Astron. Astrophys. Suppl. Ser. 147, 111-128

Atomic data from the Iron Project

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

Guo Xin Chen - Anil K. Pradhan

Send offprint request: Guo Xin Chen

Department of Astronomy, The Ohio State University, Columbus, OH 43210, U.S.A.
e-mail: chen@astronomy.ohio-state.edu

Received March 8; accepted August 22, 2000

Abstract:

Relativistic atomic structure calculations for electric dipole (E1), electric quadrupole (E2) and magnetic dipole (M1) transition probabilities among the first 80 fine-structure levels of Fe VI, dominated by configurations ${\rm 3d^3}, {\rm 3d^24s}$, and ${\rm 3d^24p}$, 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 constraints on, 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.

Key words: atomic data -- line formation -- stars: white dwarfs -- ISM: H II regions: planetary nebulae