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 10<SUP>6</SUP> C/H ≃ 140±20. <P />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. <P />Based upon an assessment of the total interstellar abundance of oxygen (i.e., gas and dust) along with comparisons to 0/H and C/H observed in B stars, we adopt an intrinsic (i.e., cosmic) abundance for carbon in the local ISM of 10<SUP>6</SUP>(C/H)<SUB>cosmic</SUB> ≍ 240±50. From the observed gas-phase C/H, we argue that the dust-phase carbon abundance must then be 50 &lt; 10<SUP>6</SUP>(C/H)<SUB>dust</SUB> &lt; 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.