[Paper Review] 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|ArXiv.org|Mar 27, 2000

Atomic and Molecular Physics8 citations

TL;DR

This paper presents relativistic Breit-Pauli atomic structure calculations for Fe VI, computing E1, E2, and M1 transition probabilities for 80 fine-structure levels. Using a collision-radiative model with fluorescent excitation, it demonstrates that including continuum fluorescent excitation is essential for accurately interpreting [Fe VI] line ratios in planetary nebulae such as NGC 6741, IC 351, and NGC 7662, enabling improved diagnostics of electron density, temperature, and central source radiation temperature with constraints on observational uncertainties.

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 3d^3, 3d^24s, and 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.

Motivation & Objective

• To compute accurate electric dipole (E1), electric quadrupole (E2), and magnetic dipole (M1) transition probabilities for Fe VI using relativistic Breit-Pauli atomic structure calculations.
• To incorporate fluorescent excitation from a UV continuum into a collision-radiative (CR) model to improve spectral line ratio predictions in photoionized environments.
• To enable consistent diagnostics of electron density, temperature, and radiation field properties in planetary nebulae using observed [Fe VI] line ratios.
• To assess and quantify observational uncertainties in line intensities by comparing model predictions with observations.
• To provide a generalizable method applicable to other astrophysical sources with intense radiation fields, such as novae and active galactic nuclei.

Proposed method

• Relativistic Breit-Pauli atomic structure calculations were performed using the SUPERSTRUCTURE code for 80 fine-structure levels of Fe VI dominated by 3d³, 3d²4s, and 3d²4p configurations.
• Experimental energy levels were used to improve the accuracy of computed transition probabilities (A-values).
• A 80-level collision-radiative (CR) model was constructed, incorporating both electron impact excitation (EIE) and fluorescent excitation (FLE) via a background UV continuum.
• R-matrix collisional data from the Iron Project were used for electron impact excitation rate coefficients.
• Line ratios were computed as functions of electron density, electron temperature, effective radiation temperature, and geometrical dilution factor to match observed nebular spectra.
• The model was applied to observed [Fe VI] line ratios in NGC 6741, IC 351, and NGC 7662 to infer physical conditions and assess observational errors.

Experimental results

Research questions

• RQ1 Can fluorescent excitation from a UV continuum significantly affect the predicted intensities of [Fe VI] optical emission lines in planetary nebulae?
• RQ2 To what extent do observed [Fe VI] line ratios in NGC 6741, IC 351, and NGC 7662 require inclusion of fluorescent excitation in CR models for consistent interpretation?
• RQ3 Can the effective radiation temperature of the central star and the emission region distance (via dilution factor) be constrained using [Fe VI] line ratios when FLE is included?
• RQ4 How do discrepancies between observed and predicted line ratios help identify and quantify observational uncertainties in line intensities?
• RQ5 Can the developed method be generalized to other astrophysical sources with strong radiation fields, such as novae and AGN?

Key findings

• Including fluorescent excitation in the CR model is essential for accurately interpreting observed [Fe VI] line ratios in planetary nebulae such as NGC 6741, IC 351, and NGC 7662.
• For NGC 6741, the observed intensity of the 5146 Å line should be increased by 80% to match model predictions, and the 5485 Å line should be decreased by 35% to achieve consistency.
• The line 5677 Å requires an 110% increase in observed intensity to match the model, indicating significant observational uncertainty.
• The electron density in the [Fe VI] region of NGC 6741 is constrained to approximately 3,000 cm⁻³ based on consistent line ratio analysis.
• The method enables estimation of both electron temperature and effective radiation temperature of the central source, with the latter constrained via the dilution factor.
• The model provides a framework to assess observational errors in line intensities by comparing predicted and observed line ratios across multiple transitions.