Radiative rates for forbidden M1 and E2 transitions of astrophysical interest in doubly ionized iron-peak elements

Abstract

Aims. Accurate and reliable atomic data for lowly ionized Fe-peak species (Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) are of paramount importance for analyzing the high-resolution astrophysical spectra currently available. The third spectra of several iron group elements have been observed in different galactic sources, such as Herbig-Haro objects in the Orion Nebula and stars like Eta Carinae. However, forbidden M1 and E2 transitions between low-lying metastable levels of doubly charged iron-peak ions have been investigated very little so far, and radiative rates for those lines remain sparse or nonexistent. We attempt to fill that gap and provide transition probabilities for the most important forbidden lines of all doubly ionized iron-peak elements. Methods. We carried out a systematic study of the electronic structure of doubly ionized Fe-peak species. The magnetic dipole (M1) and electric quadrupole (E2) transition probabilities were computed using the pseudo-relativistic Hartree-Fock (HFR) code of Cowan and the central Thomas-Fermi-Dirac-Amaldi potential approximation implemented in AUTOSTRUCTURE. This multiplatform approach allowed for consistency checks and intercomparison and has proven very useful in many previous works for estimating the uncertainties affecting the radiative data.Results. We present transition probabilities for the M1 and E2 forbidden lines depopulating the metastable even levels belonging to the 3d k and 3d k−1 4s configurations in Sc III (k = 1), Ti III (k = 2), V III (k = 3), Cr III (k = 4), Mn III (k = 5), Fe III (k = 6), Co III (k = 7), and Ni III (k = 8).

Collision strengths and transition probabilities for Co iii forbidden lines

“…As discussed above in the comparison with the work of Hansen et al (1984), the systematic nature of the difference suggests that it is due to a different value for the 3d radial quadrupole integral rather than details of the wave function expansions of individual terms. The configuration expansions in the present work and in that of Fivet et al (2016) are very similar but differ in one key aspect. We use a somewhat contracted 4d orbital to allow for the differences in the 3d orbital between the 3d 7 and 3d 6 4s configurations, while Fivet et al (2016) employ a spectroscopic 4d orbital but a contracted 5s orbital which provides flexibility to the spectroscopic 4s.…”

“…There is therefore a stepwise decay through the levels and only three relevant transition probabilities, for a 4 F 3/2 -a 4 F 5/2 , a 4 F 5/2a 4 F 7/2 and a 4 F 7/2 -a 4 F 9/2 . We are aware of only two previous calculations of transition probabilities for Co iii, one by Hansen et al (1984) and one by Nussbaumer & Storey (1988), as well as one contemporary calculation by Fivet et al (2016). Nussbaumer & Storey (1988) only give values for these three probabilities and these differ by less than 1% from our values.…”

“…With this interaction included, the quadrupole line strength involves both the 3d radial quadrupole integral and the 4d integral which is significantly larger than for the 3d. Fivet et al (2016) have made calculations of forbidden transition probabilities for the twice ionised iron-peak elements from Sc to Ni, including Co, and we compare with their results in Table 10. Their calculations were made with two different methods which we label as FQB1 and FQB2.…”

Photoionization of Co+and electron-impact excitation of Co2 +using the DiracR-matrix method

“…Excellent agreement is evident for the bulk of the transitions considered with the greatest difference of 12.7% occurring for the 3d 7 a 4 F 3/2 -3d 7 a 4 P 5/2 (4-5) transition. A much larger calculation for doubly ionized Fe-peak species has been performed recently by Fivet et al (2016) using the same suite of codes as Hansen et al (1984), and also by considering the computer package autostructure, (Eissner et al 1974;Badnell 1986), where the optimization process is carried out with a Thomas-Fermi-Dirac potential using lambda scaling parameters. Numerous doubly ionized ions of Fe, Ni and Co were considered in this publication.…”

“…This is the most sophisticated and complete report for radiative rates in Co iii to date. Within the 3d 7 complex, we vary approximately 27% on average compared with both methods of Fivet et al (2016).…”