The chemical composition of the Orion star-forming region: II. Stars,
  gas, and dust: the abundance discrepancy conundrum
  We re-examine the recombination/collisional emission line (RL/CEL) nebular
abundance discrepancy problem in the light of recent high-quality abundance
determinations in young stars in the Orion star-forming region.
  We re-evaluate the CEL and RL abundances of several elements in the Orion
nebula and estimate the associated uncertainties, taking into account the
uncertainties in the ionization correction factors for unseen ions. We estimate
the amount of oxygen trapped in dust grains for several scenarios of dust
formation. We compare the resulting gas+dust nebular abundances with the
stellar abundances of a sample of 13 B-type stars from the Orion star-forming
region (Ori\,OB1), analyzed in Papers I and III of this series.
  We find that the oxygen nebular abundance based on recombination lines agrees
much better with the stellar abundances than the one derived from the
collisionally excited lines. This result calls for further investigation. If
the CEL/RL abundance discrepancy were caused by temperature fluctuations in the
nebula, as argued by some authors, the same kind of discrepancy should be seen
for the other elements, such as C, N and Ne, which is not what we find in the
present study. Another problem is that with the RL abundances, the energy
balance of the Orion nebula is not well understood. We make some suggestions
concerning the next steps to undertake to solve this problem.
