1 Introduction

Spectral lines arising from transitions within the n = 3 complex of Mg-like Fexv have been observed in solar active regions, corona and flares as far back as in the earlier pioneering work by Edlén (1942) and then by, among others, Cowan & Widing (1973), Behring et al. (1976), Doschek et al. (1976), Dere (1978, 1982), Vernazza & Reeves (1978), Widing & Cook (1987), Thomas & Neupert (1994) and Brosius et al. (1997a). Furthermore spatially resolved coronal EUV emission-line profiles of Fexv over a solar active region have been used to detect mass outflow in the low corona (Neupert et al. 1992). Images taken in the 284 Å line have been used by Brueckner (1983) to characterize the coronal magnetic field; and simultaneous observations in the EUV spectrum (including this line) and microwave images allow the mapping of the magnetic field spatial variations suggesting the presence of coronal electric currents (Brosius et al. 1997b). Lines belonging to Fexv have also been observed in EUV spectra of the Algol eclipsing binary (Stern et al. 1995), of non-supergiant B stars (Cassinelli 1994) and of the nearby K2 dwarf $\epsilon$ Eridani (Schmitt et al. 1996).

The usefulness of these lines in devising plasma diagnostics has been extensively discussed (Mason 1975; Kastner & Mason 1978; Dere et al. 1979; Bhatia & Kastner 1980; Dufton et al. 1990; Keenan et al. 1993; Brickhouse et al. 1995). However, the modeled lines are not always in agreement with observations (see, for instance, Dere at al. 1979 and Dufton et al. 1990). The latter authors propose as possible sources of error the exclusion of fluorescence effects or poor atomic data (i.e. poor electron excitation rates due to the neglect of relativistic effects or a limited target representation in the close-coupling calculation). In the case of solar flares Feldman et al. (1992) invoke the direct excitation of the 3s3p³P$^{\rm o}_{\rm J}$levels by inner-shell ionisation of the Al-like ion in order to interpret observations; however, the coronal heating rate from bursts obtained from such an approach seems to be larger than the accepted values.

Previous electron impact excitation collision strengths for this ion have been computed at a limited number of energy points by Bhatia & Kastner (1980), Mann (1983), Christensen et al. (CNP, 1985), Bhatia et al. (BMB, 1997) and Bhatia & Mason (BM, 1997) in the distorted wave approximation. Using the sparse tabulation by Christensen et al., Pradhan (1988) has estimated rate coefficients for a 16-level system in the electron temperature range $4.8\leq \log(T/K)\leq 7.6$.Rates computed (but not listed) by Dufton et al. (1990) from a 20-point tabulation of collision strengths in an 8-state close-coupling approximation are found to be in excellent agreement ($\sim$20% or better) with those by Pradhan. The common approach adopted in all these calculations implies the assumption, as stated by Dufton et al. and BMB, that the contribution from resonances can be neglected at the high temperatures of the solar medium. The validity of this assumption has never been really tested in the case of Fexv, in particular for the non-allowed transitions that usually display small background cross sections. Some of these transitions, even though their collisional and radiative rates are considerably smaller than those of their optically allowed counterparts, are nonetheless of astrophysical interest.

In the present work we have calculated effective collision strengths (excitation rates) for the n=3 transitions of Fexv in the close-coupling approximation (Burke & Seaton 1971). Particular care is taken to resolve the resonance structure and to ensure the convergence of the partial wave expansion which can be tricky for both allowed and forbidden transitions. Relativistic effects are taken into account by means of a Breit-Pauli Hamiltonian. Therefore the present approach should lead to a more reliable collisional dataset for this ion. This work is performed as part of the systematic study of the atomic properties of Fe ions undertaken by the international collaboration known as the IRON Project (Hummer et al. 1993; Butler 1996; Mendoza 1999). A detailed comparison with the work of CNP, BMB, BM and Pradhan (1988) is also carried out in an attempt to assign accuracy ratings to the present dataset. The earlier work by Bhatia & Kastner (1980) and Mann (1983) has already been analysed by CNP and will not be included in the present discussion.