Measurements of the f-Values of the Resonance Transitions of Ni II at 1317.217A and 1370.132A

Abstract

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 1317.217A. We then compared them to absorptions from either the transitions at 1741.553A (covered in the spectra of 21 of the stars) or the one at 1370.132A (seen for the remaining 48 stars). All spectra were recorded by the either the E140M, E140H, or E230H gratings of STIS. 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 1317A line is 1.34 +/-0.019 times the one at 1741A and 0.971 +/-0.014 times that of the one at 1370A. We adopt as a comparison standard an experimentally determined f-value for the 1741A line (known to 10% accuracy), so that f(1317A) = 0.0571 +/-0.006. It follows from this f-value and our measured line-strength ratios that f(1370A) = 0.0588 +/-0.006. As an exercise to validate our methodology, we compared the 1317A transition to another Ni II line at 1454.842A and arrived at an f-value for the latter that is consistent with a previously measured experimental value to within the expected error.