Atomic data and density diagnostics for S IV.

We present a new large-scale R-matrix scattering calculation for S iv. We used the intermediate-coupling frame transformation method and applied term energy corrections. Our calculation has a much larger configuration-interaction (CI) and close-coupling (CC) expansion than previous calculations. Despite that, we find good agreement in the predicted intensities of the decays from the three 3s 3p2 4P levels around 1400 Å, important for density diagnostics. A discrepancy between the observed and predicted intensity of the 1404.8 Å line, which is known to be blended at least with an O iv transition, is still present. Significant differences compared to previous models are found instead for the 1062.7 and 1073.0 Å lines, useful for diagnostics in low density plasma such as in nebulae. Several other significant differences were also found, concerning the population of the 3s 3p 3d 4F9/2 metastable level, and the intensities of several transitions