Transition probabilities of Co II, weak lines to the ground and low metastable levels

Abstract: New branching fraction measurements based primarily on data from a cross dispersed echelle spectrometer are reported for 84 lines of Co II. The branching fractions for 82 lines are converted to absolute atomic transition probabilities using radiative lifetimes from laser induced fluorescence (LIF) measurements on 19 upper levels of the lines. A lifetime of 3.3(2) ns for the z5D0 level is used based on LIF measurements for lifetimes of the four other levels in the z5D term. Twelve of the 84 lines are weak transitions connecting to the ground and low metastable levels of Co+. Another 46 lines are strong transitions connecting to the ground and low metastable levels of Co+. For these lines log(gf) values were measured in earlier studies and, with a few exceptions, are confirmed in this study. Such lines, if unblended in stellar spectra, have the potential to yield Co abundance values unaffected by any breakdown of the Local Thermodynamic Equilibrium approximation in stellar photospheres because the ground and low metastable levels of Co+ are the primary population reservoirs of Co in photospheres of interest. Weak lines, if unblended, are useful in photospheres with high Co abundance and strong lines are useful in metal-poor photospheres. New hyperfine structure A constants for 28 levels of ionized Co from least square fits to Fourier transform spectra line profiles are reported. These laboratory data are applied to re-determine the Co abundance in metal-poor halo star HD 84937. Branching fractions and transition probabilities for 19 lines are reported for the first time.