7,451 research outputs found

    Line formation in solar granulation: V. Missing UV-opacity and the photospheric Be abundance

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    The possibility of unaccounted for opacity sources in the UV for late-type stars has often been invoked to explain discrepancies between predicted and observed flux distributions and spectral line strengths. Such missing UV-opacity could among other things have a significant impact on abundance determination for elements whose only relevant spectral features are accessible in this wavelength region, such as Be. Here, the study by Balachandran & Bell (1998) is re-visited in the light of a realistic 3D hydrodynamical solar model atmosphere and the recently significantly downward revised solar O abundance obtained with the same model atmosphere. The amount of missing UV-opacity, if any, is quantified by enforcing that the OH A-X electronic lines around 313 nm produce the same O abundance as the other available diagnostics: OH vibration-rotation and pure rotation lines in the IR, the forbidden [OI] 630.0 and 636.3 nm lines and high-excitation, permitted OI lines. This additional opacity is then applied for the synthesis of the BeII line at 313.0nm to derive a solar photospheric Be abundance in excellent agreement with the meteoritic value, thus re-enforcing the conclusions of Balachandran & Bell. The about 50% extra opacity over accounted for opacity sources can be well explained by recent calculations by the Iron Project for photo-ionization of FeI.Comment: Accepted for A&A, 7 pages. The article is also available from http://www.mso.anu.edu.au/~martin/publications.htm

    The make-up of stars

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    The chemical composition of stars contain vital clues not only about the stars themselves but also about the conditions prevailing before their births. As such, stellar spectroscopy plays a key role in contemporary astrophysics and cosmology by probing cosmic, galactic, stellar and planetary evolution. In this review I will describe the theoretical foundations of quantitative stellar spectroscopy: stellar atmosphere models and spectral line formation. I will focus mainly on more recent advances in the field, in particular the advent of realistic time-dependent, 3D, (magneto-)hydrodynamical simulations of stellar surface convection and atmospheres and non-LTE radiative transfer relevant for stars like the Sun. I will also discuss some particular applications of this type of modelling which have resulted in some exciting break-throughs in our understanding and with wider implications: the solar chemical composition, the chemical signatures of planet formation imprinted in stellar abundances, the cosmological Li problem(s) and where the first stars may be residing today.financial support from the Organizing Committee of the XVIII School, the Australian Research Council (e.g. grants FL110100012, DP120100991) and the Australian National University

    The stability of late-type stars close to the Eddington limit

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    Super-Eddington luminosities in hydrostatic model atmospheres manifest themselves by the presence of gas pressure inversions. Such inversions are not an artifact of the assumption of hydrostatic equilibrium but can also be present in hydrodynamical model atmospheres. Only for very large mass loss rates hardly realized in supergiants will the inversions be removed. Instabilities may, however, still be present in such inversions, which is investigated for both H-rich and H-deficient late-type supergiant model atmospheres. A local, non-adiabatic, linear stability analysis reveals that sound waves can be amplified due to the strong radiative forces. However, despite the super-Eddington luminosities, the efficiency of the radiative instabilities is fairly low compared to for early-type stars with growth rates of 10−5s−110^{-5} s^{-1}.Comment: 11 pages; accepted for publication in Astronomy & Astrophysic

    Line formation in solar granulation: III. The photospheric Si and meteoritic Fe abundances

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    Using realistic hydrodynamical simulations of the solar surface convection as 3D, time-dependent, inhomogeneous model atmospheres, the solar photospheric Si abundance has been determined to be log Si = 7.51 +- 0.04. This constitutes a difference of 0.04 dex compared with previous estimates based on the 1D Holweger-M\"uller (1974) model, of which half is attributable to the adopted model atmosphere and the remaining part to the improved quantum mechanical broadening treatment. As a consequence, all meteoritic abundances should be adjusted downwards by the same amount. In particular the meteoritic Fe abundance will be log Fe = 7.46 +- 0.01, in good agreement with the recently determined photospheric Fe abundance (Asplund et al. 2000b). The existing uncertainties unfortunately prevent an observational confirmation of the postulated effects of elemental migration of metals in the Sun.Comment: Accepted for A&
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