Si II emission line diagnostics

Ratios of Si II UV emission lines in a variety of objects are compared with computations based upon recently published collisional data of Dufton & Kingston. We analyse a high-quality spectrum of α Tau (K5 III) obtained by Carpenter et al. using the G270M mode of the GHRS on the Hubble Space Telescope, in addition to high-dispersion data from Skylab and IUE. Agreement betwen observation and theory is satisfactory for most lines with the exception of ratios within the 3s²3p²P⁰ → 3s3p^2,4P (UV 0.01) multiplier near 2335 Å. A blend with a line of Ni II accounts partially for the discrepancies and solves a problem with Si II) emission measures. However, a further problem remains which suggests that the computed collision strengths for the transitions 3s²3p²P⁰_1/2,3/2 → 3s3p^2,4P_5/2 may be too large by a factor of about 1.5.