The solar abundance of calcium and collision broadening of Ca i- and Ca ii-Fraunhofer lines by hydrogen

Abstract

An abundance analysis of the solar calcium spectrum is carried out using 46 lines with knownfivalues in the visible and near infrared spectral region. Resonance, forbidden and autoionizing lines are included. The solar abundance of calcium resulting from the 25 weaker, nearly dampingindependent lines only is log eca = 6.36 • 0.07, on the scale log e~ = 12. The great variety of transitions involved in the solar calcium spectrum, ranging from 0 eV lines of CaI to 7.5 eV lines of Cau and including autoionizing lines, are in reasonable agreement (Figure 1 b). Therefore notable non-LTE effects on their equivalent widths can be excluded.Together with the sodium abundance log ez~ = 6.30 determined earlier, the solar abundance ratio Ca/Na = 1.15 is obtained with an accuracy of ~ 10%. Comparison with meteorites (carbonaceous chondrites I) shows that solar and meteoritic ratio agree within these limits.The line broadening by collisions with hydrogen atoms is determined empirically from a comparison of weak and strong Fraunhofer lines of CaI and Call, thereby using the solar atmosphere as an absorption tube of comparatively well-known physical state. The damping half-widths 7J~ turn out to be larger than predicted from pure van der Waals interaction, the average enhancement factor being 3.0 for CaI and 1.7for Cau, independent of term properties to a first approximation. Regardless of the reason for this enhancement -inaccurate van der Waals theory or predominance of repulsive interactionthese results can be used in the spectroscopy of other stars.

Committee 3: Collision Cross Sections and Line Broadening

“…There is now much experimental and theoretical evidence that the broadening due to collisions with light atoms like H and He does not arise from the Van der Waals interaction but from a shorter ranger interaction. This is true at low thermal temperature for experiments with He (7,8,9,10,11,12) and is a fortiori true at stellar temperatures for the broadening by H (13,14). This has been confirmed by recent detailed calculations (15,16).…”

Differential and integrated cross sections for the electron excitation of the 41Postate of calcium atom

“…The first detailed study of calcium lines in the solar spectra was published by Holweger [4]. Many papers have been published in which the presence of Ca lines in solar and stellar spectra was confirmed.…”

Electron-impact excitation and ionization of atomic calcium at intermediate energies

“…Calcium, one of the α elements, has observable lines along a broad range of wavelengths in late-type stellar spectra. The first detailed study of calcium lines in solar spectra was published by Holweger [2]. The presence of Ca lines in both solar and stellar spectra was confirmed in many subsequent papers.…”