Identification of the 1600A feature in Lambda Bootis stars

Abstract

The nature of the absoprtion feature near 1600A in λ Bootis stars, which is one of the strongest features in these objects, is a longstanding puzzle. In this study we demonstrate that it is caused by quasimolecular absorption leading to a satellite in the Lyman α profile due to perturbations by neutral hydrogen. This feature has also been observed in the hydrogen-rich DA white dwarfs. The satellite is not observable in normal A stars because of the extremely strong distortion of the UV flux by metal lines, especially of C, Si, and Fe. Although the C abundance seems to be close to solar in λ Bootis stars, the Si, Fe, and other metal abundances are sufficiently reduced to make the feature visible. For the same reason the feature is conspicuous in metal-deficient horizontal branch stars of spectral type A.