In supervova research, the iron group elements such as Fe, Co and Ni are of particular interest. Recently, Kuchner et al. (1994) concluded that the cobalt and iron atoms observed through Type Ia supernova (SN Ia) emission lines were produced by the radioactive decay chain 56Ni 56Co 56Fe as predicted by a wide range of previous models for SN Ia explosions (see e.g. Pankey 1962; Colgate & McKee 1969; Arnett 1979, 1982; Colgate et al. 1980; Weaver et al. 1980; Chevalier 1981; Woosley & Weaver 1986). According to this model, a small fraction of the decay energy is converted into thermal energy and the expanding shell is cooled mainly through emission in forbidden lines of the first three ionization stages of Fe and Co produced from excitation of the same nuclei that are daughters of the original 56Ni (see Kirshner & Oke 1975; Kuchner et al. 1994).
Important atomic processes and reliable atomic data are needed to construct the ionization and thermal structure of Type Ia supernovae at early nebular phase as well as to solve some of the theoretical problems with SN Ia such as the "IR catastrophe" (Fransson et al. 1996). In this context, it must be noted that, while a large number of transition probabilities have been calculated in the past for the [Fe I], [Fe II], [Fe III], [Ni I], [Ni II] and [Ni III] lines (see e.g. Grevesse et al. 1971; Garstang 1957, 1958, 1962, 1964; Nussbaumer & Storey 1980, 1982, 1988a; Quinet 1996; Quinet & Le Dourneuf 1996; Quinet et al. 1996a,b), very few studies of forbidden lines belonging to low ionization stages of cobalt have been reported so far. To our knowledge, only the results obtained by Nussbaumer & Storey (1988b) for the lines within the ground terms in Co I and Co III and for the forbidden transitions involving the lowest three terms of Co II have been published. This lack of radiative data for forbidden lines of cobalt and, in particular for those belonging to Co II, has been underlined recently by Liu et al. (1997a). In their paper, it is stated that the overall discrepancy between the synthetic and observed spectra of SN Ia is partly caused by the neglect of the [Co II] lines in the spectral synthesis.
The aim of the present work is to provide a new set of accurate transition rates for forbidden lines of singly ionized cobalt. Together, the three low even configurations 3d8, 3d74s and 3d64s2 in Co II have 47 metastable levels. In our study, radiative transition probabilities for forbidden lines between these metastable levels were calculated using the relativistic Hartree-Fock (HFR) method combined with an optimization of the radial parameters based on the available observed energy levels. The most important configuration interaction (CI) and relativistic effects were included in the calculations. The results reported here can be seen as an extension of the recent investigations of forbidden transitions of astrophysical interest in iron group elements carried out in the cases of Cr II (Quinet 1997), Fe II (Quinet et al. 1996a,b), Fe III (Quinet 1996), Ni I and Ni II (Quinet & Le Dourneuf 1996).