Electron-ion Recombination of Fe XII forming Fe XI: Laboratory
  Measurements and Theoretical Calculations
  We have measured electron-ion recombination for Fe XII forming Fe XI using a
merged beams configuration at the heavy-ion storage ring TSR located at the Max
Planck Institute for Nuclear Physics in Heidelberg, Germany. The measured
merged beams recombination rate coefficient (MBRRC) for collision energies from
0 to 1500 eV is presented. This work uses a new method for determining the
absolute MBRRC based on a comparison of the ion beam decay rate with and
without the electron beam on. For energies below 75 eV, the spectrum is
dominated by dielectronic recombination (DR) resonances associated with 3s-3p
and 3p-3d core excitations. At higher energies we observe contributions from
3-N' and 2-N' core excitations DR. We compare our experimental results to
state-of-the-art multi-configuration Breit-Pauli (MCBP) calculations and find
significant differences, both in resonance energies and strengths. We have
extracted the DR contributions from the measured MBRRC data and transformed
them into a plasma recombination rate coefficient (PRRC) for temperatures in
the range of 10^3 to 10^7 K. We show that the previously recommended DR data
for Fe XII significantly underestimate the PRRC at temperatures relevant for
both photoionized plasmas (PPs) and collisionaly ionized plasmas (CPs). This is
to be contrasted with our MCBP PRRC results which agree with the experiment to
within 30% at PP temperatures and even better at CP temperatures. We find this
agreement despite the disagreement shown by the detailed comparison between our
MCBP and experimental MBRRC results. Lastly, we present a simple parameterized
form of the experimentally derived PRRC for easy use in astrophysical modelling
codes.
