Determine the origin and excitation mechanism of low-ionization structures

Determine the physical origin and dominant excitation mechanism of low-ionization structures in planetary nebulae, including whether their observed emission is primarily produced by shocks, photoionization, or another process.

Background

Low-ionization structures (LISs) in planetary nebulae exhibit enhanced emission from low-ionization species and occur in forms including knots, jets, and jet-like structures. Their physical origin and excitation processes are difficult to establish because several mechanisms—including shocks, photoionization, photoevaporation, and radiative processes—can produce related emission-line signatures.

The paper uses ultraviolet-to-near-infrared spectroscopy of the NGC 7009 ansae to assess these mechanisms. The authors find evidence that slow shocks contribute substantially to the excitation of the observed atomic, ionic, and molecular emission, but the broader origin and dominant excitation mechanism of LISs remains unresolved.

References

Although these findings provide new insight into the nature of such structures, there is still no conclusive scenario for their origin.

— X-Shooter view of planetary nebulae microstructures I: The case of NGC 7009 knots  (2610.13587 - Konstantinou et al., 9 Oct 2026) in Introduction; Section 3.1, “LISs excitation traced by the atomic and ionic emission lines”