Lyman α absorption A versus Doppler-shift velocity Δv from line centre, evaluated for our standard 3200 × 4800 simulation at t = 2 × 10⁶ s. Absorption at −50 km s⁻¹ is stronger than at +50 km s⁻¹ because of the bulk motion of charge-exchanged neutral hydrogen streaming from the star towards the observer. The line wings at larger Doppler shifts are primarily thermally broadened at T_* = 10⁶ K. The absorption A ≈ 9 per cent at Δv = ±100 km s⁻¹, in accordance with HST observations (see Fig. 5).