The cutoff level of the empirical model is somewhat arbitrary. For neutral species it is partly based on ionization (K10), but this criterion obviously does not apply to ions. With a cutoff level at 5Rp for the ions only, the M7 model yields line-integrated transit depths of 3.9%, 8%, and 5.8% in the C II 1334.5 Å, C II 1335.7 Å, and Si III lines, respectively, if 40% of silicon is Si2+. These values agree with the observed values to better than 2σ. Similarly, by extending the cutoff level of the C2 model to 5Rp, we obtained transit depths of 3.2%, 6.7%, and 4.6% in the C II 1334.5 Å, C II 1335.7 Å, and Si III lines, respectively (see Table 2). These values deviate from the observed values by 2σ, 0.9σ, and 2.6σ, respectively. The transit depths predicted by the M7 model are higher partly because the mean temperature of 8250 K is higher than the corresponding temperature in the C2 model (Paper I). This also leads to the higher Si2+/Si+ ratio that we used here.
M7 model
cutoff level
5Rp
Rp
M7 model
line-integrated transit depths
3.9%
%
M7 model
line-integrated transit depths
8%
%
M7 model
line-integrated transit depths
5.8%
%
silicon
Si2+
40%
%
C2 model
cutoff level
5Rp
Rp
C2 model
transit depths
3.2%
%
C2 model
transit depths
6.7%
%
C2 model
transit depths
4.6%
%
M7 model
mean temperature
8250 K
K