Ignition curves for carbon (lower/left group of lines) and oxygen (upper/right group of lines) in different approximations. Solid lines are obtained using Eq. (19) and the quantum correction according to Sect. 2.3; long-dashed lines: Eq. (12) and the quantum correction; dot-dashed lines: Eq. (18) without any correction. The lines are plotted heavy if the characteristic nuclear fuel burning time t_burn is shorter than 1 Myr; the lines are plotted thin if 1   Myr < t_burn < 14 Gyr. In the domain where t_burn > 14 Gyr the lines are dotted. For the carbon case we also plot the fit (Potekhin et al. 2003) to the results of Sahrling & Chabrier (1998) (the short-dashed line).