  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.
