Electron densities, temperatures and ionization rates in two interstellar clouds in front of beta Canis Majoris, as revealed by UV absorption lines observed with IMAPS

The spectrum of beta CMa (Mirzam) between 1000 and 1200 Angstrom was record ed at a wavelength resolving power lambda/Delta lambda similar to 60 000 by the Interstellar Medium Absorption Profile Spectrograph (IMAPS) during its orbital flight on the ORFEUS-SPAS II mission in 1996. New information abou t interstellar absorption lines of C II, C II*, N I and O I from this spect rum are combined with the HST results reported by Dupin & Cry (1998) to arr ive at new conclusions about the physical properties of the absorbing gas i n front of beta CMa. For two prominent velocity components centered at heli ocentric velocities of +20.0 and +30.5 km s(-1), designated by Dupin & Cry as Components C and D, respectively, we use determinations of N(C II*)/N(C II) and N(Mg I)/N(Mg II) to solve for temperatures and electron densities. From our knowledge that oxygen and nitrogen have their ionizations coupled to that of hydrogen through charge exchange reactions, we can derive the hy drogen ionizations by comparing these elements to sulfur, which is likely n ot to be depleted onto dust grains. For Component C with an approximate col umn density of neutral and ionized hydrogen N(H-total) = 6 x 10(18)cm(-2), we find that the neutral fraction n(H I)/n(H-total) = 0.25, 400 < T < 6500 K, and 0.08 < n(e) < 0.6 cm(-3), while for Component D with N(H-total) = 1. 2 x 10(19)cm(-2), we arrive at n(H I)/n(H-total) = 0.035, 8000 < T < 14 000 K, and 0.09 < n(e) < 0.2 cm(-3) The relatively large ionization fractions of H can arise if the clouds are about 130 pc away from us, so that they ar e exposed to the strong, ionizing radiation fields from epsilon and beta CM a. The presence of Si III indicates the existence of additional gas with ev en higher levels of ionization.