Physical conditions and chemical abundances in PN M 2-36. Results from
  deep echelle observations
  We present a spectrum of the planetary nebula M 2-36 obtained using the Ultra
Violet and Visual Echelle Spectrograph (UVES) at the Very Large Telescope
(VLT). 446 emission lines are detected. We perform an analysis of the chemical
composition using multiple electron temperature ($T_{e}$) and density ($n_{e}$)
diagnostics. $T_{e}$ and $n_{e}$ are computed using a variety of methods,
including collisionally excited line (CEL) ratios, O$^{++}$ optical
recombination lines (ORLs), and measuring the intensity of the Balmer jump.
Besides the classical CEL abundances, we also present robust ionic abundances
from ORLs of heavy elements. From CELs and ORLs of O$^{++}$, we obtain a new
value for the Abundance Discrepancy Factor (ADF) of this nebula, being
ADF(O$^{++})=$ 6.76 $\pm$ 0.50. From all the different line ratios that we
study, we find that the object cannot be chemically homogeneous; moreover, we
find that two-phased photoionization models are unable to simultaneously
reproduce critical \ion{O}{ii} and [\ion{O}{iii}] line ratios. However, we find
a three-phased model able to adequately reproduce such ratios. While we
consider this to be a toy model, it is able to reproduce the observed
temperature and density line diagnostics. Our analysis shows that it is
important to study high ADF PNe with high spectral resolution, since its
physical and chemical structure may be more complicated than previously
thought.
