The f-value and Branching Ratio for the Mg ii 1240 Angstroms Doublet

Details The f-value and Branching Ratio for the Mg ii 1240 Angstroms Doublet The f-value and Branching Ratio for the Mg ii 1240 Angstroms Doublet

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...195.5304s&link_type=abstract

American Astronomical Society, 195th AAS Meeting, #53.04; Bulletin of the American Astronomical Society, Vol. 31, p.1453

Astronomy and Astrophysics

Astronomy

Scientific paper

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 1240 Angstroms (3s - 4p) doublet of 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 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}/f{1240 = 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 x 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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