Atomic data for interpreting stellar spectra: isotopic and hyperfine data

Abstract

The quality of solar, stellar, and interstellar observations is now so high that we can see physical effects that have been assumed to be insignificant. In natural isotopic mixtures essentially all atomic lines are asymmetric because of isotopic and hyperfine components. In astrophysics the effects of isotopic components have been generally ignored in the case of Ti, Cr, Fe and Ni. Ignoring isotopic splitting introduces the following systematic errors: (1) the use of bisectors to determine velocity fields is incorrect; (2) Fourier analysis of line profiles to determine rotational, microturbulent, and macroturbulent velocities is incorrect; and (3) damping constants are overestimated. When the stronger components become saturated, the weak components are still on the linear part of the curve of growth. This introduces the additional systematic errors; (4) wavelengths of saturated lines are shifted; and (5) abundances determined from equivalent widths are wrong. In addition the Doppler width and Voigt profile of each isotope are significantly different. It is imperative that laboratory measurements be made to determine the isotopic splitting of every energy level of every stable isotope of every atom and ion as is now routinely done for actinides and for diatomic molecules. Hyperfine splitting should be measured as well for those isotopes with nuclear spin. Russia, the United States, and other countries should stockpile pure stable isotopes so they will be available for these measurements. Computer programs for analyzing spectra including the hyperfine and isotopic splitting already exist. We now need the laboratory data.