The strongest dust emission features in evolved carbon-rich stars — the 21 µm feature (peak at 20.1 µm), seen exclusively in carbon-rich proto-planetary nebulae, and the 30 µm feature, seen from the AGB through planetary-nebula stages and in the Magellanic Clouds — together can carry more than 20% of the total infrared luminosity of these objects, yet their carriers remain unidentified more than three decades after discovery. The MgS attribution of the 30 µm feature was challenged in 2009 by an abundance-budget analysis showing that it would demand more sulfur than the prototype source HD 56126 can supply, and the last surviving inorganic candidate for the 21 µm feature (FeO) was refuted in 2013; a fullerane (hydrogenated fullerene) origin of the 21 µm feature has since become the leading but unconfirmed hypothesis. The open problem is the definitive chemical identification of the carrier material(s), with a demonstrated formation pathway and abundance budget consistent with carbon-rich, oxygen-depleted, metal-poor outflows and with the feature's exclusive occurrence in the short proto-planetary-nebula phase.

Background

When low- and intermediate-mass stars exhaust their nuclear fuel they ascend the asymptotic giant branch (AGB, the late red-giant phase driven by shell hydrogen and helium burning), then rapidly traverse a short transitional stage before ionized planetary nebulae (PNe) form. During the AGB, internal nucleosynthesis and repeated mixing events enrich the surface with carbon and with s-process elements (heavy nuclei built by slow neutron capture); if carbon becomes more abundant than oxygen, the star and its ejecta are 'carbon-rich'. Because carbon and oxygen bind into the very stable molecule CO, essentially all oxygen is locked in CO in these envelopes, so any oxygen-bearing dust is hard to form and preserve there — a central constraint on carrier candidates. The brief transitional stage between the AGB and the PN, lasting only a few thousand years, is called the proto-planetary nebula (PPN) or post-AGB phase; its central stars are typically F–G type. The 21 µm feature was discovered by the IRAS satellite in four PPNe (Kwok, Volk & Hrivnak 1989); high-resolution spectra from the Infrared Space Observatory (ISO) later showed its peak lies at 20.1 µm, though the historical name is retained. It is one of the rarest infrared dust features: about a dozen Galactic sources were known by 2008, all carbon-rich, metal-poor, s-process-enriched PPNe with double-peaked spectral energy distributions. Its profile is universal from source to source — a steep blue wing rising to a 20.1 µm maximum and a red tail to about 23–24 µm — and it has no appreciable associated emission features, which together imply a single carrier species with uniform properties; in the prototype HD 56126 the feature emits about 103610^{36} erg/s, roughly 300 solar luminosities. The 30 µm feature, known since 1973, is far more common: dozens of Galactic carbon stars, PPNe and PNe, plus carbon-rich objects in the Large and Small Magellanic Clouds (LMC and SMC, the Milky Way's low-metallicity satellite galaxies). It was attributed to magnesium sulfide (MgS) dust in the mid-1980s, and simple MgS models (Hony, Waters & Tielens 2002) reproduced many profiles assuming a continuous distribution of ellipsoidal grain shapes. Eleven of the twelve 21 µm sources known in 2008 also showed the 30 µm feature, yet the two bands' strengths are uncorrelated, so the carriers may be related in origin but are not the same material. A valid carrier must pass three tests (Jiang, Zhang & Li 2010): (i) its laboratory or quantum-chemically computed resonance must reproduce the observed feature profile; (ii) it must predict no strong companion bands where none are observed (e.g., silicon carbide is excluded because the observed ratio of the 11.3 µm SiC band to the 21 µm band is below 0.01); and (iii) the required elemental abundance must not exceed what is available in metal-poor carbon-rich ejecta (this excluded titanium carbide nanoclusters, which need more titanium than cosmic abundance allows). Beyond these tests, a viable candidate must plausibly form and survive in a carbon-rich, oxygen-depleted envelope. By 2008, essentially every inorganic candidate had failed one of these tests — except iron monoxide, FeO, which has abundant constituent elements and no companion bands, but whose formation in an environment where all oxygen should be locked in CO was (and is) unexplained. Organic (carbon-bearing) candidates such as polycyclic aromatic hydrocarbons (PAHs, large benzene-ring molecules responsible for several well-known infrared bands), hydrogenated fullerenes, and hydrogenated amorphous carbon remained insufficiently tested, mainly because their predicted associated bands are hard to quantify. For the 30 µm feature, laboratory MgS spectra depend strongly on particle shape (lab-condensed MgS is roundish or cubic rather than ellipsoidal), and how sulfur and magnesium partition among MgS, iron sulfides, and silicates in carbon-rich and low-metallicity outflows was unquantified, leaving MgS unconfirmed rather than established.

Problem Statement

Identify the carrier(s) of the unidentified 21 µm and 30 µm dust emission features of carbon-rich post-AGB objects, together with the formation pathway that produces the carrier in carbon-rich, oxygen-depleted (CO-locked) circumstellar environments. The common scientific objective is a definitive chemical identification, meaning a proposed material or molecule for the 21 µm feature that simultaneously: (a) reproduces the universal observed profile — a 20.1 µm peak with a steep blue wing and red tail to 23–24 µm (the band spans ~17.4–23.4 µm), invariant across the known Galactic and Magellanic Cloud emitters (31 as of 2024: 20 Galactic, 9 LMC, 2 SMC) — through its laboratory-measured or quantum-chemically computed infrared properties (including the correct treatment for nanoparticles or stochastically heated grains); (b) predicts no associated emission bands stronger than the existing observational upper limits (for example, the observed 11.3 µm-to-21 µm flux ratio is below 0.01); (c) requires no more of its constituent elements than is available in metal-poor, carbon-rich post-AGB ejecta; (d) has a demonstrated formation route and survival under carbon-rich, oxygen-depleted conditions; and (e) explains the feature's exclusive appearance during the short proto-planetary nebula phase (its absence on the AGB and among planetary nebulae) in a way consistent with its correlation with AGB-phase mass loss. The coupled companion sub-question for the 30 µm feature — part of the same identification objective because 11 of 12 (at least) 21 µm sources also emit it — is to determine whether magnesium sulfide, whether bare or as a coating on SiC or carbon grains, can satisfy the sulfur/magnesium abundance budget (including at Magellanic-Cloud metallicities), the particle-shape assumptions needed to fit observed profiles, and the absence of unobserved sub-structure, or to identify the actual 30 µm carrier instead. Supporting results — new emitter surveys that pin down the environmental conditions (metallicity, s-process enrichment, evolutionary timing), laboratory spectroscopy and dust-condensation experiments on candidate materials, and radiative-transfer/dust-condensation models coupling particle shape, temperature-dependent optical properties, and circumstellar chemistry — are examples of the research lines that contribute to this single objective, since each exists to test or supply the identification; the scientific objective is the identification of the carrier(s) of both features, supported by observational evidence together with laboratory-measured or theoretically computed infrared properties.

References

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