A Morpho-kinematic Study of Galactic High-ADF PNe Based on the VLT/UVES Deep Spectroscopy

Abstract

We report detailed analyses of deep, high-resolution spectra of three Galactic planetary nebulae (PNe) with high abundance discrepancy factors (ADFs), Hf2-2, M1-42 and NGC6153, obtained with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the 8.2m Very Large Telescope (VLT). These spectra were carefully reduced, including rigorous absolute flux calibration, yielding detections of ~410-800 emission lines in each PN. Plasma diagnostics and abundance calculations were critically performed using nebular lines. In all three PNe, the electron temperatures derived using the collisionally excited lines (CELs) are higher than those yielded by the HI Balmer and Paschen jumps, while the temperatures yielded by the OII and NII optical recombination lines (ORLs) are very low, <2000 K, indicating that the heavy-element ORLs probe cold nebular regions. The ORL abundances of N, O and Ne are systematically higher than the corresponding CEL values, confirming high ADFs in the three objects. Position-velocity (PV) diagrams were created, and spatio-kinematical studies show that CELs come from the outer nebular regions, while the ORL-emitting regions are close to nebular center. Additionally, the velocity indicated by CEL line-splitting decreases with ionization potential, which was not obvious in ORLs. These spatial and kinematic differences support two distinct components of ionized gas: a cold, metal-rich component and a warmer component with normal metallicity. Heavy elements are strongly enriched in the cold gas, while its H^+ fraction is low but still produces significant HI emission, affecting CEL abundance estimates.