---
title: LISs in Planetary Nebulae
url: https://www.emergentmind.com/topics/low-ionization-structures-liss
type: topic
---

# LISs in Planetary Nebulae

Searching arXiv for recent and foundational papers on low-ionization structures in planetary nebulae.
Low-ionization structures (LISs) are small-scale nebular features in planetary nebulae (PNe) that are prominent in emission from low-ionization species such as [N II], [S II], [O II], and [O I]. They include knots, filaments, jets, clumps, ansae, and related condensations embedded within rims, shells, and haloes, and they are found in essentially all morphological classes of PNe. Their importance follows from the fact that they are sites where fast collimated outflows, shocks, photoevaporation, and strong UV irradiation interact with dense nebular material, so they preserve information on nebular shaping, kinematic history, and the balance between radiative and mechanical excitation [2409.00377, 2508.20240, 2308.04577].

## 1. Definition, taxonomy, and observational identity

LISs are defined observationally by their enhanced low-ionization emission relative to the surrounding nebula. Across the literature synthesized in recent statistical and object-by-object studies, they appear as compact knots, filaments, jets, jet-like systems, ansae, arcs, and isolated condensations, and are often identified in [N II] $\lambda\lambda 6548,6584$, [S II] $\lambda\lambda 6716,6731$, [O II] $\lambda 3727$, and sometimes [O I] $\lambda 6300$ [2308.04577, 1509.05346].

They span a wide kinematic range. One review-oriented observational study describes LISs as compact structures with velocities from about $30\ \mathrm{km\ s^{-1}}$ to $350\ \mathrm{km\ s^{-1}}$, and later work continues to treat them as a heterogeneous family that includes both slowly moving peripheral knots and highly collimated fast outflows [1604.04763, 1611.06578]. Terminology such as FLIER-like features and fast-moving low-ionization emission regions is used in parts of the literature, but current work generally treats these morphologies collectively as LISs when their defining property is enhanced low-ionization emission [2003.01737, 1707.09650].

This heterogeneous morphology is not a classification nuisance but a physical clue. A plausible implication is that LISs do not arise from a single formation channel; rather, they sample multiple dynamical regimes that later become processed by the same PN radiation field.

## 2. Plasma properties and the density problem

The most robust global result is that LISs have lower electron densities than the rims or shells of their host nebulae. In the statistical analysis of 33 PNe containing 104 LISs and 88 rims/shells, LISs have a median electron density of about $\sim 1700~\mathrm{cm^{-3}}$, while rims/shells have $\sim 2700~\mathrm{cm^{-3}}$; the interquartile ranges are $800\!-\!2700~\mathrm{cm^{-3}}$ for LISs and $1800\!-\!5000~\mathrm{cm^{-3}}$ for rims/shells [2308.04577, 2508.20240]. Long-slit and IFU studies of individual nebulae reinforce the same pattern: LISs are characterized by lower or at most equal $N_e$ than associated rims and shells, and in NGC 3242 the lower density is localized to the LISs while an increase is observed at the rim [2209.10988, 2503.16662].

By contrast, temperatures are much less distinctive. The broad consensus is that $T_e[\mathrm{N\,II}]$ does not show significant variations between LISs and surrounding nebular components, while $T_e[\mathrm{O\,III}]$ may be slightly higher in LISs but with uncertainties large enough that object-by-object significance is weak [2308.04577, 2209.10988]. In NGC 3242, the ordering derived from multiple diagnostics is
$$
T_{\rm e}([\mathrm{S\,III}]) > T_{\rm e}([\mathrm{N\,II}]) > T_{\rm e}(\mathrm{H\,I}) \approx T_{\rm e}(\mathrm{PJ}) > T_{\rm e}(\mathrm{He\,I}),
$$
with temperatures generally higher in the inner nebular structures rather than specifically enhanced in the LISs [2503.16662].

Chemical abundances are likewise not a primary discriminator. Statistical and spatially resolved studies consistently report no significant LIS/rim difference in total abundances of He, N, O, Ne, Ar, Cl, and S, with only limited suggestions of slightly higher nitrogen abundances in some knots of NGC 5307 [2308.04577, 1707.09650, 1509.05346]. The low-ionization brightness of LISs therefore does not require a chemically distinct ejecta component.

