Bulletin of the American Physical Society
41st Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 55, Number 5
Tuesday–Saturday, May 25–29, 2010; Houston, Texas
Session B1: Atomic and Molecular Physics in Early Universe |
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Chair: Daniel Savin, Columbia University Room: Imperial East |
Wednesday, May 26, 2010 10:30AM - 11:00AM |
B1.00001: Primordial gas chemistry and the formation of the first stars Invited Speaker: Astronomical observations using large ground-based and space-borne telescopes have probed cosmic history all the way from the present-day Universe to an epoch when the age of the Universe was less than a tenth of its present age. The Universe at a very early epoch can be seen as the present-day cosmic microwave background radiation. The remaining frontier lies in between, where the first stars are predicted to have formed. I review our current knowledge of the primordial star formation process. I summarize the key open questions to be addressed with theoretical studies. There are a few important chemical processes of which the reaction rates are uncertain. Recent theoretical studies suggest that cooling by HD molecules plays a crutial role in the formation of a particular type of primordial stars. I show a few illustrative examples of this process. Including radiative transfer in a promordial gas is a grand challenge problem in numerical cosmology. I review the recent development. Finally, I discuss prospects for future observations against which we can test the current theoretical models. [Preview Abstract] |
Wednesday, May 26, 2010 11:00AM - 11:30AM |
B1.00002: Molecular hydrogen formation in the early universe: new implications from laboratory measurements Invited Speaker: Molecular hydrogen plays a central role in the cooling of primordial gas and the formation of the first stars and galaxies. The creation of molecular hydrogen in the early universe during the epoch of first star formation proceeds predominantly through the associative detachment (AD) reaction H$^{-}$ + H $\rightarrow$ H$_{2}$ + e$^{-}$. Despite being the most fundamental anion-neutral reaction in physics and chemistry, no agreement has yet been reached between theory and experiment for this process. The uncertainty in the H$_{2}$ creation rate severely limits our understanding of the formation of the first stars and protogalaxies. To address this issue we have developed a new merged beams apparatus to measure the H$_{2}$ AD rate coefficient as a function of the collision energy. Kinematical compression for fast velocity-matched beams allows us to achieve collision energies from 4meV to 1eV. We will describe the experimental approach and present the first energy-resolved measurement of the H$_{2}$ associative detachment reaction which we use to derive an experimentally confirmed thermal rate coefficient. We will also present the results of new cosmological models, demonstrating the implications of our measurements for the evolution of primordial gas in an initially ionized protogalactic halo. [Preview Abstract] |
Wednesday, May 26, 2010 11:30AM - 12:00PM |
B1.00003: Low-Energy Mutual Neutralization Studies for Early Universe Hydrogen Chemistry Invited Speaker: Low-energy interactions between light ions, as they occur in low density plasmas, are ideally studied under merged-beam conditions. This was the motivation for building the dual-source setup in operation at UCL, Louvain-la-Neuve, since the early eighties. Although initially developed for the study of charge exchange~[1], mutual neutralization and transfer ionization, this machine has produced a host of total cross section measurements for a wide variety of associative ionization and other reactive processes involving charged reactants, from H$^+$ to CO$^+$, in collision with H$^-$, D$^-$, C$^-$ and O$^-$~[2]. A recent paper by Glover et al.~[3] has revived the interest for mutual neutralization studies, by stressing the need of the astrophysical community for a precise determination of the low-energy cross section of the H$^+$/H$^-$ reaction. The mutual neutralization acts as a sink for negative ions which otherwise dominate the primordial formation of H$_2$ by associative detachment with ground state H. Absolute measurements in the range 5 meV to 5 eV are needed to rule out earlier experimental work~[4] contradicting the most recent theoretical predictions~[5]. Our setup is currently modified to incorporate coincident imaging techniques, giving access to differential cross sections besides the branching among accessible neutral channels. Mutual neutralization reactions of H$^-$ with H$_2^+$ and H$_3^+$ will also be investigated, for the role they play in laboratory plasmas~[6].\\ [4pt] [1] S. Sz\"ucs, M. Karemera, M. Terao, and F. Brouillard, \emph{J. Phys. B} \textbf{17}, 1613 (1983).\\ [0pt] [2] E. A. Naji et al., \emph{J. Phys. B} \textbf{31}, 4887 (1998), A. Le Padellec et al., \emph{J. Chem. Phys.}, \textbf{124}, 154304 (2006) and references therein.\\ [0pt] [3] S. C. Glover, D. W. Savin, and A.-K. Jappsen , \emph{Astrophys. J.} \textbf{640}, 553 (2006). \\ [0pt] [4] J. Moseley, W. Aberth, and J. R. Peterson, \emph{Phys. Rev. Lett.} \textbf{24}, 435 (1970).\\ [0pt] [5] M. Stenrup, {\AA}. Larson, and N. Elander, \emph{Phys. Rev. A} \textbf{79}, 012713 (2009).\\ [0pt] [6] M. J. J. Eerden et al., \emph{Phys. Rev. A} \textbf{51}, 3362 (1995). [Preview Abstract] |
Wednesday, May 26, 2010 12:00PM - 12:30PM |
B1.00004: Atomic physics and the cosmological 21 cm signal Invited Speaker: Upcoming low-frequency radio interferometers, such as MWA and SKA, offer the prospect of using 21 cm tomography to map the evolution of hydrogen reionization. The existence of a detectable signal is dependent upon the existence of a background of Lyman alpha photons able to decouple the 21 cm spin temperature from that of the CMB. In this talk, I will discuss the details of the relevant atomic physics and compare the results of simulations and analytical calculations of the effect of inhomogeneities in the Lya and X-ray background on the 21 cm power spectrum. [Preview Abstract] |
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