Spectral Diagnoses of Chromospheres and Winds in A-Type Stars

Abstract

AbstractSo far, neither chromospheres nor stellar winds have been directly detected in main-sequence A stars. While radiative diffusion requires extremely weak stellar winds to reproduce chemical anomalies (10−15 to 10−12M⊙yr−1), two independent direct searches for mass loss set up upper limits to 10−10 M⊙yr−1, which is still several orders of magnitude higher. We discuss some new recent possibilities to detect chromospheres which arise thanks to new NLTE model atmospheres. In the near future, some progress is also expected from new observations of Lyman α with HST and from the increased sensitivity of ROSAT in the X-ray domain.