The FERRUM Project: Experimental transition probabilities of [Fe II] and astrophysical applications

¹ Atomic Astrophysics, Lund Observatory, Lund University, Box 188, 221 00 Lund, Sweden
² Physics Department, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
³ Department of Applied Mathematics and Theoretical Physics, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland
⁴ NASA/Goddard Space Flight Center, Code 681, Greenbelt, MD 20771, USA
⁵ Department of Physics, Lund Institute of Technology, Box 118, 221 00 Lund, Sweden
⁶ Physics Department, Royal Institute of Technology, AlbaNova University Center, 106 91 Stockholm, Sweden

Corresponding author: H. Hartman, Henrik.Hartman@astro.lu.se

Received: 10 September 2002
Accepted: 25 October 2002

Abstract

We report on experimental transition probabilities for thirteen forbidden [Fe II] lines originating from three different metastable levels. Radiative lifetimes have been measured of two metastable states by applying a laser probing technique on a stored ion beam. Branching ratios for the radiative decay channels, i.e. M1 and E2 transitions, are derived from observed intensity ratios of forbidden lines in astrophysical spectra and compared with theoretical data. The lifetimes and branching ratios are combined to derive absolute transition probabilities, A-values.
We present the first experimental lifetime values for the two Fe II levels a⁴G_9/2 and b²H_11/2 and A-values for 13 forbidden transitions from a⁶S_5/2, a⁴G_9/2 and b⁴D_7/2 in the optical region. A discrepancy between the measured and calculated values of the lifetime for the b²H_11/2 level is discussed in terms of level mixing. We have used the code CIV3 to calculate transition probabilities of the a⁶D-a⁶S transitions.
We have also studied observational branching ratios for lines from 5 other metastable Fe II levels and compared them to calculated values. A consistency in the deviation between calibrated observational intensity ratios and theoretical branching ratios for lines in a wider wavelength region supports the use of [Fe II] lines for determination of reddening.