Atomic and Molecular Negative Ions in Stellar Atmospheres

Late-type stars show continuous absorption from other sources besides the H ^– ion. These sources begin to appear in the Sun, and increase in strength towards later spectral types, especially in carbon stars. On the basis of recent theoretical and laboratory work on atomic and molecular negative ions, the concentrations of $$\text{C}^{-},\,\text{O}^{-},\,\text{OH}^{-}\,\text{and}\,\text{CN}^{-}$$ are roughly calculated in giants and dwarfs of various compositions and their contributions to the continuous absorption coefficient assessed. O ^– is generally unimportant; C ^– is likely to be important in stars with very high C abundance such as R Cr B ; OH ^– may be important in the late M and S stars; and CN ^– perhaps contributes more than C ₃ to the violet absorption in late N stars. Observational verification of these conclusions will require accurate spectrophotometry over a wide range of wave-length, in addition to further laboratory work.