A spectroscopic study of 14 Comae and other A-type shell stars

The physical conditions in the circumstellar shells of the well-known shell star 14 Comae and similar stars are investigated. A shell line list is presented, and the column densities of shell ions are obtained. It is found that the shell material lies approximately three stellar radii above the stellar photosphere. There is little evidence for rapid expansion or collapse of the shell of 14 Com, but for two other stars shell column densities have changed markedly on a time scale of about 2 years. Excitation and ionization equilibrium calculations indicate that shell hydrogen is predominantly neutral. The position of late A-type shell stars in the H-R diagram is consistent with a view that all rapidly rotating evolved A stars have the potential to become shell stars.