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

Abstract

The spectrum of beta CMa (Mirzam) between 1000 and 1200 A was recorded at a wavelength resolving power ~ 60 000 by the Interstellar Medium Absorption Profile Spectrograph (IMAPS) during its orbital flight on the ORFEUS-SPAS II mission in 1996. New information about interstellar absorption lines of CII, CII*, NI and OI from this spectrum are combined with the HST results reported by Dupin & Gry (1998) to arrive at new conclusions about the physical properties of the absorbing gas in front of beta CMa. For two prominent velocity components centered at heliocentric velocities of +20.0 and +30.5 km/s, designated by Dupin & Gry as Components C and D respectively, we use determinations of N(CII*)/N(CII) and N(MgI)/N(MgII) 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 hydrogen ionizations by comparing these elements to sulfur, which is likely not to be depleted onto dust grains. For Component C with an approximate column density of neutral and ionized hydrogen N(Htotal)=6 10^{18} cm^-2, we find that the neutral fraction n(HI)/n(Htotal)=0.25, 400<T<6500 K, and 0.08<n(e)<0.6 cm^-3, while for Component D with N(Htotal)=1.2 10^{19}cm^-2, we arrive at n(HI)/n(Htotal)=0.035, 8000<T<14000 K, and 0.09<n(e)<0.2cm^-3. The relatively large ionization fractions of H can arise if the clouds are about 130 pc away from us, so that they are exposed to the strong, ionizing radiation fields from epsilon and beta CMa. The presence of SiIII indicates the existence of additional gas with even higher levels of ionization.