Electron impact collision strengths in Si IX, Si X, and Si XI

Abstract

Electron impact collision strengths among 560 levels of Si IX, 320 levels of Si X, and 350 levels of Si XI have been calculated using the Flexible Atomic Code of Gu [M.F. Gu, Astrophys. J. 582 (2003) 1241]. Collision strengths X at 10 scattered electron energies, namely 10, 50, 100, 200, 400, 600, 800, 1000, 1500, and 2000 eV, are reported. Assuming a Maxwellian energy distribution, effective collision strengths Ç are obtained on a finer electron temperature grid of 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 MK, which covers the typical temperature range of astrophysical hot plasmas. Additionally, radiative rates A and weighted oscillator strengths gf are given for the more probable transitions among these levels. Comparisons of our results with available predictions reported in earlier literature are made and the accuracy of the data is assessed. Most transitions exhibit a good agreement, but large differences in gf appear for a few cases, which are due to the different configuration interactions included in different theoretical calculations. For excitations among levels of the ground and lower excited configurations, large discrepancies of Ç may have resulted from the consideration of resonance effects in earlier works.

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

“…2d) which connects the two sets. Excellent agreement is found between our background collision strengths and the DW results of Liang et al (2007).…”

“…This is most likely due to resonances attached to n = 3 which were not described by Keenan et al Furthermore, at high temperatures, we observed incongruous behaviour in their effective collision strengths for many dipole transitions. For the n = 2 → 3 excitation rates, the present results are higher than previously available Sampson 1994 andLiang et al 2007) non-resonant DW data, and can be so by more than an order of magnitude for many transitions at low temperatures ( < ∼ 5 × 10 5 K). Here, the difference peaks in the range 1 × 10 4 < ∼ T e (K) < ∼ 1 × 10 5 , and so is of likely importance for photoionized plasmas.…”

“…For excitation to levels of the n = 3 configurations, only DW calculations (Zhang & Sampson 1994;Liang et al 2007) are reported in the literature, to the best of our knowledge. The comparison with Zhang & Sampson's results indicate that there are 66% and 28% of the total (n = 3) transitions with the persent Υ being over 20% and a factor of 2 larger, respectively, at a temperature of 8.10 × 10 5 K (see Fig.…”

“…For example, in collision dominated plasmas, such as stellar coronae, supernova remnants etc., Si 9+ has a peak abundance in equilibrium at 1.3 × 10 6 K (Mazzotta et al 1998;Bryans et al 2006) using ionization Liang et al (2007). The inset panels illustrate the Maxwellian-averaged effective collision strength (Υ) from the present ICFT R-matrix (filled boxes linked by solid lines) and Liang et al's (2007) fac DW (filled circles linked by solid lines) calculations, and data we derived from the original Ω's of Zhang & Sampson (1994, denoted ZS94, opened star symbols).…”

“…For excitations to higher levels, belonging to n = 3 configurations, results of a DW calculation by Zhang & Sampson (1994) were included in the CHIANTI-5.2 database. By using a larger configuration interaction (CI) target, Liang et al (2007) calculated results for excitations up to n = 5 using the flexible atomic code fac of Gu (2003).…”