Ultraviolet emission lines of Si II in cool star and solar spectra

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, 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-3s3p^{2 4}P^J' intercombination multiplet of Si II at ~ 2335 Å are significantly reduced, as are those for ratios containing the 3s²3p ²PJ-3s3p^{2 2}DJ ~ transitions at ~1816 Å. 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. 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.

Measurement of Fast Electrons Spectra Generated by Interaction between Solid Target and Peta Watt Laser

Fast electron energy spectra have been measured for a range of intensities between 1018 Wcm−2 and 1021 Wcm−2 and for different target materials using electron spectrometers. Several experimental campaigns were conducted on peta watt laser facilities at the Rutherford Appleton Laboratory and Osaka University. In these experimental campaigns, the pulse duration was varied from 0.5 ps to 5 ps. The laser incident angle was also changed from normal incidence to 40° in p-polarized. The results show a reduction from the ponderomotive scaling on fast electrons over 1020 Wcm−2.