Atomic data for astrophysics: Fe xii soft X-ray lines^⋆

G. Del Zanna¹, P. J. Storey², N. R. Badnell³ and H. E. Mason¹

¹ DAMTP, Centre for Mathematical Sciences, Wilberforce Road, Cambridge, CB3 0WA, UK
e-mail: g.del-zanna@damtp.cam.ac.uk
² Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
³ Department of Physics, University of Strathclyde, Glasgow, G4 0NG, UK

Received: 12 February 2012
Accepted: 30 May 2012

Abstract

We present new large-scale R-matrix (up to n = 4) and distorted-wave (DW, up to n = 6) scattering calculations for electron collisional excitation of Fe xii. The first aim is to provide accurate atomic data for the soft X-rays, where strong decays from the n = 4 levels are present. As found in previous work on Fe x, resonances attached to n = 4 levels increase the cross-sections for excitations from the ground state to some n = 4 levels, when compared to DW calculations. Cascading from higher levels is also important. We provide a number of models and line intensities, and list a number of strong unidentified lines. The second aim is to assess the effects of the large R-matrix calculation on the n = 3 transitions. Compared to our previous (n = 3) R-matrix calculation, we find overall excellent agreement to within a few percent, however a few key density diagnostic EUV intensities differ by about 60% at coronal densities. The new atomic data result in lower electron densities, resolving previous discrepancies with solar observations.