IMPROVED ATOMIC DATA FOR HO ii AND NEW HOLMIUM ABUNDANCES FOR THE SUN AND THREE METAL-POOR STARS

Improved energy levels and hyperfine structure constants for a selected set of Ho ii levels were measured from spectra recorded Q1 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, log10 ðHoÞ 1⁄4 þ0:51 0:10, is consistent with its meteoritic value, log10 ðHoÞ 1⁄4 þ0:49 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. Subject headings: atomic data — stars: abundances — Sun: abundances