The abundance of interstellar carbon

We present an analysis of the abundance of interstellar carbon toward six stars within 600 pc of the Sun based upon high signal-to-noise (S/N ≃ 400-600) data of the weak intersystem line C II] λ2325 Å obtained with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. We find that the measured gas-phase C/H shows no dependence on either direction or the physical condition of the gas and that 106 C/H ≃ 140 ± 20. Our sample of sight lines includes examples exhibiting a wide range of extinction variation, including features traditionally associated with carbon grains (e.g., the 2175 Å bump). The lack of any statistically significant variation in the observed gas-phase C/H among these sight lines strongly suggests that the source of this variation in extinction is probably not variations in the dust abundance of carbon. Based upon an assessment of the total interstellar abundance of oxygen (i.e., gas and dust) along with comparisons to O/H and C/H observed in B stars, we adopt an intrinsic (i.e., cosmic) abundance for carbon in the local ISM of 10⁶(C/H)_cosmic ≃ 240 ± 50. From the observed gas-phase C/H, we argue that the dust-phase carbon abundance must then be 50 < 10⁶(C/H)_dust < 150. This amount of solid carbon can explain such extinction features as the 2175 Å bump (i.e., graphite and/or PAHs) but puts severe restrictions on the availability of carbon in grains to explain the total optical/UV dust opacity.