The central tension is that many formation models require total densities well above $10^4~\mathrm{cm^{-3}}$, and in some scenarios as high as $10^{6-7}~\mathrm{cm^{-3}}$, whereas the ionized gas traced by optical diagnostics is systematically less dense than the surrounding nebula [2508.20240]. Current work increasingly interprets this not as a contradiction, but as evidence that optical diagnostics sample only the ionized skin while a neutral or molecular component remains hidden.

## 3. Molecular and neutral counterparts

Near-infrared imaging has established that LISs can contain molecular gas. Deep NIRI@Gemini observations detected H$_2$ emission in the LISs of K 4-47 and NGC 7662, showing that both highly moving and slowly moving LISs can host molecular material [1604.04763, 1611.06578]. Subsequent very deep narrow-band H$_2$ images of NGC 7009 and NGC 6543 detected H$_2$ emission in LISs in both nebulae, with H$_2$ 1-0 S(1) surface brightnesses of $(0.46\text{--}2.9)\times10^{-4}\ \mathrm{erg\ s^{-1}\ cm^{-2}\ sr^{-1}}$ in NGC 7009 and $(0.29\text{--}0.48)\times10^{-4}\ \mathrm{erg\ s^{-1}\ cm^{-2}\ sr^{-1}}$ in NGC 6543 [2003.01737].

A larger H$_2$ imaging survey later nearly doubled the number of host PNe with confirmed LIS/H$_2$ associations. In 19 LISs across five Galactic PNe, the warm H$_2$ masses range from $0.4$ to $10\times10^{-7}\,M_{\odot}$, with an average of $4.6\times10^{-7}\,M_{\odot}$, and the excited H$_2$ molecular mass is between 200 and 5000 times lower than the corresponding ionized gas mass [2508.20240]. This was used to argue that the systematically lower electron density in LISs is linked to the presence of H$_2$ molecular gas.

Neutral atomic gas has now been added to this picture. In NGC 7009, the first spatially resolved detection of [C I] $\lambda 8727$ from the outer pair of LISs showed intense [C I] emission where He I $\lambda 8733$ is absent, while the host nebula shows the opposite behavior [2409.00371]. In NGC 3242, MUSE data revealed [C I] $\lambda 8727$ primarily emitted from the LISs, spatially associated with [O I] $\lambda 6300$ and [N I] $\lambda 5198,5200$ [2503.16662]. Because [C I] $\lambda 8727$ traces the transition zone between ionized and neutral gas, these detections support a layered structure in which LISs are not purely ionized condensations but contain neutral and plausibly molecular material [2409.00371, 2503.16662].

Taken together, the H$_2$ and [C I] results have shifted the interpretation of LISs from purely optical low-ionization features toward partially molecular, stratified condensations with ionized exteriors and neutral or molecular interiors.

## 4. Excitation mechanisms and diagnostic frameworks

The central interpretive problem is whether LIS emission is powered mainly by stellar photoionization, by shocks, or by both. Early spectroscopic work on five PNe argued that LISs are mainly shock-excited, introducing diagnostic diagrams based on $\log(f_{\rm shocks}/f_{\star})$ and proposing the empirical regimes
- $\log(f_{\rm shocks}/f_{\star}) > -1$ for shock-excited structures,
- $\log(f_{\rm shocks}/f_{\star}) < -2$ for photo-ionized structures,
- $-2 < \log(f_{\rm shocks}/f_{\star}) < -1$ for a transition zone [1509.05346].

Later analyses became more cautious. A spatially resolved study of six PNe concluded that shocks are not negligible but that their inferred importance depends strongly on orientation [2209.10988]. A larger statistical study comparing LISs with photoionization and shock grids from Cloudy and MAPPINGS V found substantial overlap between the mechanisms in classical low-ionization diagrams and concluded that shocks cannot be the main source of excitation for most LISs of PNe [2308.04577]. In that work, one of the more useful shock indicators is
$$
\log\left(\frac{[\mathrm{O\,III}]\,4363}{[\mathrm{O\,III}]\,5007}\right) \ge -1.5,
$$
provided the density is below the regime where [O III] $\lambda 5007$ is collisionally de-excited [2308.04577].

