2015ApJ...809..120M

We probe the role of carbon in the ultraviolet (UV) extinction by examining the relations between the amount of carbon required to be locked up in dust with the extinction bump and the far-UV extinction rise, based on an analysis of the extinction curves along 16 Galactic sightlines for which the gas-phase carbon abundance is known and the extinction bump exhibits variable strengths and widths. We derive from the Kramers-Kronig relation which relates the wavelength-integrated extinction to the total dust volume. This approach is less model-dependent since it does not require the knowledge of the detailed optical properties and size distribution of the dust.We also derive from fitting the observed UV/optical/near-infrared extinction with a mixture of amorphous silicate and graphite. We find that the carbon depletion tends to correlate with the strength of the bump, while the abundance of silicon depleted in dust shows no correlation with the bump. This supports graphite or polycyclic aromatic hydrocarbon molecules as the possible carrier of the bump. We also see that shows a trend of correlating with, where R_V is the total-to-selective extinction ratio, suggesting that the far-UV extinction is more likely produced by small carbon dust than by small silicate dust.