Near-Threshold Dielectronic Recombination Studies of Si-Like Ions

We present results of dielectronic recombination (DR) calculations for the Si-like isoelectronic sequence and the important S$^{2+}$ case in particular. A perturbative, multi-configuration approach is used, and uncertainties in the energy positions of low-lying resonances are investigated. Multi-configuration Hartree-Fock calculations are also performed for energy positions of near-threshold bound and resonance states. This work is motivated by the astrophysical importance of the S$^{2+}$ DR rate in determining the sulfur ionization balance in the Orion nebula, a photoionized plasma corresponding to low-energy electrons. The computed DR rate coefficients comprise part of the assembly of the DR data base required in the modeling of dynamic finite density plasmas.