Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...20916801s&link_type=abstract
2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #168.01; Bulletin of the American Astronomical Society,
Other
Scientific paper
New laboratory radiative lifetimes accurate to about ±5% for 41 odd-parity levels of Hf II have been combined with FTS-based emission-line branching fractions to determine transition probabilities for 150 lines of Hf II. Approximately half of these new transition probabilities overlap and are in excellent agreement with recent independent measurements by Lundqvist et al. (2006). Our new laboratory data include all known strong Hf II transitions in wavelength regimes accessible to ground-based spectroscopy of cool stars. These gf-values are applied to refine the hafnium photospheric solar abundance and to determine hafnium abundances in 10 metal-poor giant stars with enhanced r-process abundances (those with[Fe/H] +0.5). For the Sun we derive log ɛ(Hf) = 0.88 ± 0.08 from four lines; the uncertainty is due to the intrinsic weakness of the Hf II lines, and their usually significant blending by other spectral features. We find that the r-process-rich stars exhibit constant Hf/La and Hf/Eu abundance ratios, log ɛ(Hf/La) = -0.13 ± 0.02 (σ = 0.06) and log ɛ(Hf/Eu) = +0.04 ± 0.02 (σ = 0.06). The observed average stellar abundance ratio of Hf/Eu and La/Eu is larger than previous estimates of the solar system r-process-only value, arguing for a larger r-process contribution to Hf and La abundances. The newly determined Hf values could be employed as part of the chronometer pair, Th/Hf, to determine radioactive stellar ages.
This work has been supported by the National Science Foundations through grants AST-0506324 to JEL and EADH, AST-0307495 to CS, and AST-0307279 to JJC.
Cowan John J.
Den Hartog Elizabeth A.
Ivans Inese I.
Labby Z. E.
Lawler James E.
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