The Role of H₂ Molecules in Primordial Star Formation

Summary

H₂ and HD molecules provide the cooling needed for the fragmentation and collapse of the first structures in the universe. In this review, we describe the main chemical and physical processes occurring in the primordial gas after the recombination epoch. We also highlight the areas where improvements in the determination of reaction rates and excitation coefficients are necessary to reduce the remaining uncertainties in the predictions of the numerical models. The interaction of primordial molecules with the CBR and the role of H₂ and HD cooling in the early universe are discussed. Finally, we comment on the results of recent simulations of the fragmentation and collapse of primordial clouds, with an emphasis on the typical mass scale of the first objects.

Introduction

The formation of the first stellar objects in the universe is a fascinating, yet little understood process. Although we have now observational data on bright quasars and galaxies out to redshifts of about 5 (corresponding to 10⁹ yr after the Big Bang) and on the density fluctuations at redshifts about 1000 (or an age of ∼ 10⁶ yr), there is no direct evidence as to when and how the first structures formed. This unique epoch and the nature of the primordial objects define what has been called the end of the “Dark Ages” (Rees 1999).

According to Big Bang cosmology, there must have been an epoch in the history of the universe during which the original gas mixture was altered by the manufacture of heavy elements inside stars.