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B.E. Wood, J.L. Linsky
Using the echelle-A grating of the Goddard High-Resolution Spectrograph, we have observed the C II lambda lambda 1335, 1336 emission lines of the nearby (d = 13.3 pc) star system Capella (G8 III + G1 III). Interstellar C II absorption features are detected within both stellar emission lines. The ground-state and excited-state C II column densities derived from these absorption lines imply an electron density of ne = 0.11 ^{+0.12}_{-0.06} cm-3 in the local interstellar medium (LISM). Unlike previous derivations of ne from Mg II/Mg I ratios, the density suggested by the C II lines is independent of assumptions about ionization equilibrium. Current estimates of the H I density in the LISM are in the range 0.1--0.2 cm-3. The He I/H I ratio toward the white dwarf G191-B2B, which is only 7 deg from Capella, has been measured to be He I/H I = 0.068--0.082 from the Extreme-Ultraviolet Explorer data. These results indicate hydrogen and helium ionization fractions toward Capella of X(H) = 0.45 +/- 0.25 and X(He) = 0.57 +/- 0.23, respectively, confirming that hydrogen and helium are substantially ionized in the LISM.
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