Browsing by Author "Rollinde, Emmanuel"

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    Cosmological cosmic rays and the observed Li6 plateau in metal poor halo stars
    (2005-01-01) Rollinde, Emmanuel; Elisabeth, Vangioni-Flam; Olive, Keith A
    Very recent observations of the 6Li isotope in halo stars reveal a 6Li plateau about 1000 times above the predicted BBN abundance. We calculate the evolution of 6Li versus redshift generated from an initial burst of cosmological cosmic rays (CCRs) up to the formation of the Galaxy. We show that the pregalactic production of the 6Li isotope can account for the 6Li plateau observed in metal poor halo stars without additional overproduction of 7Li. The derived relation between the amplitude of the CCR energy spectra and the redshift of the initial CCR production puts constraints on the physics and history of the objects, such as pop III stars, responsible for these early cosmic rays. Consequently, we consider the evolution of 6Li in the Galaxy. Since 6Li is also produced in Galactic cosmic ray nucleosynthesis, we argue that halo stars with metalIicities between [Fe/H) = -2 and -1, must be somewhat depleted in 6Li.
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    Density structure around quasars from optical depth statistics
    (2005-05-01) Rollinde, Emmanuel; Srianand, R.; Chand, Hum; et al.
    We present a method for studying the proximity effect and the density structure around redshift z=2-3 quasars. It is based on the probability distribution of Lyman-α pixel optical depths and its evolution with redshift. We validate the method using mock spectra obtained from hydrodynamical simulations, and then apply it to a sample of 12 bright quasars at redshifts 2-3 observed with UVES at the VLT-UT2 Kueyen ESO telescope. These quasars do not show signatures of associated absorption and have a mean monochromatic luminosity of 5.4 × 1031 h−2 erg s−1 Hz−1 at the Lyman limit. The observed distribution of optical depth within 10 h−1Mpc from the QSO is statistically different from that measured in the general intergalacticmedium at the same redshift. Such a change will result from the combined effects of the increase in photoionisation rate above the mean UV-background due to the extra ionizing photons from the quasar radiation (proximity effect), and the higher density of the IGM if the quasars reside in overdense regions (as expected from biased galaxy formation). The first factor decreases the optical depth whereas the second one increases the optical depth, but our measurement cannot distinguish a high background from a low overdensity. An overdensity of the order of a few is required if we use the amplitude of the UV-background inferred from the mean Lyman-α opacity. If no overdensity is present, then we require the UV-background to be higher, and consistent with the existing measurements based on standard analysis of the proximity effect.