Evidence of a High Carbon Abundance in the Local Interstellar Cloud

The nature of the local interstellar cloud (LIC) is highly constrained by the combination of in situ heliospheric and line-of-sight data toward nearby stars. We present a new interpretation of the LIC components of the absorption-line data toward CMa, based on recent atomic data that include new rates for the Mg⁺ to Mg⁰ dielectronic recombination rate, and using in situ measurements of the temperature and density of neutral helium inside of the heliosphere. With these data we are able to place interesting limits on the gas-phase abundance of carbon in the LIC. If the C/S abundance ratio is solar, ~20, then no simultaneous solution exists for the N(Mg I), N(Mg II), N(C II), and N(C II*) data. The combined column density and in situ data favor an abundance ratio A_C/A_S = 47. We find that the most probable gas-phase C abundance is in the range 400-800 ppm with a lower limit of ~330. We speculate that such a supersolar abundance could have come to be present in the LIC via destruction of decoupled dust grains. Similar enhanced C/H ratios are seen in very low column density material, N(H) < 10¹⁹ cm^-2, toward several nearby stars.