Atomic Transition Probabilities for Transitions of Si I and Si II and the Silicon Abundances of Several Very Metal-Poor Stars

Abstract

We report new measurements of branching fractions for 20 UV and blue lines in the spectrum of neutral silicon (Si I) originating in the 3$s^2$3$p$4$s$ ${}^3$P${}_{1,2}^{\mathrm{o}}$, ${}^1$P${}_1^{\mathrm{o}}$ and 3$s$3$p^3$ ${}^1$D${}_{1,2}^{\mathrm{o}}$ upper levels. Transitions studied include both strong, nearly pure LS multiplets as well as very weak spin-forbidden transitions connected to these upper levels. We also report a new branching fraction measurement of the ${}^4$P${}_{1/2}$ - ${}^2$P${}_{1/2,3/2}^{\mathrm{o}}$ intercombination lines in the spectrum of singly-ionized silicon (Si II). The weak spin-forbidden lines of Si I and Si II provide a stringent test on recent theoretical calculations, to which we make comparison. The branching fractions from this study are combined with previously reported radiative lifetimes to yield transition probabilities and log($gf$)s for these lines. We apply these new measurements to abundance determinations in five metal-poor stars.

Accepted for publication in the Astrophysical Journal Supplement Series (26 pages, 6 figures, 5 tables; machine-readable versions of Tables 3 and 4 will be available from the publisher)

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