Empirical determination of Einstein A-coefficient ratios of bright [Fe   II] lines
  The Einstein spontaneous rates (A-coefficients) of Fe^+ lines have been computed by several authors, with results that differ from each other up to 40%. Consequently, models for line emissivities suffer from uncertainties which in turn affect the determination of the physical conditions at the base of line excitation. We provide an empirical determination of the A-coefficient ratios of bright [Fe II] lines, which would represent both a valid benchmark for theoretical computations and a reference for the physical interpretation of the observed lines. With the ESO-VLT X-shooter instrument between 3,000 A, and 24,700 A, we obtained a spectrum of the bright Herbig-Haro object HH1. We detect around 100 [Fe II] lines, some of which with a signal-to-noise ratio > 100. Among these latter, we selected those emitted by the same level, whose de-reddened intensity ratio is a direct function of the Einstein A-coefficient ratios. From the same X-shooter spectrum, we got an accurate estimate of the extinction toward HH1 through intensity ratios of atomic species, HI, recombination lines and H_2 ro-vibrational transitions. We provide seven reliable A-ooefficient ratios between bright [Fe II] lines, which are compared with the literature determinations. In particular, the A-coefficient ratios involving the brightest near-infrared lines (12570A/16440A and 13209A/16440A) are better in agreement with the predictions by Quinet et al. (1996) Relativistic Hartree-Fock model. However, none of the theoretical models predicts A-coefficient ratios in agreement with all our determinations. We also show that literature data of near-infrared intensity ratios better agree with our determinations than with theoretical expectations. 
T. Giannini, S. Antoniucci, B. Nisini, D. Lorenzetti, J. M. Alcala',   F. Bacciotti, R. Bonito, L. Podio, and B. Stelzer
astro-ph.SR