2006ApJ...637..548J

We have retrieved high-resolution UV spectra of 69 hot stars from the HST archive and determined the strengths of the interstellar Ni II absorption features at 1317.217 Å arising from the ground 3d⁹²D_5/2 electronic state to the 3d⁸(¹G)4p²F°_5/2excited level. We then compared them to absorptions to either the 3d⁸(³F)4p²D°_5/2or 3d⁸(³P)4p²P°_3/2 upper levels occurring at, respectively, λ=1741.553 Å (covered in the spectra of 21 of the stars) and 1370.132 Å (seen for the remaining 48 stars). All spectra were recorded by the either the E140M, E140H, or E230H gratings of the Space Telescope Imaging Spectrograph. By comparing the strengths of the two lines in each spectrum and evaluating a weighted average of all such comparisons, we have found that the f-value of the 1317 Å line is 1.34±0.019 times the one at 1741 Å, and 0.971±0.014 times the one at 1370 Å. We adopt as a comparison standard an experimentally determined f-value for the 1741 Å line (known to 10% accuracy), so that f(1317)=0.0571±0.006. It follows from this f-value and our measured line-strength ratios that f(1370)=0.0588±0.006. As an exercise to validate our methodology, we compared the 1317 Å transition to another Ni II line at 1454.842 Å to the 3d⁸(¹D)4p²D°_5/2level and arrived at an f-value for the latter that is consistent with a previously measured experimental value to within the expected error.