Improved Atomic Transition Probabilities for UV and Optical Lines of Hf II and Determination of the Hf Abundance in Two Metal-poor Stars*

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Abstract
We report new branching fraction measurements for 199 UV and optical transitions of Hf ii. These transitions range in wavelength (wavenumber) from 2068 to 6584 Å (48,322–15,183 cm−1) and originate in 17 odd-parity upper levels ranging in energy from 38,578 to 53,227 cm−1. The branching fractions are combined with radiative lifetimes reported in an earlier study to produce a set of transition probabilities and log(gf) values with accuracy ranging from 5% to 25%. Comparison is made to transition probabilities from the literature where such data exist. We use these new transition probabilities to derive improved Hf abundances in two metal-poor stars. HD 196944 is enhanced in s-process elements, and we derive log ε (Hf) = −0.72 ± 0.03 (σ = 0.09) from 12 Hf ii lines. HD 222925 is enhanced in r-process elements, and we derive log ε (Hf) = 0.32 ± 0.03 (σ = 0.11) from 20 Hf ii lines. These measurements greatly expand the number of potentially useful Hf ii lines for analysis in UV and optical spectra.
Funding Information
  • NSF ∣ MPS ∣ Division of Astronomical Sciences (AST-1814512)
  • National Aeronautics and Space Administration (GO-14765)
  • National Aeronautics and Space Administration (GO-15657)
  • NSF ∣ MPS ∣ Division of Astronomical Sciences (AST-1815403)
  • Joint Institute for Nuclear Astrophysics - Center for the Evolution of the Elements (PHY 14-30152)

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