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R-matrix electron-impact excitation data for B-like Si and its application in cool stars

Liang, G.Y. and Whiteford, A.D. and Badnell, N.R. (2009) R-matrix electron-impact excitation data for B-like Si and its application in cool stars. Astronomy and Astrophysics, 499 (3). pp. 943-954. ISSN 0004-6361

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We present the results of an intermediate coupling frame-transformation R-matrix calculation for the electron-impact excitation of Si9+. The target and close-coupling expansions are both taken to be the 125 fine-structure levels (58 LS terms) belonging to the configurations 2sx2py (x+y = 3) and 2s2p3l (+ = 2, l = s, p, and d). Due to the additional resonances included in our calculation, we find significant differences at low temperatures with the widely used n = 2 2 excitation rates, also obtained with the R-matrix method, as well as with the n = 2 3 excitation rates calculated by using the distorted wave (DW) approximation. We present a list of prominent transition lines and comparisons with SERTS and Hinode/EIS EUV spectra of the solar corona, SUMER observations for the quiet sun, as well as Chandra LETG and Rocket soft X-ray spectra of the Procyon corona and solar flares, respectively. Line emissivities of some transitions are enhanced up to 40% when compared with those obtained from using the previous atomic data at the same electron density (1.6 109 cm-3) and temperature (1.3 106 K). The comparison with Chandra LETG observation of Procyon reveals that the 3s-2p line flux was significantly underestimated (by a factor of 4-5) in previous analyses. Some EUV and soft X-ray emission line ratios are -sensitive and -insensitive. Estimated electron densities from them shift downwards due to the new resonant-enhanced excitation data used in the present modelling.