Expansion and Halo Density Budget of the Owl Nebula M97

This paper computes a self-contained budget for M97 (NGC 3587) from published observational values, with every derived number produced by the deposited computation script. The Owl Nebula shows two dark cavities in projection, interpreted as the evacuated lobes of a barrel-like shell, surrounded by a faint extended halo. The main shell expansion speed and radius give a dynamical age, while the halo records the earlier asymptotic-giant-branch wind. Comparing the shell pressure with the halo density tests the standard picture in which the bright shell is the swept-up and ionised portion of that earlier wind rather than a separate ejection. The headline results are: R_pc = 0.28; v_exp_km_s = 17; dynamical_age_yr = 16105; electron_density_cm3 = 350; thermal_pressure_Pa = 9.66e-11; ionised_mass_Msun = 0.372; kinetic_energy_J = 1.07e+38; emitting_volume_fraction = 0.4; sound_speed_km_s = 11.7; mach_number = 1.45. The shell age is longer than in compact nebulae and the present pressure is modest, consistent with a shell that has done most of its work against the surrounding wind. The stored kinetic energy is a small fraction of the energy radiated by the central star over the shell lifetime. The halo interpretation survives the budget test: the shell mass that follows from the density and volume is of the order the progenitor wind could have supplied over the relevant interval. 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.