Characteristic combustion time for helium, τ_nuc, as a function of temperature for ρ = 5.28 × 10³   g   cm^-3. Also shown are the free-fall time, τ_ff, the sound-crossing time (over the smoothing length h), τ_sonic, and the time spent by the disk to complete one orbit, τ_orb. The curves correspond to model C in Table 2.