Browsing by Subject "winds"

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First Stars XVI. HST/STIS Abundances Of Heavy Elements In The Uranium-Rich Metal-Poor Star CS 31082-001
(2013-02) Mello, C. Siqueira; Spite, M.; Barbuy, B.; Spite, F.; Caffau, E.; Hill, V.; Wanajo, S.; Primas, F.; Plez, B.; Cayrel, R.; Andersen, J.; Nordstrom, B.; Sneden, Christopher; Beers, T. C.; Bonifacio, P.; Francois, P.; Molaro, P.; Sneden, Christopher
Context. The origin and site(s) of the r-process nucleosynthesis is(are) still not known with certainty, but complete, detailed r-element abundances off er our best clues. The few extremely metal-poor (EMP) stars with large r-element excesses allow us to study the r-process signatures in great detail, with minimal interference from later stages of Galactic evolution. CS 31082-001 is an outstanding example of the information that can be gathered from these exceptional stars. Aims. Here we aim to complement our previous abundance determinations for third-peak r-process elements with new and improved results for elements of the first and second r-process peaks from near-UV HST/STIS and optical UVES spectra. These results should provide new insight into the nucleosynthesis of the elements beyond iron. Methods. The spectra were analyzed by a consistent approach based on an OSMARCS LTE model atmosphere and the Turbospectrum spectrum synthesis code to derive abundances of heavy elements in CS 31082-001, and using updated oscillator strengths from the recent literature. Synthetic spectra were computed for all lines of the elements of interest to check for proper line intensities and possible blends in these crowded spectra. Our new abundances were combined with the best previous results to provide reliable mean abundances for the first and second-peak r-process elements. Results. We present new abundances for 23 neutron-capture elements, 6 of which - Ge, Mo, Lu, Ta, W, and Re - have not been reported before. This makes CS 31082-001 the most completely studied r-II star, with abundances for a total of 37 neutron-capture elements. We also present the first NLTE + 3D abundance of lead in this star, further constraining the nature of the r-process.
HST-COS Observations Of AGNs. II. Extended Survey Of Ultraviolet Composite Spectra From 159 Active Galactic Nuclei
(2014-10) Stevans, Matthew L.; Shull, J. Michael; Danforth, Charles W.; Tilton, Evan M.; Stevans, Matthew L.
The ionizing fluxes from quasars and other active galactic nuclei (AGNs) are critical for interpreting their emissionline spectra and for photoionizing and heating the intergalactic medium. Using far-ultraviolet (FUV) spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we directly measure the rest-frame ionizing continua and emission lines for 159 AGNs at redshifts 0.001 < Z(AGN) < 1.476 and construct a composite spectrum from 475 to 1875 A. We identify the underlying AGN continuum and strong extreme ultraviolet (EUV) emission lines from ions of oxygen, neon, and nitrogen after masking out absorption lines from the HI Ly alpha forest, 7 Lyman-limit systems (N-HI, 10(17.2) Cm-2) and 214 partial Lyman-limit systems (14.5 < log N-HI < 17.2). The 159 AGNs exhibit a wide range of FUV/EUV spectral shapes, F-V, proportional to v(proportional to v)(4, typically with -2 <= alpha(v), <= 0 and no discernible continuum edges at 912 angstrom (HI) or 504 angstrom (He I). The composite rest-frame continuum shows a gradual break at lambda(br) approximate to 1000 angstrom, with mean spectral index alpha(v) = -0.83 +/- 0.09 in the FUV (1200-2000 angstrom) steepening to alpha(v), = -1.41 +/- 0.15 in the EUV (500-1000 angstrom). We discuss the implications of the UV flux turnovers and lack of continuum edges for the structure of accretion disks, AGN mass inflow rates, and luminosities relative to Eddington values.