A Study of the Determination of Solar Atmospheric Abundances

Abstract

A study is made of the determination of solar atmospheric abundances from the observed intensities of faint Fraunhofer lines. The method of weighting functions, which utilizes the results of the observed limb-darkening, is considered in detail. An alternative method is proposed, the method of the Planckian gradient, which makes no recourse to limb-darkening observations and which is entirely dependent upon the physical characteristics of the solar atmospheric model considered. Throughout the paper reference is made to the case of nitrogen (ionization potential = 14.54 volts) primarily to compare results obtained by the above two methods. Nine absorption lines are considered and the nitrogen abundance is determined from each of the nine lines. Appreciable discrepancies are found between the results obtained by the two methods. Theoretical considerations lead to the conclusion that in the case of nitrogen, and in general for all elements of high ionization potential, the method of weighting functions is quite unreliable and the Planckian gradient method should be used. Three atmospheric models are considered, and, for nitrogen, the mean results obtained by the latter method are (on the basis log AH = 12.00) Claas model: log AN = 7.71, Vitense model: log AN = 7.81, Swihart model: log AN = 7.93 (cf. Hunaerts 9.02, Unsöld 8.61).