2016MNRAS.455.3405L

Recent atomic physics calculations for Siii are employed within the cloudy modelling code to analyse Hubble Space Telescope (HST) STIS ultraviolet spectra of three cool stars, β Geminorum, α Centauri A and B, as well as previously published HST/GHRS observations of α 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 3s23p 2PJ-3s3p2 4P_J^′ intercombination multiplet of Siii at ∼ 2335 Å are significantly reduced, as are those for ratios containing the 3s23p 2PJ-3s3p2 2D_J^′ transitions at ∼ 1816 Å. This is primarily due to the effect of the new Siii 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. for the intercombination lines. New measurements of transition probabilities for Siii 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.