Near-infrared diagnostics have added new leverage but not complete uniqueness. For molecular gas, the framework based on
$$
R(\mathrm{H}_2)=\frac{\mathrm{H}_2\ 1-0\ S(1)}{\mathrm{H}_2\ 2-1\ S(1)}, \qquad
R(\mathrm{Br}\gamma)=\frac{\mathrm{H}_2\ 1-0\ S(1)}{\mathrm{Br}\gamma}
$$
was applied to LISs in NGC 7009 and NGC 6543. High $R(\mathrm{H}_2)$ often suggests thermal or shock excitation, and low values often suggest UV fluorescence, but the observations and model grids overlap too strongly to identify a unique dominant mechanism in general [2003.01737].

For atomic gas, a pilot [Fe II] survey targeted the [Fe II] $1.644~\mu$m line as a shock tracer. Because the narrowband filter includes both [Fe II] $1.644~\mu$m and H I 12-4 at $1.640~\mu$m, the theoretical H I 12-4 flux was computed from either Br$\gamma$ or H$\beta$ using Case B recombination for $n_e = 10^4\ \mathrm{cm^{-3}}$ and $T_e = 10^4\ \mathrm{K}$, with
$$
12\!-\!4 / Br\gamma \approx 0.19,
$$
and then subtracted from the observed narrowband fluxes [2409.00377]. The resulting diagnostic,
$$
R(\mathrm{Fe})=\frac{[\mathrm{Fe\,II}]\ 1.644\,\mu \mathrm{m}}{Br\gamma},
$$
is interpreted such that low values favor UV photoionization, high values favor shock excitation, and intermediate values indicate a mixture of both [2409.00377].

The net result is not a single universal excitation law. Instead, current diagnostics show that LISs occupy a continuum from predominantly photoionized to clearly shock-dominated, with many objects in an intermediate regime.

## 5. Empirical archetypes in well-studied nebulae

A small set of nebulae has become archetypal because their LISs occupy different parts of the excitation sequence.

| Object | Diagnostic result | Interpretation |
|---|---|---|
| K 4-47 | H$_2$ 1-0 S(1)/2-1 S(1) $\sim$ 7–10 | shock interaction |
| NGC 7662 | H$_2$ 1-0 S(1)/2-1 S(1) = 2–3 | photo-ionization by the central star |
| NGC 6543 | $R(\mathrm{Fe}) < 0.15$ | UV radiation from the central star is likely dominant |
| NGC 7009 | outer LISs $R(\mathrm{Fe}) < 0.25$ | combination of UV photoionization and shock contribution |
| IC 4634 | $R(\mathrm{Fe}) \sim 1$ | shock excitation |
| NGC 6571 | $2 < R(\mathrm{Fe}) < 7$ | shock excitation as the main driver |

These empirical cases come from direct imaging of molecular and atomic tracers associated with LISs [1604.04763, 2409.00377]. K 4-47 represents the fast, highly collimated, shock-dominated end of the class: H$_2$ emission comes from the walls of bipolar outflows and from a pair of knots at their tips, and the system has been interpreted as a jet or bullet interacting with surrounding asymptotic giant branch material [1611.06578]. NGC 7662 represents the slower, predominantly photoionized end: H$_2$ is found in peripheral LISs but not in the main shell, and only four knots are detected in H$_2$ $v=2-1$ S(1), giving line ratios between 2 and 3.5 [1611.06578].

NGC 7009 and NGC 6543 are especially important because they contain both molecular and atomic tracers. H$_2$ was detected in LISs in both nebulae, but the 2020 analysis concluded that the diagnostic diagram could not decide uniquely between UV or shocks as the mechanism behind the molecular emission [2003.01737]. In NGC 7009, the later [C I] $\lambda 8727$ detection in the outer LIS pair provided direct evidence that these LISs are photodominated regions, while the [Fe II] results for the same nebula indicate a moderate regime in which both mechanisms probably contribute [2409.00371, 2409.00377]. This juxtaposition does not represent a contradiction; it indicates that different tracers sample different layers and processes within the same LIS.

