Experimental Results for H 2 Formation from H − and H and Implications for First Star Formation

• Published in: Science (American Association for the Advancement of Science) 2010-07, Vol.329 (5987), p.69-71
• Main Authors: Kreckel, H., Bruhns, H., Čížek, M., Glover, S. C. O., Miller, K. A., Urbain, X., Savin, D. W.
• Format: Article
• Language: English
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.1187191

Formation of molecular hydrogen through electron-expelling collisions of H atoms and H − anions is regarded as a key step in the cooling process that led to assembly of the first stars in the early universe. Kreckel et al. (p. 69 ; see the Perspective by Bromm ) performed highly precise laboratory measurements of the rate of this reaction at a range of different energies. The study required construction of a dedicated apparatus to carefully tune the relative velocity of merged atom and ion beams. The data validated prior theoretically calculated reaction cross sections, which were then extended for use in cosmological models. Precise measurements of molecular hydrogen formation rates help improve models of star assembly in the early universe. During the epoch of first star formation, molecular hydrogen (H 2 ) generated via associative detachment (AD) of H − and H is believed to have been the main coolant of primordial gas for temperatures below 10 4 kelvin. The uncertainty in the cross section for this reaction has limited our understanding of protogalaxy formation during this epoch and of the characteristic masses and cooling times for the first stars. We report precise energy-resolved measurements of the AD reaction, made with the use of a specially constructed merged-beams apparatus. Our results agreed well with the most recent theoretically calculated cross section, which we then used in cosmological simulations to demonstrate how the reduced AD uncertainty improves constraints of the predicted masses for Population III stars.