IUCAA Preprints
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Item Physical conditions in the ISM towards HD185418(2005-06-01) Shaw, Gargi; Srianand, R.We have developed a complete model of the hydrogen molecule as part of the spectral simulation code Cloudy. Our goal is to apply this to spectra of high-redshift star-forming regions where H2 absorption is seen, but where few other details are known, to understand its implication for star formation. The microphysics of H2 is intricate, and it is important to validate these numerical simulations in better-understood environments. This paper studies a well-defined line-of-sight through the Galactic interstellar medium (ISM) as a test of the microphysics and methods we use. We present a self-consistent calculation of the observed absorption-line spectrum to derive the physical conditions in the ISM towards HD185418, a ine-of-sight with many observables. We deduce density, temperature, local radiation field, cosmic ray ionization rate, chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density, similar abundances, and require a cosmic ray flux enhanced over the Galactic background value, consistent with enhancements redicted by MHD simulations.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.