In Paper I we noted that the stellar XUV flux, or the corresponding alternative heat source, would have to be 5–10 stronger than the average solar flux to produce a mean temperature between 8000 and 9000 K. Under such circumstances, the predicted transit depths in the C II and Si III lines would obviously be even higher than the values predicted by the SOL2 model. Indeed, higher temperatures broaden absorption in the wings of the line profiles and may help to explain the in-transit flux differences better (see Fig. 7). However, the energy input and temperature in the model cannot be increased without bound. Higher temperatures and flux lead to more efficient ionization of the neutral species, and as a result the transit depths in the H Lyman α and O I lines begin to decrease. Also, mass loss rates of 109–1010 kg s−1 lead to the loss of 10–100% of the planet’s mass over the estimated lifetime of the system, and this probably limits reasonable energy inputs to less than ∼10 times the solar average on HD209458b.
stellar XUV flux, or the corresponding alternative heat source
stronger
5–10
mean temperature
between 8000 and 9000 K
K
mass
loss rates
109–1010 kg s−1
kg s−1
planet’s mass
loss
10–100%
%
solar average on HD209458b
energy inputs
less than ∼10 times
times