An Empirical Test of the Mg II λ1240 Doublet Branching Ratio and Oscillator Strength

Abstract

We empirically confirm the theoretical branching ratio and oscillator strength for the weak Mg II 3s-4p doublet at 1240 Å as determined by Theodosiou & Federman. We use the independent methods of apparent optical depth analysis for the sight lines toward μ Col, γ Ara, and ρ Leo and profile component fitting for the sight line toward μ Col in order to determine the branching ratio. We find f₁₂₃₉/f₁₂₄₀=1.74+/-0.06, in agreement with the theoretical value of 1.78+/-0.03. Profile fitting for the line of sight toward μ Col gives an f-value for the doublet of 9.71+/-0.32x10^-4, which agrees with both the theoretical value of Theodosiou & Federman and the empirical value of Fitzpatrick. Based on observations obtained with the NASA/ESA Hubble Space Telescope through the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NASA-26555.