  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.
