AAS 195th Meeting, January 2000
Session 53. ISM: Ionized Gas
Display, Thursday, January 13, 2000, 9:20am-6:30pm, Grand Hall

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[53.04] The f-value and Branching Ratio for the Mg {\sc ii} 1240Å\ Doublet

U.J. Sofia (Whitman College), D. Fabian (Univ. of Wisconsin-Madison), J.C. Howk (The Johns Hopkins Univ.)

Magnesium is an important element to study in the interstellar medium (ISM) because it is both abundant and reactive. These characteristics make Mg a substantial component of interstellar dust specifically silicate and oxide grains. A testament to the importance of Mg is the recent work that has gone into refining the f-values of the transitions used to determine its abundances in the interstellar medium (Cardelli et al. 1991; Sofia et al. 1994; Fitzpatrick 1997; Fleming et al. 1998; Godefroid & Fischer 1999; Theodosiou & Federman 1999; Sofia, Fabian & Howk 1999).

The 1240Å\ (3s - 4p) doublet of \ion{Mg}{2} is the primary absorption used for determining the interstellar abundance of Mg in the neutral ISM. The oscillator strength for this doublet is difficult to measure experimentally because the lifetimes for the transitions are not controlled by de-excitations to the ground state. The theoretical determination of the f-value is difficult because of substantial radial overlap integral cancellations.

Godefroid & Fischer and Theodosiou & Federman have recently suggested that the branching ratio of the (3s - 4p) doublet is not 2-to-1 as previously assumed (G & F find a higher ratio while T & F find a smaller ratio). We empirically determine the branching ratio and oscillator strength for the weak \ion{Mg}{2} 3s - 4p doublet in order to compare it to the theoretical values. We use the independent methods of apparent optical depth analysis and profile component fitting in order to determine the branching ratio. We find f1239/f1240 = 1.74 ± 0.06, in agreement with Theodosiou & Federman's theoretical value of 1.78 ± 0.03. The profile fitting analysis gives an f-value for the doublet of 9.71 ± 0.32 \times 10-4 which agrees with both the theoretical value of Theodosiou & Federman and the empirical value of Fitzpatrick (1997).

We acknowledges support from the HST grant GO-06686.01 and the NASA LTSARP Grant NAG5-8249 through Whitman College.

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