New Atomic Data for Trans-Iron Elements and Their Application to
  Abundance Determinations in Planetary Nebulae
  [Abridged] Investigations of neutron(n)-capture element nucleosynthesis and
chemical evolution have largely been based on stellar spectroscopy. However,
the recent detection of these elements in several planetary nebulae (PNe)
indicates that nebular spectroscopy is a promising new tool for such studies.
In PNe, n-capture element abundance determinations reveal details of s-process
nucleosynthesis and convective mixing in evolved low-mass stars, as well as the
chemical evolution of elements that cannot be detected in stellar spectra. Only
one or two ions of a given trans-iron element can typically be detected in
individual nebulae. Elemental abundance determinations thus require corrections
for the abundances of unobserved ions. Such corrections rely on the
availability of atomic data for processes that control the ionization
equilibrium of nebulae. Until recently, these data were unknown for virtually
all n-capture element ions. For the first five ions of Se, Kr, and Xe -- the
three most widely detected n-capture elements in PNe -- we are calculating
photoionization cross sections and radiative and dielectronic recombination
rate coefficients using the multi-configuration Breit-Pauli atomic structure
code AUTOSTRUCTURE. Charge transfer rate coefficients are being determined with
a multichannel Landau-Zener code. To calibrate these calculations, we have
measured absolute photoionization cross sections of Se and Xe ions at the
Advanced Light Source synchrotron radiation facility. These atomic data can be
incorporated into photoionization codes, which we will use to derive ionization
corrections (hence abundances) for Se, Kr, and Xe in ionized nebulae. These
results are critical for honing nebular spectroscopy into a more effective tool
for investigating the production and chemical evolution of trans-iron elements
in the Universe.
