The Iron Project: R-Matrix calculations for opacities~II.: Photoionization and oscillator strengths of iron ions Fe~XVII, Fe~XVIII, and Fe~XIX

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 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 $\sim$ $2.11 \times 10^6$K and electron density N$_e$ = $3.1 \times 10^{22}$cc. 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, 1174 levels for Fe XVIII, and 1626 for Fe XIX~up to $n\leq$ 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 wide Seaton resonances due to PEC (photoexcitation-of-core) formed via high-lying core excitations with $\Delta n=1$ that significantly impact bound-free opacity.