EPSILON-CANIS-MAJORIS AND THE IONIZATION OF THE LOCAL CLOUD

The Lyman continuum radiation from the brightest extreme ultraviolet ( EUV) source, the B2 II star epsilon Canis Majoris (Adara), is so inten se that it dominates the local stellar EUV radiation field at waveleng ths longer than 450 Angstrom and therefore sets a lower limit to the i onization of hydrogen in the Local Cloud. Using the EUV (70-730 Angstr om) spectrum of epsilon CMa taken with the Extreme Ultraviolet Explore r Satellite (EUVE) and simple models that extrapolate this spectrum to the Lyman edge at 912 Angstrom, we have determined the local interste llar hydrogen photoionization parameter Gamma solely from epsilon CMa to be 1.1 x 10(-15) s(-1). This figure is a factor of 7 greater than p revious estimates of Gamma calculated for all nearby stars combined (B ruhweiler and Cheng 1988). Using measured values of the density and te mperature of neutral interstellar hydrogen gas in the Local Cloud, we derive a particle density of ionized hydrogen n(H+) and electrons n(e) of 0.015-0.019 cm(-3) assuming ionization equilibrium and a helium io nization fraction of less than 20%. These values correspond to a hydro gen ionization fraction, X(H) from 19% to 15%, respectively. The range of these derived quantities is due to the uncertainties in the local values of the neutral hydrogen and helium interstellar densities deriv ed from both (1) solar backscatter measurements of Ly alpha lines of-h ydrogen and helium (1216 and 584 Angstrom), and (2) the average neutra l densities along the line of sight to nearby stars. The local proton density produced by epsilon CMa is enough to allow the ionization mech anism of Ripken and Fahr (1983) to work at the heliopause and explain the discrepancy between the neutral hydrogen density derived from sola r backscatter measurements and line-of-sight averages to nearby stars. A large value of electron density in the Local Cloud of n(e) similar to 0.3-0.7 cm(-3) (T = 7000 K) has recently been reported by Lallement et al. (1994) using observations of Mg II and Mg I toward Sirius A. W e show that if such a high value exists, it cannot result from the EUV stellar radiation field and, therefore, must be due to a strong diffu se source of EUV radiation.