Carbon and oxygen depletion and extinction in the translucent cloud toward HD 24534 (X Persei)

Recent studies of the gas-phase abundances of carbon and oxygen in diffuse clouds have suggested that the depletions of both elements are invariant and independent of extinction curve properties. We show, however, that in the line of sight toward HD 24534 (X Persei), the depletion of carbon is at least a factor of 3 greater than in the diffuse clouds previously surveyed. For HD 24534, the interstellar gas-phase C/H ratio is lower than the mean of diffuse sight lines by a factor of about 3.5. We conclude that the carbon depletion may be enhanced in translucent molecular clouds and also that carbon begins to show an enhanced level of depletion at lower extinctions than oxygen, which is also discussed. We argue that it is imperative to measure abundances and depletions in translucent and denser clouds in order to understand the interaction between gas and dust as the thicknesses and densities of clouds increase.