Electron-impact excitation of Co²⁺

Electron-impact excitation cross sections and Maxwellian-averaged rate coefficients are calculated from the ground 3d⁷⁴F to the first 3d⁷⁴P and the second 3d⁷²G excited terms in Co²⁺ in close coupling with 8 and 27 terms; much larger transition arrays were first explored in a distorted-wave approximation. Both transitions are important elements of the emission spectrum of type Ia supernovae. We find that the two excitations are dominated by autoionizing resonances attached to higher-lying excited terms in the 3d⁷, 3d⁶4s, 3d⁶4p, and 3d⁶4d configurations. Those LS terms that are strongly excited from the ground 3d⁷⁴F, as found by distorted-wave calculations, are included in a large R-matrix calculation using asymptotics based on multichannel quantum defect theory.