Gas-phase iron abundances and depletions in translucent interstellar lines of sight from Far Ultraviolet Spectroscopic Explorer observations of FeII lines

Far Ultraviolet Spectroscopic Explorer ( FUSE) wavelength coverage includes several weak- to moderate-strength lines of Fe II allowing the determination, through curve-of-growth analysis, of accurate Fe II abundances and hence iron depletions. We have analyzed Fe II absorption lines toward 18 of the reddened stars included in the FUSE survey of molecular hydrogen abundances in translucent clouds. Our analysis is based on equivalent width measurements and curves of growth, aided by the fact that some of the observed lines are weak enough to be on the linear part of the curve of growth. In interpreting our abundance and depletion results, we have combined our data with those of an earlier survey of interstellar iron abundances and depletions in diffuse clouds, based on Copernicus data. The principal result of our survey is that iron depletions, known from earlier work to increase with average line-of-sight density for diffuse clouds, do not continue to increase with either density or extinction in translucent clouds; i.e., there is no significant trend of increasing depletion with increasing extinction or molecular fraction. This may be due to the fact that our data set does not probe lines of sight with greater average volume densities than those that were covered by the previous Copernicus-based survey of iron depletions. We conclude by reevaluating the definition of translucent clouds, based on the lack of enhanced iron depletions in our sample.