Center-Limb Effects on Solar Abundances.
Abstract
Several investigators have pointed out that deviations from local thermodynamic equilibrium (LTE) can introduce large differences between the electron temperature and the excitation temperature. Accordingly, for photospheric models that neglect deviations from LTE one might expect large discrepancies in the abundance of an element when the calculations are based on lines that are formed in different layers of the photosphere, i.e.,lines observed at different points on the solar disk and lines of different excitation potentials. The purpose of this investigation has been to determine whether or not such discrepancies exist for elements in the iron group, and if they do exist, to determine their magnitude. The elements of the iron group were chosen for investigation because (a) for these elements a number of faint and medium-strong lines of different excitation potential are observed in the solar spectrum, and (b) reliable experimental relative f values are available. The lines were observed at the three positions on the solar disk ~-- 1.0, 0.5, and 0.3 with the vacuum spectrograph at the MeMath-Hulbert Observatory by Dr. Mohler and his associates. The equivalent widths of blended lines were determined by the Utrecht method. A homogeneous model of the photosphere, assuming LTE, was employed in which the recently observed temperature distributions are included. The theoretical calculations of the equivalent widths by the weighting function method were carried out with the aid of the IBM 704 computer of The University of Michigan. Individual curves of growth were calculated for each element for various sets of wavelengths and excitation potentials. The observed lines of each element were separated into groups according to their excitation potentials. For each element and for each position on the solar disk, the individual groups of lines were plotted separately on a curve of growth diagram and individual abundances derived. No systematie variation of the abundances from center to limb or from lower to higher excitation lines could be detected. This result indicates that the assumption of LTE in the photospheric model does not introduce great discrepancies in the solar abundances of the elements in the iron group. This work was supported in part by Contract Nonr-1224(19), NR 015-320 with the Office of Naval Research.
- Publication:
-
The Astronomical Journal
- Pub Date:
- 1962
- DOI:
- 10.1086/108668
- Bibcode:
- 1962AJ.....67T.277M