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

TL;DR

The paper presents a meticulous reanalysis of deep VLT/UVES spectra for three high-ADF Galactic planetary nebulae (Hf 2-2, M 1-42, NGC 6153), combining 1D and 2D (PV) diagnostics to dissect morpho-kinematic structures. By using absolute flux calibration, extensive line identifications, and diagnostics from CELs, HI recombination, He I, and heavy-element ORLs, it demonstrates a consistent pattern: a cold, metal-rich plasma component dominates ORL emission while a warmer, standard-metallicity plasma dominates CEL emission. The work derives temperatures and densities that reveal two coexisting phases with distinct spatial and kinematic properties, and it quantifies abundance discrepancy factors (ADFs) and the mass ratios of heavy elements between these phases, supporting the dual-abundance scenario. The findings have significant implications for the interpretation of chemical abundances in ionized nebulae and the cooling physics of metal-rich gas, and they motivate future 3D high-resolution spectroscopy to fully map the cold component’s extent and origin, including its relationship to central binary activity. Overall, the study tightens the link between ADFs, cold metal enrichment, and nebular kinematics, providing a concrete, spatially resolved framework for understanding how ORL-dominated gas coexists with CEL-dominated gas in high-ADF PNe. Math: Temperatures and densities are discussed with symbols Te, ne, ADF, and mass ratios Mc/Mw, each derived under carefully chosen plasma conditions and corrected for recombination or charge-exchange effects where needed.

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.

Figures (35)

• Figure 1: VLT UVES slit-view images of Hf 2-2 (panels a and b), M 1-42 (panel c), and NGC 6153 (panel d), showing the red-arm slit positions during spectroscopic observations with Dichroic 2. Panels (a) and (b) show the slit positions for the first and second science exposures of Hf 2-2, respectively; note that there is a slight offset in slit position (perpendicular to the direction of slit length) between the two exposures. Slit width is 2; north is up and east to the left.
• Figure 2: Central star positions (dashed lines) and extraction regions (shaded area between two translucent lines) of Hf 2-2 (upper panel), M 1-42 (middle panel) and NGC 6153 (lower panel). Spectra of field stars near M 1-42 are largely ruled out.
• Figure 3: CELs plasma diagnostic diagram of Hf 2-2 (left), M 1-42 (middle) and NGC 6153 (right). Numbers of the same colour labeled near curves represent the diagnostic line ratios with the IDs in Table \ref{['temden']}. The solid and dashed lines are the temperature and density diagnostics. The auroral lines used in the [N ii] and [O iii] temperature diagnostics have been corrected to remove the recombination excitation contributions.
• Figure 4: The VLT UVES spectra (grey lines) of Hf 2-2 (left),M 1-42 (middle) and NGC 6153 (right) in the vicinity of Balmer jump/discontinuity, with radial velocity and extinction corrected. The black dashed lines are fittings of continua on both sides the Balmer jump and extrapolated towards 3643 Å.
• Figure 5: Same as Figure \ref{['fig:BJ']}, but the UVES spectra and continuum fittings near the Paschen discontinuity at 8204 Å.