Autoionizing levels in Si II and identification of the 1400 Å feature of Ap-Si stars.

Abstract

The author reports four new high energy levels of Si II belonging to the core-excited terms 3s3p(¹P)3d²D⁰ and ²P⁰. They are determined from the classification of six laboratory UV emission lines. A broadening due to autoionization is detected for two of them. These results fix the energy of the strongly autoionizing 3s3p(¹P)3d²F⁰ level of Si II, to about ±1000 cm^-1, and lead to explain the broad 1400 Å absorption feature of Ap-Si stars as one strong transition toward this state. A synthetic calculation of this autoionizing line is compared with the IUE spectrum of the silicon star HD 34452.