Hyperfine structure and interstellar curves of growth

Abstract

The effects of the hyperfine structure (HFS) that is present in some interstellar absorption lines are investigated in the case of a single absorbing cloud. If the respective total equivalent widths of two or more unresolved HFS multiplets measured in relatively low-resolution spectra are analysed specifically by means of a curve of growth, the column density N(X) and the linewidth parameter b(X) inferred for absorbing species X in the cloud will generally be in error if the HFS is ignored. The fundamental physical effect is the reduced line saturation that arises because the total column density is divided among the HFS levels of the ground atomic level, each of which generally gives rise to an HFS line at a different wavelength. For nuclear spins / = 3/2 and / = 5/2, theoretical curves of growth are calculated for some of the resonance lines of some alkali atoms, for each of four illustrative choices of the parameter α = A/b, the ratio of the HFS splitting in the ground atomic level to the linewidth. Applications of the results to interstellar absorption by NaI, KI and Al III are emphasized. HFS is, fortunately, unimportant for most interstellar lines, however. Among the 35 elements that have been detected in diffuse clouds via interstellar absorption in the ultraviolet/optical spectral region, the most abundant isotope of each of 25 of these shows no HFS, because I = 0 or, in the relevant ground atomic level, J = 0.