Star formation in the early universe.

다국어 초록 (Multilingual Abstract)

We investigate the formation of the first stars in the universe. In the context of hierarchical models of structure formation, these Population III stars are expected to form in high or peaks of mass ∼10<super>6</super><math> <f> <rm><mit>M<inf>⊙</inf></mit></rm></f> </math>, collapsing at redshifts ≃20−30. We present an exploratory survey, based on numerical simulations using the SPH method. The main results are: (1) Just before the onset of gravitational instability, the primordial gas attains a characteristic temperature of a few 100 K, and a density of 10<super>3</super>−10<super>4</super>cm<super>−3</super>, with corresponding Jeans mass <italic>M_J</italic> of ∼10<super> 3</super><math> <f> <rm><mit>M<inf>⊙</inf></mit></rm></f> </math>. These characteristic values have robust explanation in the microphysics of H₂ cooling, related to the minimum temperature that can be reached with the H₂ coolant, and to the critical density at which the transition takes place between levels being populated according to NLTE, and according to LTE. The gas fragments into clumps with initial masses close to <italic> M_J</italic>. This result is remarkably insensitive to the initial conditions, and suggests that the first stars might have been quite massive. (2) The later evolutionary stages, during which the clumps grow in mass due to accretion and merging with other clumps, are quite sensitive to the initial conditions. The key process in building up very massive clumps, with masses up to a few times 10<super>4</super><math> <f> <rm><mit>M<inf>⊙</inf></mit></rm></f> </math>, is merging. (3) We follow the collapse of a clump up to central densities of ∼10<super>14</super>cm<super>−3</super>. Three-body reactions are very efficient in converting the hydrogen into fully molecular form. A central core of ∼10<super>2</super><math> <f> <rm><mit>M<inf>⊙</inf></mit></rm></f> </math> is in a state of free-fall, leaving behind an extended envelope with an isothermal profile. No further subfragmentation is seen. (4) We calculate the generic spectral signature of a population of massive stars at high redshifts. The production rate of ionizing radiation per stellar mass by stars more massive than ∼100<math> <f> <rm><mit>M<inf>⊙</inf></mit></rm></f> </math> is larger by ∼1 order of magnitude for hydrogen and He I, and by ∼2 orders of magnitude for He II, than the emission from a Salpeter IMF.