Very Large Telescope deep echelle spectroscopy of Galactic planetary
  nebulae NGC 6153, M 1-42 and Hf 2-2
  We present deep spectroscopy of three Galactic planetary nebulae (PNe) with
large abundance discrepancy factors (ADFs): NGC6153, M1-42 and Hf2-2. The
spectra were obtained with VLT/UVES and cover the whole optical range
(3040-11,000 A) with a spectral resolution of ~20,000. For all three PNe,
several hundred emission lines were detected and identified, with more than 70
per cent of them as permitted lines. Most of these permitted lines are excited
by recombination. Numerous weak optical recombination lines (ORLs) of O II, C
II, N II and Ne II were detected in the spectra and accurate fluxes measured.
Line flux tables were compiled and ready for use by the community of nebular
astrophysics. These ORLs were critically analyzed using the effective
recombination coefficients recently calculated for the optical recombination
spectrum of N II and O II under the physical conditions of photoionized gaseous
nebulae. Plasma diagnostics based on the heavy element ORLs were carried out
using the new atomic data. Elemental abundances derived from the ORLs were
systematically higher than those derived from the collisionally excited lines
(CELs) by a factor of ~10, 22 and 80 for NGC6153, M1-42 and Hf2-2,
respectively. The electron temperatures derived from the heavy element ORLs are
systematically lower than those derived from the CELs. These ORL versus CEL
abundance and temperature discrepancies, previously observed in the three PNe
through deep spectroscopy with medium to low spectral resolution, are thus
confirmed by our analysis of the deep echelle spectra using the new atomic
data.
