Hydrogen loss and O₂ pressure for Proxima-b for an initial time of t₀ = 3 Myr. Top panel: hydrogen loss computed with method (1) and (2) in full red lines and with method (3) in colored dashed lines. The gray areas correspond to the amount of hydrogen created by photolysis for two different efficiencies ϵ_α = 1.0 (all the incoming energy is used for photolysis) and ϵ_α = 0.1 (only 10% of the incoming energy is used). The edges of the gray areas were calculated using two different assumptions on the wavelength range important for photolysis: the lower edge corresponds to the energy flux in the H Ly α band (Table 2), the upper edge corresponds to the energy flux in a wider band: 10–170 nm. The vertical lines represent the time at which the planet reaches the 1.5 S_⊕ HZ inner edge, the 0.9 S_⊕ HZ inner edge and the age of the system (4.8 Gyr). Bottom panel: O₂ pressure building up in the atmosphere computed with method (3).