1 Introduction

Intercombination transitions are observed in the spectra of a wide variety of astronomical bodies where they may be used as plasma diagnostics to estimate the electron density, temperature and chemical abundances. In this context, the intercombination transitions involving the ${\rm 2s2p}\sp3$$\sp5{\rm S}\sp{\rm o}_2$ metastable level in ions of the carbon isoelectronic sequence, in particular those down to the ${\rm 2s}\sp2{\rm 2p}\sp2\ \sp3{\rm P}_{1,2}$ ground term, are among the most interesting in laboratory astrophysics. Their complex properties present an opportunity to test the state of the art in observation, theory and experiment. For instance, by making use of a systematic configuration interaction method (SCIV3), Brage et al. (1997) and Fleming & Brage (1997) have recently reported that in N II and O III the branching ratio

$$B={A(\sp5{\rm S}\sp{\rm o}_2\ - \ \sp3{\rm P}_2)\over A(\sp5{\rm S}\sp{\rm o}_2\ -\ \sp3{\rm P}_1)}$$ (1)

shows a reduction of approximately $20\%$ from the expected value of $\sim 3$ for low Z (Ellis & Martinson 1984), which Fleming & Brage have attributed in the case of O III to the contribution from the relativistic two-body Breit interaction. This is an important conclusion because the Breit formulation although assumed correct has not been fully verified by experiment. Since B is a measurable quantity and the departure from the first-order value is large, an experimental benchmark would be invaluable. A recent measurement of the 2143/2139 Å emission doublet from N II in a low-pressure inductively coupled plasma by Curry et al. (1997) resulted in a value for B of $2.27\pm 0.23$, somewhat lower than obtained by the SCIV3 method of Brage et al. (1997), but in agreement within the experimental errors. Moreover, the former compare their results with the discordant values by Musielok et al. (1996) and Bridges et al. (1996) obtained in an atmospheric pressure wall-stabilised arc discharge ($B=2.24\pm 0.06$ and $B=2.45\pm 0.04$, respectively). Following a discussion of the experimental difficulties, Curry et al. favour their low-pressure source and a higher resolution (compared to other experiments) as it permits to reduce line-broadening and unfold weak blends. The situation is similar for the radiative lifetimes of the $\sp5$S$\sp{\rm o}_2$ level along the sequence. In the case of N II and O III, there is excellent agreement (better than 1%) between the theoretical estimates by Brage et al. (1997) and Fleming & Brage (1997) and the values that have emerged from the ion-trap experiments (Calamai & Johnson 1991; ,Johnson et al. 1984, 1991), which contrasts with the wide scatter (as large as a factor of 2) found in previous calculations. However, a recent and very accurate ($\sim$0.5%) measurement by Träbert et al. (1998) for N II in a heavy-ion storage ring results in a significantly higher value thus leaving the pursuit of the benchmark still open.

The emission from N II($\sp5{\rm S}\sp{\rm o}_2$) is an important feature in the Earth's aurora and dayglow (Torr & Torr 1985; Siskind & Barth 1987; Bucsela & Sharp 1989) where the level is populated by photodissociative ionization of N₂ (Dalgarno et al. 1981; Victor & Dalgarno 1982). From the observed doublet Bucsela & Sharp have obtained a value of $B=1.72\pm 0.24$which is significantly lower than both the latest laboratory and theoretical estimates. The O III doublet at 1666/1600 Å is frequently observed in astronomical sources of low to medium density, and in some conditions the branching ratio is found to depend on optical depth (Kastner & Bhatia 1989). Furthermore, in the case of symbiotic stars the observed O III intercombination ratios are noticeably higher than the theoretical value; this unusual effect has been interpreted by Kastner et al. (1989) as the result of Bowen pumping. The corresponding lines in Fe XXI (271/242 Å) were first identified in the EUV spectra of solar flares by Dere (1978) although any conclusion about the magnitude of B, which is expected to be close to unity, is spoilt by the blending with a strong line of Fe XIV.

Previous datasets for the intercombination transitions of the carbon sequence have been computed by Cheng et al. (MCDF, 1985) in a Multiconfiguration Dirac-Fock approach; by Froese Fischer & Saha (MCHF, 1985) in the well established Breit-Pauli Multiconfiguration Hartree-Fock method; by Bhatia (1982), Bhatia et al. (1987), Bhatia & Kastner (1993), Bhatia & Doschek (1993a, 1993b, 1993c, 1995) and Mason & Bhatia (1978) (to be hereafter referred to as SSTR) using the atomic structure code SUPERSTRUCTURE by Eissner et al. (1974); and by Aggarwal (1986), ,Aggarwal et al. (1997a, 1997b) and Bell et al. (1995) (referred to as CIV3) with the CIV3 program of Hibbert (1975). Comparisons and assessments of these datasets have yet to be made in order to determine much needed accuracy ratings.

In the on-going IRON Project (IP, Hummer et al. 1993), we are interested in computing atomic data, namely radiative and collisional rates, in isoelectronic sequences for astrophysical plasma diagnostics. Although the emphasis is on the iron-group elements due to the needs of recent space missions, e.g. the Solar and Heliospheric Observatory (SOHO), good accord with the available detailed calculations for the lower members (C, N and O say) is always a requisite in order to ensure good representations of electron correlation effects and relativistic couplings at higher Z. Electron impact excitation data involving the $\sp5{\rm S}\sp{\rm o}_2$ level in ions of the carbon sequence have been reported within the IP by Lennon & Burke (1994). We are concerned here with the corresponding radiative rates, and in the context of the current discussion of radiative lifetimes and branching ratios, we are interested in studying isoelectronic trends in particular those of the Breit operators. We attempt to end up also with a ranked radiative dataset more accurate than previous work and one that will be included in the IP public databases. Note that a complete list of papers in the IP series can be found at the URL http://www.am.qub.ac.uk/projects/iron/papers/papers. html. In Sect. 2 we describe the present computational method and in Sect. 3 results are discussed in the light of extensive comparisons. Conclusions are summarised in Sect. 4.