  Recent atomic physics calculations for Si II are employed within the Cloudy
modelling code to analyse Hubble Space Telescope (HST) STIS ultraviolet spectra
of three cool stars, Beta-Geminorum, Alpha-Centauri A and B, as well as
previously published HST/GHRS observations of Alpha-Tau, plus solar quiet Sun
data from the High Resolution Telescope and Spectrograph. Discrepancies found
previously between theory and observation for line intensity ratios involving
the 3s$^{2}$3p $^{2}$P$_{J}$--3s3p$^{2}$ $^{4}$P$_{J^{\prime}}$
intercombination multiplet of Si II at 2335 Angs are significantly reduced, as
are those for ratios containing the 3s$^{2}$3p $^{2}$P$_{J}$--3s3p$^{2}$
$^{2}$D$_{J^{\prime}}$ transitions at 1816 Angs. This is primarily due to the
effect of the new Si II transition probabilities. However, these atomic data
are not only very different from previous calculations, but also show large
disagreements with measurements, specifically those of Calamai et. al. (1993)
for the intercombination lines. New measurements of transition probabilities
for Si II are hence urgently required to confirm (or otherwise) the accuracy of
the recently calculated values. If the new calculations are confirmed, then a
long-standing discrepancy between theory and observation will have finally been
resolved. However, if the older measurements are found to be correct, then the
agreement between theory and observation is simply a coincidence and the
existing discrepancies remain.
