The FERRUM project: Transition probabilities for forbidden lines in [Fe ii] and experimental metastable lifetimes

Context. Accurate transition probabilities for forbidden lines are important diagnostic parameters for low-density astrophysical plasmas. In this paper we present experimental atomic data for forbidden [Fe ii] transitions that are observed as strong features in astrophysical spectra. Aims. To measure lifetimes for the 3d(G)4s a 4G11/2 and 3d (D)4s b 4D1/2 metastable levels in Fe ii and experimental transition probabilities for the forbidden transitions 3d a 4F7/2,9/2 – 3d (G)4s a 4G11/2. Methods. The lifetimes were measured at the ion storage ring facility CRYRING using a laser probing technique. Astrophysical branching fractions were obtained from spectra of Eta Carinae, obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope. The lifetimes and branching fractions were combined to yield absolute transition probabilities. Results. The lifetimes of the a 4G11/2 and the b 4D1/2 levels have been measured and have the following values, τ = 0.75 ± 0.10 s and τ = 0.54 ± 0.03 s respectively. Furthermore, we have determined the transition probabilities for two forbidden transitions of a 4F7/2,9/2 – a 4G11/2 at 4243.97 and 4346.85 Å. Both the lifetimes and the transition probabilities are compared to calculated values in the literature.