The impact of strong recombination on temperature determination in
  planetary nebulae
  The long-standing difference in chemical abundances determined from optical
recombination lines and collisionally excited lines raises questions about our
understanding of atomic physics, as well as the assumptions made when
determining physical conditions and chemical abundances in astrophysical
nebulae. Here, we study the recombination contribution of [O III] 4363 and the
validity of the line ratio [O III] 4363/4959 as a temperature diagnostic in
planetary nebulae with a high abundance discrepancy. We derive a fit for the
recombination coefficient of [O III] 4363 that takes into account the radiative
and dielectronic recombinations, for electron temperatures from 200 to 30,000
K. We estimate the recombination contribution of [O III] 4363 for the planetary
nebulae Abell 46 and NGC 6778 by subtracting the collisional contribution from
the total observed flux. We find that the spatial distribution for the
estimated recombination contribution in [O III] 4363 follows that of the O II
4649 recombination line, both peaking in the central regions of the nebula,
especially in the case of Abell 46 which has a much higher abundance
discrepancy. The estimated recombination contribution reaches up to 70% and 40%
of the total [O III] 4363 observed flux, for Abell 46 and NGC 6778,
respectively.
