2015MNRAS.447..817B

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, Te(O+2) is smaller than Te(N+) and possibly Te(S+), the ionized mass is ≃ 0.07M_☉ and the progenitor zero-age main-sequence mass is 1.2-2M_☉. 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, HeII 4686/Hei 5876, [OII]3726/[OIII]5007 and [OII]3726/3729, but not the other density sensitive line ratios. Including fluorescence and recombination, we found good agreement with observed CII and CIII line intensities, OII and OIII model intensities which are 2.6 and 7.8 times smaller than observed and large erratic differences with observations in CIII, NII and NIII lines. UV and IR data suggest that [OII] 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/cm3. 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.