1 Introduction

Previous calculations on the low-ionization stages of iron have been carried out by the group at Ohio State for Fe II (Zhang & Pradhan 1995b), Fe III (Zhang 1996), and Fe IV (Zhang & Pradhan 1997). Whereas these ionization states of iron are the dominant species in gaseous nebulae and photoionized H II regions in general, Fe VI emission lines could also form in somewhat hotter sources such as from hot white dwarfs (Jordan et al. 1995). The [Fe VI] lines are observed in the optical spectra of relatively hot H II regions, such as the high-excitation planetary nebula NGC 6741 studied by Hyung & Aller (1997). Numerical simulation of spectral emission from such complex atomic species requires non-local thermodynamic equilibrium (NLTE) calculations involving many atomic levels (Koester 1995).

With the exception of two calculations two decades ago, there are no other calculations of electron impact excitation of Fe VI in literature (see the review by Pradhan 1994). It is difficult to calculate the relativistic effects together with the electron correlation effects in this complex atomic system and coupled channel calculations necessary for such studies are very computer intensive. The two previous sources for the excitation rates of Fe VI are the close coupling (CC) calculations by Garstang et al. (1978) and the distorted-wave (DW) calculations by Nussbaumer & Storey (1978). Although the Garstang et al. (1978) calculations were in the CC approximation, they used a very small basis set and did not obtain the resonance structures; their results are given only for the averaged values. The Nussbaumer & Storey (1978) calculations were in the DW approximations that does not enable a treatment of resonances. Therefore neither set of calculations included resonances or the coupling effects due to higher configurations. Owing primarily to these factors we find that the earlier data are in lower by several factors (discussed later) when compared to the new Fe VI rates presented herein.

The earlier studies on Fe III (Zhang & Pradhan 1995a) and Fe IV (Zhang & Pradhan 1997) using the nonrelativistic (NR) and Breit-Pauli R-matrix (BPRM) methods show that the relativistic effects are small for the forbidden transitions between the low-lying levels, and that the resonances and the coupling effects arising from a large coupled-channel wave function expansion dominate the collision strengths. However, as the ion charge increases the relativistic effects become more significant. It is therefore necessary to carry out detailed calculations to determine precisely the extent of these effects in conjunction with the already complex electron correlation effects, as manifested in particular in autoionising resonances. Similar to the calculations described by Zhang (1996, IP Paper XVIII), we have carried out several sets of NR and BPRM calculations, including a 19-level BPRM calculation (or 19BP for brevity), a 8-term NR calculation (8CC for brevity), and a large 34-term (80 fine structure levels) NR CC calculation (hereafter 34CC) for Fe VI neglecting the relativistic intermediate coupling effects. In addition, we also investigate the effect on low-lying transitions by using the relativistic term coupling approximation (Eissner et al. 1974) that is often employed in close coupling and distorted wave calculations to deal with complicated ions where a full BP calculation may be difficult.

The Maxwellian-averaged rate coefficients or effective collision strengths are calculated and tabulated over a temperature range in which Fe VI is most abundant in astrophysical sources. A brief description of the computations and the results are given in the following section.

The present work is part of an international collaboration referred to as the IRON Project (Hummer et al. 1993, Paper I) to obtain accurate electron-impact excitation rates for fine-structure transitions in atomic ions. A full list of the papers in this Atomic Data from the IRON Project series published to-date is given in the references. A complete list of papers including those in press can be found at http://www.am.qub.uk/projects/iron/papers/, where abstracts are also given for each paper. Information on other works by the authors and collaborators, including photoionization and recombination of ions of iron and other elements, can be found at http://www-astronomy.mps.ohio-state.edu/$\sim$pradhan/.