A NEW MEASUREMENT OF THE ELECTRON-DENSITY IN THE LOCAL INTERSTELLAR-MEDIUM

Using the echelle-A grating of the Goddard High-Resolution Spectrograp h, we have observed the C II lambda lambda 1335, 1336 emission lines o f the nearby (d = 13.3 pc) star system Capella (G8 III + G1 III). Inte rstellar C Ir absorption features are detected within both stellar emi ssion lines. The ground-state and excited-state C II column densities derived from these absorption lines imply an electron density of n(e) = 0.11(-0.06)(+0.12) cm(-3) in the local interstellar medium (LISM). U nlike previous derivations of n(e) from Mg II/Mg I ratios, the density suggested by the C II lines is independent of assumptions about ioniz ation equilibrium. Current estimates of the H I density in the LISM ar e in the range 0.1-0.2 cm(-)3. The He I/H I ratio toward the white dwa rf G191-B2B, which is only 7 degrees from Capella, has been measured t o be He I/H I = 0.068-0.082 from the Extreme-Ultraviolet Explorer data . These results indicate hydrogen and helium ionization fractions towa rd Capella of X(H) = 0.45 +/- 0.25 and X(He) = 0.57 +/- 0.23, respecti vely, confirming that hydrogen and helium are substantially ionized in the LISM.