Ionized gas diagnostics from protoplanetary discs in the Orion Nebula
  and the abundance discrepancy problem
  We present results from integral field spectroscopy with PMAS. The observed
field contains: five protoplanetary discs (also known as proplyds), the
high-velocity jet HH 514 and a bowshock. Spatial distribution maps are obtained
for different emission line fluxes, the c(H{\beta}) coefficient, electron
densities and temperatures, ionic abundances of different ions from
collisionally excited lines (CELs), C2+ and O2+ abundances from recombination
lines (RLs) and the abundance discrepancy factor of O2+, ADF(O2+). We find that
collisional de-excitation has a major influence on the line fluxes in the
proplyds. If this is not properly accounted for then physical conditions
deduced from commonly used line ratios will be in error, leading to unreliable
chemical abundances for these objects. We obtain the intrinsic emission of the
proplyds 177-341, 170-337 and 170-334 by a direct subtraction of the background
emission, though the last two present some background contamination due to
their small sizes. A detailed analysis of 177-341 spectra reveals the presence
of high-density gas (3.8\times10^5 cm^-3) in contrast to the typical values
observed in the background gas of the nebula (3800 cm^-3). We also explore how
the background subtraction could be affected by the possible opacity of the
proplyd. We construct a physical model for the proplyd 177-341 finding a good
agreement between the predicted and observed line ratios. Finally, we find that
the use of reliable physical conditions returns an ADF(O2+) about zero for the
intrinsic spectra of 177-341, while the background emission presents the
typical ADF(O2+) observed in the Orion Nebula. We conclude that the presence of
high-density ionized gas is severely affecting the abundances determined from
CELs and, therefore, those from RLs should be considered as a better
approximation to the true abundances.
