Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007smma.conf..115m&link_type=abstract
Spectroscopic methods in modern astrophysics. Proceedings of the conference held 13-15 September 2006, Moscow, Russia. Eds. L. M
Astronomy and Astrophysics
Astrophysics
Scientific paper
Non-local thermodynamical equilibrium (NLTE) line formation for neutral and singly-ionized calcium is considered through a range of spectral types when the Ca abundance varies from the solar value down to [Ca/H] = -5. Departures from LTE significantly affect the profiles of the Ca I lines. At [Ca/H] ≥ -2, NLTE abundance correction of individual lines may be small in absolute value due to the different influence of NLTE effects on the line wings and the line core. At lower Ca abundances, NLTE leads to systematically depleted total absorption in the line and positive abundance corrections, exceeding +0.5 dex for Ca I λ4226 at [Ca/H] = -4.9. In contrast, NLTE effects strengthen the Ca II lines and lead to negative abundance corrections. For the astrophysically important Ca II triplet at 8498 A, 8542 A, and 8662 A, they grow in absolute value with decreasing Ca abundance exceeding 0.4 dex in metal-poor stars with [Fe/H] ≤-3. Theoretical results are applied to determine Ca abundances in the Sun and seven metal-poor stars on the basis of plane-parallel LTE model atmospheres using high S/N and high-resolution spectra at visual and near-IR wavelengths. For each object, NLTE leads to consistent abundances from the lines of two ionization stages, Ca I and Ca II. The derived absolute solar Ca abundance (from Ca I and Ca II lines) is log\varepsilon_{Ca,&sun;} = 6.38±0.01. We inspect the possibility of using CaI / CaII line-strength ratios as indicators of surface gravity for extremely metal-poor stars.
Korn Andreas
Mashonkina Lyudmila
Przybilla Norbert
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