Physical conditions and chemical abundances in PN M 2-36. Results from deep echelle observations

Abstract: 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.