Transition probabilities and line ratios for Fe VI with fluorescent
excitation in planetary nebulae |

Guo Xin Chen and Anil K. Pradhan |

Astronomy & Astrophysics Supplement Series, 2000, Vol.147, pp.111-128 |

Relativistic atomic structure calculations for electric dipole (*E*1),
electric quadrupole (*E*2) and magnetic dipole (*M*1) transition
probabilities 80 fine-structure levels of Fe VI, dominated by
configurations 3d, 3d4s, and 3d4p, are carried out using the
Breit-Pauli version of the code SUPERSTRUCTURE. Experimental energies are
used to improve the accuracy of these transition probabilities. Employing
the 80-level collision-radiative (CR) model with these dipole and forbidden
transition probabilities, and Iron Project *R*-matrix collisional data,
we present a number of [Fe VI] line ratios applicable to spectral
diagnostics of photoionized H II regions. It is shown that continuum
fluorescent excitation needs to be considered in CR models in order to
interpret the observed line ratios of optical [Fe VI] lines in
planetary nebulae NGC 6741, IC 351 and NGC 7662. The analysis leads to
parametrization of line ratios as function of, and as of constraints of, the
electron density and temperature, as well as the effective radiation
temperature of the central source and a geometrical dilution factor. The
spectral diagnostics may also help ascertain observational uncertainties.
The method may be generally applicable to other objects with intensive
background radiation fields, such as novae and active galactic nuclei. The
extensive new Iron Project radiative and collisional calculations enable a
consistent analysis of many line ratios for the complex iron ions.