Browsing by Subject "stellar atmospheres"

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    High-precision Atomic Physics Laboratories in Space: White Dwarfs and Subdwarfs
    (2018-07) Landstorfer, A.; Löbling, L.; Rauch, T.; Werner, K.; Quinet, P.; Castanheira, B.; Vanderbosch, Z.; Montgomery, M.
    Stellar atmospheres are prime laboratories to determine atomic properties of highly ionized species. Reliable opacities are crucial ingredients for the calculation of stellar atmospheres of white dwarfs and subdwarfs. A detailed investigation on the precision of many iron-group oscillator strengths is still outstanding. To make progress, we used the Hubble Space Telescope Imaging Spectrograph to measure high-resolution spectra of three hot subdwarfs that exhibit extremely high iron-group abundances. The predicted relative strengths of the identified lines are compared with the observations to judge the quality of Kurucz’s line data and to determine correction factors for abundance determinations of the respective elements.
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    Neon And CNO Abundances For Extreme Helium Stars-A Non-LTE Analysis
    (2011-02) Pandey, Gajendra; Lambert, David L.; Lambert, David L.
    A non-LTE (NLTE) abundance analysis was carried out for three extreme helium stars (EHes): BD+10 degrees 2179, BD-9 degrees 4395, and LS IV+6 degrees 002, from their optical spectra with NLTE model atmospheres. NLTE TLUSTY model atmospheres were computed with H, He, C, N, O, and Ne treated in NLTE. Model atmosphere parameters were chosen from consideration of fits to observed He I line profiles and ionization equilibria of C and N ions. The program SYNSPEC was then used to determine the NLTE abundances for Ne as well as H, He, C, N, and O. LTE neon abundances from Ne I lines in the EHes: LSE 78, V 1920 Cyg, HD 124448, and PV Tel, are derived from published models and an estimate of the NLTE correction applied to obtain the NLTE Ne abundance. We show that the derived abundances of these key elements, including Ne, are well matched with semi-quantitative predictions for the EHe resulting from a cold merger (i.e., no nucleosynthesis during the merger) of an He white dwarf with a C-O white dwarf.