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

¹ Department of Physics, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden e-mail: jonas.gurell@physto.se
² Lund Observatory, Lund University, Box 43, 22100 Lund, Sweden
³ Department of Physics, Royal Institute of Technology, AlbaNova University Center, 10691 Stockholm, Sweden

Received: 3 September 2009
Accepted: 2 October 2009

Abstract

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 [] transitions that are observed as strong features in astrophysical spectra.

Aims. We measure lifetimes for the 3d⁶(³G)4s a ⁴G_11/2 and 3d⁶(³D)4s b ⁴D_1/2 metastable levels in and experimental transition probabilities for the forbidden transitions 3d⁷ a ⁴F–3d⁶(³G)4s a ⁴G_11/2.

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 ⁴G_11/2 and the b ⁴D_1/2 levels have been measured and have the following values,  s and  s respectively. Furthermore, we have determined the transition probabilities for two forbidden transitions of a ⁴F–a ⁴G_11/2 at 4243.97 and 4346.85 Å. Both the lifetimes and the transition probabilities are compared to calculated values in the literature.