Silicon lines as spectral diagnostics - The effect of charge transfer

Abstract

The paper discusses the importance of charge transfer as an ionization source in astrophysical plasmas. The effect of the rapid charge transfer reactions Si(+) + H(+) yields Si(+2) + H - 2.74 eV and SI(2+) + He(+) yields Si(+3) + He - 8.88 eV on the ionization equilibrium of silicon is calculated. The dominant ionization stage of silicon in a coronal plasma is shown to be Si(+2) for temperatures as low as 20,000 K in contrast to the temperature of 35,000 K determined from earlier ionization equilibrium calculations. The Si(+3) abundance is also substantially modified. For the quiet-sun transition region, the effect of charge transfer upon the emissivity of the Si III lambda 1892 emission line is calculated. The peak emissivity occurs at a temperature a factor of 2 lower and is an order of magnitude higher than implied by calculations neglecting the silicon charge-transfer process. The results have widespread importance for the interpretation of solar, stellar, interstellar, and extragalactic silicon lines.