Photoionization resonances of SiII in stellar spectra.

Abstract

We have studied the influence of the Si^+^ photoionization on stellar atmospheres. Recent calculations using the R-matrix code provide detailed cross-sections with autoionization resonances for the first 50 energy levels of Si^+^. These photoionization cross-sections are included in the calculation of the far ultraviolet synthetic spectrum of A and late B-type stars. The opacity from the six lowest excited states of Si^+^ provides observable features in the emergent spectrum of a solar-composition A0 star. The photoionization from higher states of Si^+^ should be included when modeling hotter or silicon-rich stars. These new cross-sections increase dramatically the opacity in the far UV spectrum of Ap Si stars and allow to reproduce the most characteristic UV features of these stars, as illustrated by a comparison with IUE data for the hot Ap Si star HD 34452. Broad features in its spectrum are definitely assigned to Si ii autoionization resonances, and the flux deficiency below 130nm is well matched with a predicted continuum edge near this wavelength. The successful stellar spectrum synthesis brings a strong support to the R-matrix photoionization calculations, which cannot otherwise be compared to laboratory data.