The unidentified infrared emission feature peaking at about 20.1 micrometers (the "21 micron feature") appears exclusively in carbon-rich post-asymptotic-giant-branch (post-AGB) stars during their short protoplanetary nebula phase, carries up to roughly 8% of a source's total infrared power, and has resisted carrier identification since its discovery in 1989. Mishra, Li and Jiang (2016) showed that the feature's strength correlates with the mass-loss rate of the preceding AGB phase and proposed that the carrier condenses during the AGB phase but requires ultraviolet photons, available only after the star leaves the AGB, to be excited or liberated; however, their radiative-transfer modeling had to exclude the carrier entirely because its composition, optical constants, and excitation regime remain unknown. The open problem is to identify the material responsible for the 21 micron feature, determine its optical constants or a quantitative emission model, and establish its excitation mechanism — stochastically heated nanospecies versus grains in thermal equilibrium — in a way that simultaneously reproduces the observed feature profile across all known sources, predicts no undetected secondary emission features, respects elemental abundance limits, and explains why the feature emerges only in post-AGB stars and disappears before the planetary nebula stage.

Background

Stars of roughly 0.8–8 solar masses end their lives by ascending the asymptotic giant branch (AGB), a phase of intense mass loss in which the star ejects its envelope, condensing dust in the cool outflow. When the mass loss ceases, the star contracts and heats up while the ejected envelope drifts outward: this brief transition (a few thousand years) between the AGB and the planetary nebula (PN) stage is called the post-AGB or protoplanetary nebula (PPN) phase. In 1989, Kwok, Volk and Hrivnak discovered a strong emission band peaking at about 20.1 micrometers in four PPNe; despite the peak being closer to 20.1 than 21 micrometers, it has always been called the "21 micron feature". It is now seen in about 27 carbon-rich PPNe in the Milky Way and the Magellanic Clouds (companion galaxies of the Milky Way with roughly 1/4 and 1/10 of its metal content), always with a strikingly similar profile: peak near 20.1 micrometers and full width at half maximum of about 2.2–2.3 micrometers. The feature can carry up to about 8% of the total infrared output of a source, implying its carrier is made of an abundant element — most plausibly carbon, since all 21 micron sources are carbon-rich. The same sources also show the "unidentified infrared" (UIR) emission bands at 3.3–11.3 micrometers, which are conventionally attributed to polycyclic aromatic hydrocarbon (PAH) molecules — large ring-shaped carbon molecules with hydrogen rims — as well as another unidentified band at 30 micrometers whose popular candidate carrier, magnesium sulfide (MgS) dust, is disputed because reproducing the observed flux apparently requires more sulfur than the objects have. More than a dozen carrier candidates for the 21 micron feature have been tested quantitatively. Zhang, Jiang and Li (2009) showed from elemental-abundance constraints that titanium carbide (TiC) nanoclusters, fullerenes coordinated with titanium, silicon disulfide, doped silicon carbide, and SiO2-coated SiC all require more Ti, S, or Si than available, and that carbon-silicon mixtures and iron oxides produce secondary emission features that are not observed; nano-sized iron monoxide (FeO) initially survived this test, but Li, Liu and Jiang (2013) later showed that stochastically heated FeO yields a feature too broad to match the observations and demands more iron than the prototype source HD 56126 contains. Here "stochastic" or "transient" heating means that a very small particle (nanometer-sized) lacks the heat capacity to maintain a steady temperature and instead undergoes temperature spikes after absorbing individual stellar photons, in contrast to larger grains that settle into thermal equilibrium. Within this setting, Mishra, Li and Jiang (2016) modeled the spectral energy distributions of the 21 micron sources with the 2-DUST axisymmetric radiative-transfer code (which distinguishes the mass-loss rate of the AGB phase from that of the final "superwind" pulsed ejection) and found that the 21 and 30 micron feature fluxes correlate with the AGB-phase mass-loss rate. They concluded that the carriers probably condensed in the AGB outflow, and proposed that the 21 micron carrier is excited — or chemically liberated — only by ultraviolet photons that post-AGB stars emit but AGB stars do not, explaining why the feature is absent in AGB stars and destroyed again in planetary nebulae. Crucially, their models could not include the feature carriers at all, for exactly the reasons that define the open problem: the carrier's mineralogical or molecular identity is unknown, so no optical constants (the wavelength-dependent complex refractive indices that determine how a material absorbs and emits radiation) exist for it, and its excitation regime — nanospecies under stochastic heating or equilibrium grains — is undetermined. Reviews as late as 2020 (Volk, Sloan and Kraemer) summarize that "many carriers have been proposed, but each has issues, and none have gained broad support", and work through 2024 continues to test new candidate families (hydrogenated fullerenes, "fulleranes", i.e. C60 molecules with attached hydrogen atoms) without reaching a confirmed identification.

Problem Statement

Identify the carrier of the 21 micron (∼20.1 μm\sim 20.1\,\mu\mathrm{m}) emission feature observed in carbon-rich post-AGB stars, and establish its excitation mechanism. The problem requires establishing (i) the specific material, molecule, or mineral (or a definite mixture) responsible, with its optical constants or an equivalent quantitative emission model over the relevant wavelength range; (ii) by quantitative modeling, that this carrier reproduces the observed feature — peak at ∼20.1 μm\sim 20.1\,\mu\mathrm{m}, full width at half maximum ∼2.2\sim 2.2–2.3 μm2.3\,\mu\mathrm{m}, and its strikingly invariant profile — across the known 21 micron sources, given their stellar parameters and the archived infrared spectra from the Infrared Space Observatory (ISO) and the Spitzer Space Telescope; (iii) that the same carrier model does not predict secondary emission features (for example in the 13–17 μm\mu\mathrm{m} range) beyond the observed limits in these spectra; (iv) that the elemental abundance the carrier requires does not exceed what is available in these carbon-rich environments; and (v) the excitation regime — a nano-sized species undergoing stochastic single-photon heating versus a larger grain in thermal equilibrium — and its consistency with the feature's exclusive emergence during the short post-AGB (protoplanetary nebula) phase, including or replacing the proposed ultraviolet-photon trigger of Mishra, Li & Jiang (2016), with its absence in AGB stars and planetary nebulae. The parallel identification of the 30 μm\mu\mathrm{m} feature carrier (where magnesium sulfide remains disputed) and the extension of the mass-loss correlation to the low-metallicity Magellanic-Cloud 21 micron sources are related open questions that lie outside the scope of this problem.

References

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