2007 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/334
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Item On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei(2007-10-27) Shaw, Gargi; Stancil, P. C.; Ferland, G. J.; et al.The spatial distribution of the cosmic-ray flux is important in understanding the Interstellar Medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate (ζ=χCRζstandard) in diffuse clouds compared to the standard value, ζstandard (=2.5x10-17 s-1), measured toward dense molecular clouds. In an earlier work we reported an enhancement χCR =20 towards HD185418. McCall et al. have reported χCR =48 towards ζ Persei based on the observed abundance of H3+ while Le Petit et al. found χCR ≈ 10 to be consistent with their models for this same sight line. Here we revisit ζ Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of χCR deduced from the H3 + column density, N(H3 +), in the diffuse region of the sightline depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with χCR ≈ 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.Item Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions(2007-07-30) Noterdaeme, P.; Ledoux, C.; Petitjean, Patrick; et al.Aims.We probe the physical conditions in high-redshift damped Lyman-α systems (DLAs) using the observed molecular fraction and the rotational excitation of molecular hydrogen. Methods. We search for Lyman- and Werner-band absorption lines of molecular hydrogen in the VLT/UVES spectra of background QSOs at the redshift of known DLAs. Results. We report two new detections of molecular hydrogen in the systems at zabs = 2.402 and 1.989 toward, respectively, HE 0027−1836 and HE 2318−1107, discovered in the course of the Hamburg-ESO DLA survey. We also present a detailed analysis of our recent H2 detection toward Q2343+125. All three systems have low molecular fractions, log f ≤ −4, with f = 2N(H2)/(2N(H2) + N(H)). Only one such H2 system was known previously. Two of them (toward Q2343+125 and HE 2318−1107) have high-metallicities, [X/H] > −1, whereas the DLA toward HE 0027−1836 is the system with the lowest metallicity ([Zn/H] = −1.63) among known H2-bearing DLAs. The depletion patterns for Si, S, Ti, Cr, Mn, Fe and Ni in the three systems are found to be very similar to what is observed in diffuse gas of the Galactic halo. Molecular hydrogen absorption from rotational levels up to J = 5 is observed in a single well-defined component toward HE 0027−1836. We show that the width (Doppler parameter) of the H2 lines increases with increasing J and that the kinetic energy derived from the Doppler parameter is linearly dependent on the relative energy of the rotational levels. There is however no velocity shift between lines from different rotational levels. The excitation temperature is found to be 90 K for J = 0 to J = 2 and ∼500 K for higher J levels. Single isothermal PDR models fail to reproduce the observed rotational excitations. A two-component model is needed: one component of low density (∼50 cm−3 ) with weak illumination (χ = 1) to explain the J ≤ 2 rotational levels and another of high density (∼500 cm−3 ) with strong illumination (χ = 30) for J ≥ 3 levels. However, the juxtaposition of these two PDR components may be ad-hoc and the multicomponent structure could result either from turbulent dissipation or C-shocks.