PROBING THE ROLE OF CARBON IN ULTRAVIOLET EXTINCTION ALONG GALACTIC SIGHT LINES

Abstract

We report previously undetermined interstellar gas and dust-phase carbon abundances along 15 Galactic sight lines based on archival data of the strong 1334.5323 Å transition observed with the Space Telescope Imaging Spectrograph. These are combined with previously reported carbon measurements along six sight lines to produce a complete sample of interstellar C ii measurements determined with the 1334 Å transition. Our data set includes a variety of Galactic disk environments characterized by different extinctions and samples paths ranging over three orders of magnitude in average density of hydrogen (〈n(H)〉). Our data support the idea that dust, specifically carbon-based grains, are processed in the neutral interstellar medium. We, however, do not find that the abundance of carbon in dust or the grain-size distribution is related to the strength of the 2175 Å bump. This is surprising, given that many current models have polycyclic aromatic hydrocarbons as the bump-producing dust.