Electron impact excitation rates for transitions in Mg V

Energy levels, radiative rates (A-values), and lifetimes, calculated with the GRASP code, are reported for an astrophysically important O-like ion, Mg V. Results are presented for transitions among the lowest 86 levels belonging to the 2s²2p⁴, 2s2p⁵, 2p⁶, and 2s²2p³3ℓ configurations. There is satisfactory agreement with earlier data for most levels and transitions, but scope remains for improvement. Collision strengths are also calculated, with the DARC code, and the results obtained are comparable for most transitions (at energies above thresholds) with earlier work using the DW code. In the threshold region, resonances have been resolved in a fine energy mesh to determine values of effective collision strengths (Y) as accurately as possible. Results are reported for all transitions at temperatures up to 10⁶ K, which should be sufficient for most astrophysical applications. However, a comparison with earlier data reveals discrepancies of up to two orders of magnitude for over 60% of transitions, at all temperatures. The reasons for these discrepancies are discussed in detail. (author)