Abstract
Iron is the dominant heavy element that plays an important role in radiation transport in stellar interiors. Owing to its abundance and large number of bound levels and transitions, iron ions determine the opacity more than any other astrophysically abundant element. A few iron ions constitute the abundance and opacity of iron at the base of the convection zone (BCZ) at the boundary between the solar convection and radiative zones, and are the focus of the present study. Together, Fe xvii , Fe xviii and Fe xix contribute 85% of iron ion fractions 20%, 39% and 26% respectively, at the BCZ physical conditions of temperature T ∼ 2.11 × 106K and electron density Ne = 3.1 × 1022cc. We report heretofore the most extensive R-matrix atomic calculations for these ions for bound-bound and bound-free transitions, the two main processes of radiation absorption. We consider wavefunction expansions with 218 target or core ion fine structure levels of Fe xviii for Fe xvii , 276 levels of Fe xix for Fe xviii , in the Breit-Pauli R-matrix (BPRM) approximation, and 180 LS terms (equivalent to 415 fine structure levels) of Fe xx for Fe xix calculations. These large target expansions which includes core ion excitations to n=2,3,4 complexes enable accuracy and convergence of photoionization cross sections, as well as inclusion of high lying resonances. The resulting R-matrix datasets include 454 bound levels for Fe xvii , 1,174 levels for Fe xviii , and 1,626 for Fe xix up to n ≤ 10 and l=0 - 9. Corresponding datasets of oscillator strengths for photoabsorption are: 20,951 transitions for Fe xvii , 141,869 for Fe xviii , and 289,291 for Fe xix . Photoionization cross sections have obtained for all bound fine structure levels of Fe xvii and Fe xviii , and for 900 bound LS states of Fe xix . Selected results demonstrating prominent characteristic features of photoionization are presented, particularly the strong Seaton PEC (photoexcitation-of-core) resonances formed via high-lying core excitations with ∆n = 1 that significantly impact bound-free opacity.
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