Back to the Abstracts Index Page
J. V. Vallerga and B. Y. Welsh
Eureka Scientific, 2452 Delmer St., Oakland, CA 94602
The Lyman continuum radiation from the brightest extreme ultraviolet (EUV) source, the B2 II star Epsilon CMa (Adara), is so intense that it dominates the local stellar EUV radiation field at wavelengths longer than 450 Å and therefore sets a lower limit to the ionization of hydrogen in the Local Cloud. Using the EUV (70 -- 730 Å) spectrum of Epsilon CMa taken with the Extreme Ultraviolet Explorer Satellite (EUVE) and simple models that extrapolate this spectrum to the Lyman edge at 912 Å, we have determined the local interstellar hydrogen photoionization parameter, Gamma, solely from Epsilon CMa to be 1.1E-15/s. This figure is a factor of 7 greater than previous estimates of Gamma calculated for all nearby stars combined (Bruhweiler&Cheng 1988). Using measured values of the density and temperature of neutral interstellar hydrogen gas in the Local Cloud, we derive a particle density of ionized hydrogen, nH+, and electrons, n_e, of 0.015 -- 0.019/ cm^3, assuming ionization equilibrium and a helium ionization fraction of less than 20%. These values correspond to a hydrogen 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 derived from both (a) solar backscatter measurements of Lyman alpha lines of hydrogen and helium (1216 Å and 584 Å), and (b) the average neutral densities along the line-of-sight to nearby stars. The local proton density produced by Epsilon CMa is enough to allow the ionization mechanism of Ripken & Fahr (1983) to work at the heliopause and explain the discrepancy between the neutral hydrogen density derived from solar backscatter measurements and line-of-sight averages to nearby stars. A large value of electron density in the Local Cloud of n_e ~ 0.3 -- 0.7 /cm^3 (T~ 7000 K) has recently been reported by Lallement et al. (A&A, 168, 225, 1994) using observations of Mg II and Mg I toward Sirius A. We 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 diffuse source of EUV radiation.
Back to the Abstracts Index Page
