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 3s ²3p4s ³P^o _1,2,¹P^o ₁, and 3s3p ^{3 1}D^o _1,2 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 ⁴P_1/2-²P^o _1/2,3/2 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) values for these lines. We apply these new measurements to abundance determinations in five metal-poor stars. ^∗ Based on observations made with the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Other data have been obtained from the European Southern Observatory (ESO) Science Archive Facility; and the Keck Observatory Archive, which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute, under contract with NASA. These data are associated with HST programs GO-7402, GO-14161, and GO-14232; ESO programs 66.D-0636(A), 073.D-0024(A), and 095.D-0504(A); and Keck program H41aH.