The spectrum of \beta CMa ( Mirzam ) between 1000 and 1200Å was recorded at a wavelength resolving power \lambda / \Delta \lambda \sim 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 C ii , C ii * , N i and O i 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 ^ { -1 } , designated by Dupin & Gry as Components C and D , respectively , we use determinations of N ( \mbox { C { \sc ii } } * ) / N ( \mbox { C { \sc ii } } ) and N ( \mbox { Mg { \sc i } } ) / N ( \mbox { Mg { \sc 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 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 ( { H } _ { total } ) = 6 \times 10 ^ { 18 } { cm } ^ { -2 } , we find that the neutral fraction n ( \mbox { H { \sc 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 \times 10 ^ { 19 } { cm } ^ { -2 } , we arrive at n ( \mbox { H { \sc 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 are exposed to the strong , ionizing radiation fields from \epsilon and \beta CMa .
The presence of Si iii indicates the existence of additional gas with even higher levels of ionization .