Echelle spectroscopy and photoionization modelling of the entire planetary nebula NGC 6210

Abstract

High-resolution spectroscopy of NGC 6210, show that recombination line abundances of O⁺² and Ne⁺² are two to three times larger than forbidden line abundances, T_e(O⁺²) is smaller than T_e(N⁺) and possibly T_e(S⁺), the ionized mass is ≃0.07 M_⊙ and the progenitor zero-age main-sequence mass is 1.2-2 M_⊙. If electrons are in equilibrium, recombination lines are likely produced in a cold low-density medium. A photoionization model (cloudy 13.03) with a positive density gradient and two exciting sources, reproduced most of the optical spectrum, the temperature distribution of the nebula, He ii 4686/He i 5876, [O ii]3726/[O iii]5007 and [O ii]3726/3729, but not the other density sensitive line ratios. Including fluorescence and recombination, we found good agreement with observed C ii and C iii line intensities, O ii and O iii model intensities which are 2.6 and 7.8 times smaller than observed and large erratic differences with observations in C iii, N ii and N iii lines. UV and IR data suggest that [O ii] emission is from a region ∼ 6 times less dense than assumed by the model and that IR lines are produced in a medium where the density is ∼1000 cm^-3. There may be a lower density medium beyond a dense ring described by the photoionization model, as well as a cold low-density component. A multicomponent model is required to reproduce the entire spectrum of NGC 6210.