Other
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........33d&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, SANTA CRUZ, 1993.Source: Dissertation Abstracts International, Volume: 54-02, Section
Other
8
K Giant
Scientific paper
State-of-the-art model atmospheres are closely examined for a sample of 9 bright K-giant stars. For one star, HD 122563, considered a standard for its wealth of data in the literature, the best available spectroscopic data and input parameters were used. We find a serious inconsistency in the atmosphere theory: in three, maybe four of the stars in the sample, the temperature obtained from excitation and ionization equilibria differs by up to 300 K from the temperature obtained from accurate photometric sources. This error shows up as a slope in the derived Fe abundances vs. excitation potential when the photometric temperatures are used. No clear correlation is found between the temperature discrepancy and star characteristics. Several possible causes of error are examined for HD 122563, the standard star. The analysis is carried out with different codes making use of various sets of atomic parameters: all yield comparable slopes. Assumptions made in tile atmosphere code are tested: none of them affect the slope in a significant way. Non-LTE corrections are estimated, and a non-homogeneous atmosphere model is computed yielding no effect on the slope. An analysis similar to the one performed in HD 122563 yields no significant slope in the solar Fe I abundances, ruling out the possibility of systematic errors in the gf values. A more comprehensive survey of solar Fe abundances is also performed, yielding greater understanding of previously discrepant solar (Fe/H) values in the literature. While these avenues do not lead to an explanation of the slope problem, they show that the slope is not an artifact of the data or of the analysis. Assuming that all elements behave like iron, we estimate possible errors of ~+/- 0.2 dex for HD 122563 in the abundances of elements other than Fe, depending on the excitation potential of the lines adopted in the analysis. Ratios of element abundances calculated on lines with comparable excitation potential should not be affected. The magnitude of these errors does not affect the basic trends in abundance ratios vs. metallicity: they are comparable to the observed and as yet unexplained scatter that plagues the same trends.
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