Atomic data for opacity calculations. XXIII. The aluminium isoelectronic sequence

Abstract

As a contribution to the international Opacity Project, large numbers of term energies, oscillator strengths and photoionization cross sections have been computed for Al-like ions of astrophysical interest (Al I, Si II, S IV, Ar VI, Ca VIII and Fe XIV), using an ab initio close-coupling approach in LS-coupling. Careful comparisons with previous theoretical and experimental work show that our collisional approach allows a systematic and accurate description of plunging configurations, series perturbations and cancellations typical of these three-valence-electron systems. Even for highly ionized systems such as Fe XIV, the neglect of relativistic effects introduces limited inaccuracies ( approximately 5%) in the average term energies, and the gross features of the distribution of oscillator strengths are accurately delineated. The main contribution of the present study lies in the volume of accurate new data made available to the public through the TOPbase atomic database: 1935 computed multiplets compared to 225 observed ones, 56242 oscillator strengths and photoionization cross sections from 1296 initial states.