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Issue A&A
Volume 398, Number 2, February I 2003
Page(s) 447 - 454
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20021678



A&A 398, 447-454 (2003)
DOI: 10.1051/0004-6361:20021678

Possible flakes of molecular hydrogen in the early Universe

D. Pfenniger1 and D. Puy1, 2

1  Observatory of Geneva, University of Geneva, 1290 Sauverny, Switzerland
    e-mail: Daniel.Pfenniger@obs.unige.ch
2  Institute of Theoretical Physics, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
    e-mail: puy@physik.unizh.ch

(Received 27 September 2002 / Accepted 3 November 2002 )

Abstract
The thermochemistry of H 2 and HD in non-collapsed, non-reionized primordial gas up to the end of the dark age is investigated with recent radiation-matter and chemical reaction rates taking into account the efficient coolant HD, and the possibility of a gas-solid phase transition of H 2. In the standard big-bang model we find that these molecules can freeze out and lead to the growth of flakes of solid molecular hydrogen at redshifts $z \approx 6{-}12$ in the unperturbed medium and under-dense regions. While this freezing caused by the mere adiabatic cooling of the expanding matter is less likely to occur in collapsed regions due to their higher than radiation background temperature, on the other hand the super-adiabatic expansion in voids strongly favors it. Later reionization (at $z \approx 5{-}6$) eventually destroys all these H 2 flakes. The possible occurrence of H 2 flakes is important for the degree of coupling between matter and radiation, as well as for the existence of a gas-grain chemistry at the end of the dark age.


Key words: cosmology: early Universe -- cosmology: theory

Offprint request: D. Pfenniger, Daniel.Pfenniger@obs.unige.ch




© ESO 2003


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