2004ApJ...604..850L

Improved energy levels and hyperfine structure constants for a selected set of Ho II levels were measured from spectra recorded using the 1 m Fourier transform spectrometer (FTS) at the US National Solar Observatory. Branching fractions for the strong blue-UV lines from these levels were also measured from the FTS data and combined with earlier radiative lifetimes from time-resolved laser-induced fluorescence measurements to determine accurate absolute transition probabilities for 22 lines of Ho II. These new laboratory measurements, along with a recently reported partition function for ionized Ho, were used to improve Ho abundance determinations in the Sun and three metal-poor Galactic halo giant stars. The derived solar photospheric holmium abundance, log₁₀ε(Ho)=+0.51±0.10, is consistent with its meteoritic value, log₁₀ε(Ho)=+0.49±0.02. In each of the metal-poor, neutron-capture-rich program stars, the holmium abundance relative to those of other rare earth elements agrees well with solar system rapid-neutron-capture abundance values.