## 6. Internal structure and formation scenarios

Several recent studies converge on a layered structural model. In NGC 7009, the radial stratification places moderate-ionization lines closer to the central star, low-ionization lines farther out, and H$_2$ emission still farther out at the molecular edge [2409.00371]. In NGC 3242, low-ionization and neutral lines such as [N II], [S II], [Fe II], [O I], and [C I] peak at the LIS positions, while high-ionization lines peak closer to the center or around the rim [2503.16662]. This is consistent with a mini-PDR-like or transition-layer geometry rather than a uniform ionized clump.

The preferred physical picture in several papers is that LISs are photoevaporated dense molecular clumps. In that scenario, the clump is dense enough to shield interior H$_2$ from UV destruction, the central-star radiation drives a photoevaporative flow from the surface, low-ionization emission arises in the skin of this flow, and the molecular core remains farther out and is traced by H$_2$ [2409.00371]. The [C I] $\lambda 8727$ line is particularly valuable because it probes the transition zone between fully ionized and neutral gas and, in the wording of the NGC 7009 study, “goes deeper into the molecular gas” than classical optical low-ionization lines [2409.00371].

The [Fe II] imagery adds a dynamical refinement to this model. Spatial offsets among [Fe II], H$_2$, and Br$\gamma$ in some nebulae were interpreted as especially suggestive of photoevaporating clumps with embedded shocks rather than simple direct shock interaction alone [2409.00377]. In that picture, UV photons ionize the clump surface, drive a photoevaporative flow, and generate a slow reverse shock that can liberate Fe from dust and produce [Fe II] emission [2409.00377]. This suggests that radiative and mechanical processes can coexist within a single LIS but dominate in different spatial zones.

Formation remains open. The literature summarized in recent surveys allows density enhancements produced by instabilities, stagnation points, asymptotic giant branch fossil condensations, jet-driven structures, and binary-related shaping, while emphasizing that present-day observables are further modified by shielding, photoevaporation, and the evolving UV field [2508.20240, 1509.05346].

## 7. Open questions and current synthesis

Despite nearly 30 years of detailed study, the origin and nature of LISs remain unresolved [2308.04577]. Three questions dominate the current literature.

The first is the density problem: LISs are observed to have lower electron densities than the surrounding rims and shells, yet model expectations often require dense condensations. The detection of H$_2$ in an increasing number of LISs strongly supports the interpretation that optical $N_e$ measures only the ionized fraction and misses a larger neutral or molecular reservoir [2508.20240].

The second is excitation ambiguity. H$_2$ ratios, classical optical diagnostic diagrams, and even some [C I] constraints do not uniquely separate UV and shocks across the full class, because photoionization and shock models overlap substantially in the relevant line-ratio space [2003.01737, 2308.04577]. The [Fe II] $1.644~\mu$m survey has sharpened the issue by providing the first direct detections of [Fe II] associated with LISs in four out of five surveyed PNe and by showing object-to-object variation from UV-dominated to shock-dominated regimes [2409.00377]. Still, no single tracer appears sufficient on its own.

The third is internal structure. Recent [C I], H$_2$, and [Fe II] detections favor a stratified model in which LISs are dense, partially molecular, externally illuminated, and in some cases photoevaporating [2409.00371, 2409.00377]. This suggests that the term “low-ionization structure” is observationally accurate but physically incomplete: the low-ionization optical skin is only one layer of a more complex condensation.

The current synthesis is therefore conservative but specific. LISs are not chemically distinct sub-populations; they are small-scale nebular condensations with systematically low electron densities in their ionized skins, frequent molecular and neutral counterparts, and excitation that ranges from photoionization-dominated to shock-dominated depending on local kinematics, UV field, density structure, and geometry [1707.09650, 2209.10988, 2308.04577]. A plausible implication is that progress will continue to come from combining spatially resolved optical IFU data with narrow-band near-infrared imaging and spectroscopy, because the decisive information lies in how ionized, partially ionized, neutral, and molecular layers are arranged within individual LISs.

Source: https://www.emergentmind.com/topics/low-ionization-structures-liss