Recombination Timescale and Ionisation Persistence in NGC 6826

This paper computes a self-contained budget for NGC 6826 from published observational values, with every derived number produced by the deposited computation script. NGC 6826, sometimes called the Blinking Planetary, has fast low-ionisation emission regions embedded in a bright elliptical shell. A basic question for any nebula whose central star varies or whose ionising output may decline is how long the ionisation would persist if the supply of ionising photons stopped. That persistence time is the recombination timescale, set by the electron density and the case-B recombination coefficient at the nebular temperature. Comparing it with the expansion time shows whether the nebula responds quickly or slowly to changes in its star. The headline results are: electron_density_cm3 = 4e+03; electron_temperature_K = 1e+04; alpha_B_cm3_s = 2.6e-13; recombination_time_yr = 30.5; expansion_age_yr = 4889; ratio_age_to_trec = 160; stromgren_radius_pc = 0.0731. The recombination time is short compared with the expansion age, by more than two orders of magnitude. The nebula is therefore in photoionisation equilibrium in the practical sense: its ionisation structure tracks the present stellar output rather than preserving a memory of earlier output. Any genuine change in the stellar ionising flux would be reflected in the nebular spectrum within decades, which makes this object a responsive monitor of its central star. The calculation is presented with its inputs, relations and sensitivities stated openly, so that any revised distance, density or stellar parameter can be substituted and the verdict recomputed directly.