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

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 a4G9/2 and b2H11/2 and A-values for 13 forbidden transitions from a6S5/2, a4G9/2 and b4D7/2 in the optical region. A discrepancy between the measured and calculated values of the lifetime for the b2H11/2 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.