On the search for transition zone lines in late A type stars.

Abstract

Observations of the 1400-1800 A region in late A type stars are presented, which were obtained using the IUE satellite at low and high resolution. The calibrated energy distribution is derived and presents a large variation with wavelength in the spectral range due to Si I photoionization discontinuities. Emission lines formed in the transition region do not appear, although upper limits on the emission in these lines are established. A schematic model is proposed for the outer layers in A type stars, which suggests that the chromosphere in solar type stars is likely absent in A stars, while the NLTE photosphere could be immediately connected to the transition region, joining the upper photosphere to the corona. In addition, the amount of matter in the transition zone is found to be small.