The [CII] 157.7 $\mu$m Line and [OI] 145.5 $\mu$m Line

The [CII] 157.7 $\mu$m and [OI] 145.5 $\mu$m lines are shown in Figure 5.9. CII appears in absorption towards Sgr B2 due to the cold clouds along the line of sight. This absorption traces the outer two layers of the PDR and should be similar to the absorption seen in HI (although HI does not extend as deeply into each cloud). There is also an emission component at the velocity of Sgr B2 itself. The 145.5 $\mu$m line appears in emission at the velocity of Sgr B2. The [CII] line has been modelled by C. Vastel (see Vastel et al., 2002). She accounted for the emission at the velocity of Sgr B2 by assuming that the [OI] 145 $\mu$m line emission occurred in the same region. This is likely because higher temperatures are required for [OI] emission at 145 $\mu$m than at 63 $\mu$m. This means that the 145 $\mu$m emission is probably related to the CII layers rather than cooler layers within the Sgr B2 complex. There is no emission at 145 $\mu$m in the line of sight clouds, showing that these are too cold to populate the atomic oxygen $^{3}$P$_{0}$ level. The fit parameters for the 145 $\mu$m line were used with the velocities and line widths of HI to model the shape of the [CII] line. The final fits, after convolution with the LWS FP resolution element are shown in Figure 5.9 and are described in more detail by Vastel et al. (2002). The optical depth and column densities of CII components below $+15$ km s$^{-1}$ are shown in Table 5.3.

Figure 5.9: [OI] 145 $\mu$m and [CII] 157 $\mu$m lines shown with fit from Vastel et al. (2002).