Electron-impact excitation of Cr?ii - A theoretical calculation of collision and effective collision strengths for forbidden transitions

摘要

Context. Absorption or emission lines of Cr?ii are observed in a wide variety of astrophysical spectra and accurate atomic data are urgently needed to interpret these lines. Many of these data are impossible to measure experimentally and a full theoretical treatment is the only means by which these data can be obtained. Aims. In this paper, we present collision strengths and effective collision strengths for electron-impact excitation of Cr?ii for forbidden transitions among the lowest-lying 74 fine-structure levels. Effective collision strengths have been computed for 18 individual electron temperatures of astrophysical importance, ranging from 2000–100?000?K. Methods. The parallel suite of R-matrix packages, RMATRX II, which has recently been extended to allow for the inclusion of relativistic effects, has been used in the present work to compute the collision strengths and effective collision strengths for electron-impact excitation of Cr?ii. We concentrate in this publication on low-lying forbidden lines among the lowest 74 jj fine-structure levels with configurations 3d5 and 3d44s, although atomic data has been evaluated for all 39?060?transitions among the 280?jj levels of configurations 3d5, 3d44s and 3d44p. This work constitutes the largest evaluation ever performed for this ion involving 1932 coupled channels. Results. Collision and effective collision strengths are presented for all transitions among the lowest 74 Jπ states of Cr?ii and comparisons made with the work of Bautista et al. (2009). While the effective collision strengths agree well for some transitions, significant discrepancies exist for others. We believe that the present atomic data represents the most accurate, most sophisticated and most complete data set for electron-impact excitation of Cr?ii and we would recommend them to astrophysicists and plasma physicists in their application work. We would expect that the effective collision strengths presented for the important low-lying forbidden lines are accurate to within 15%.