Atomic data for neutron-capture elements I. Photoionization and
  recombination properties of low-charge selenium ions
  We present multi-configuration Breit-Pauli AUTOSTRUCTURE calculations of
distorted-wave photoionization (PI) cross sections, and total and partial
final-state resolved radiative recombination (RR) and dielectronic
recombination (DR) rate coefficients for the first six ions of the trans-iron
element Se. These calculations were motivated by the recent detection of Se
emission lines in a large number of planetary nebulae. Se is a potentially
useful tracer of neutron-capture nucleosynthesis, but accurate determinations
of its abundance in photoionized nebulae have been hindered by the lack of
atomic data governing its ionization balance. Our calculations were carried out
in intermediate coupling with semi-relativistic radial wavefunctions. PI and
recombination data were determined for levels within the ground configuration
of each ion, and experimental PI cross-section measurements were used to
benchmark our results. For DR, we allowed dn=0 core excitations, which are
important at photoionized plasma temperatures. DR is the dominant recombination
process for each of these Se ions at temperatures representative of
photoionized nebulae (~10^4 K). To estimate the uncertainties of these data, we
compared results from three different configuration-interaction expansions for
each ion, and tested the sensitivity of the results to the radial scaling
factors in the structure calculations. We find that the internal uncertainties
are typically 30-50% for the direct PI cross sections and ~10% for the computed
RR rate coefficients, while those for low-temperature DR can be considerably
larger (from 15-30% up to two orders of magnitude) due to the unknown energies
of near-threshold autoionization resonances. The results are suitable for
incorporation into photoionization codes used to numerically simulate
astrophysical nebulae, and will enable robust determinations of nebular Se
abundances.
