The IRON Project (IP, see the general description by Hummer et al. 1993) is an international
collaboration primarily concerned with the computation of reliable
electron excitation rates for ions of astrophysical interest, with an emphasis
on the heavier species belonging to the iron group. The IP has been
motivated by data requirements from recent ambitious satellite-borne telescope
missions such as the Infrared Space Observatory (ISO), the Space Infrared
Telescope Facility (SIRTF) and the Solar and Heliospheric Observatory
(SOHO). Another aim of the IP is to produce quality radiative data such as
oscillator strengths, transition probabilities and photoionisation cross
sections to help satisfy users' needs. Indeed, the practical value of the
future IP public databases will be greatly enhanced if, for example,
accurate collisional and radiative rates are included for the same transitions.
The policy of the IP is to calculate new data rather than to compile results
from existing work. In fact, it is frequently difficult to assign
accuracy ratings to older theoretical datasets. Also, the evolution of
computers and the development of computational methods are an encouragement to
revisit important cases. In the present report, the forbidden transitions
within the ground configuration of the carbon (2s
2p
) and oxygen
(2s
2p
) isoelectronic sequences are considered. It should be noted
that there is no previous full-scale study of the oxygen sequence by means of
the code SUPERSTRUCTURE.
The forbidden transitions in C-like ions were previously considered by Garstang (1968), Nicolaides et al. (1971), Nussbaumer (1971), Kastner et al. (1977), Mason & Bhatia (1978), Nussbaumer & Rusca (NR, 1979), Cheng et al. (1979), Baluja & Doyle (1981), and more recently by Froese Fischer & Saha (FS1, 1985) and Hibbert et al. (1993). For ions in the oxygen sequence, computations have been reported by Garstang (1968), Nicolaides et al. (1971), Kastner et al. (1977), Cheng et al. (1979), Bhatia et al. (1979), Froese Fischer & Saha (FS2, 1983), Baluja & Zeippen (BZ, 1988) and Gaigalas et al. (1994). Note that the references given above are the most directly relevant to the present work. For a more complete bibliography see Biémont & Zeippen (1991, 1996). Since we are concerned here with radiative datasets for complete sequences rather than for specific systems, we will focus our comparisons on the extensive datasets generated by NR, FS1, BZ and FS2. It is worth noting that the general agreement among these datasets is found to be unsatisfactory: only 76% of the A-values in NR and FS1 (C sequence) and 71% in BZ and FS2 (O sequence) agree to within 10%. The worse differences are as large as an order of magnitude. It is important to try and shed some light on the reasons for such discrepancies. The extensive study by Cheng et al. (1979) will not be compared in detail with the present work because these authors restrict correlation in their physical models to the n=2 complex. They were mainly concerned with treating relativic effects "fully'' in a Dirac-Fock formalism. As discussed in BZ, their resulting transition probabilities are accurate for higher atomic numbers, but lack reliability for the lighter elements at the neutral end of a given isoelectronic sequence.
The present attempt aims at generating datasets of high reliability (i.e. with accuracy ratings of 10% or better) to be incorporated in the IP databases. We make use of the computer program SUPERSTRUCTURE (Eissner et al. 1974; Nussbaumer & Storey 1978; Eissner 1991), which allows for configuration interaction (CI) and Breit-Pauli (BP) relativistic effects. This code has been extensively applied in the computation of atomic spectra. The numerical method is described in Sect. 2, followed by an analysis of the present results in Sect. 3. Concluding remarks are given in Sect. 4.