup> Z<sub>sun</sub>, where Z<sub>sun</sub> denotes the metallicity of the sun. Thermal instabilities are also present in gas-clouds with metallicities, Z &#8805; 10<sup>-4</sup> Z<sub>sun</sub>. I use the methodology introduced here to perform a series of adaptive mesh refinement hydrodynamic simulations of pre-enriched primordial star-formation with varying metallicities. For metallicities below Z<sub>cr</sub>, only massive, singular objects form, nearly identical to the metal-free case. For metallicities well above Z<sub>cr</sub>, efficient cooling rapidly lowers the gas temperature to the temperature of the cosmic microwave background (CMB), which is significantly higher in the distant past. The gas is physically unable to radiatively cool below the CMB temperature, and thus, becomes very thermally stable. For moderately high metallicities, Z &#8805; 10<sup>-3.5</sup> Z<sub>sun</sub>, this occurs early in the evolution of the gas-cloud, when the central density is still relatively low. The resulting cloud-cores show little or no fragmentation, and have mass-scales of a few hundred M<sub>sun</sub>. On the other hand, if the metallicity is only slightly above Z<sub>cr</sub>, the cloud slowly cools without ever reaching the CMB temperature. In this case, the minimum cloud temperature is achieved at much higher densities than in the high-metallicity case, resulting in mass-scales of just a few M<sub>sun</sub>. This implies that not 2, but 3 star-formation modes were present in the early universe: primordial (high-mass), metallicity-regulated (low-mass), and CMB-regulated (moderate-mass). As the universe evolves to the current epoch, the CMB-regulated mode will slowly transition into the metallicity-regulated mode, as the CMB temperature gradually decreases. This suggests that the metallicity-regulated mode, which is fully dominant today, may not become so until the CMB temperature drops below the observed temperatures of molecular clouds, ~10 K, which does not occur until redshift, z ~ 2.