Els Peeters

We find that the aromatic and aliphatic emission bands in four carbon-rich post-asymptotic giant branch stars with class D emission arise from a single carrier population: extensive aliphatic decoration of an aromatic skeleton, i.e. molecules with a coherent aromatic backbone and an unusually high aliphatic content. This conclusion is based on JWST NIRSpec and MIRI/MRS spectroscopy, covering 2.88–28.7 µm. The aliphatic 3.4 µm feature is stronger than the aromatic 3.3 µm feature, reversing the ratio seen in sources with typical polycyclic aromatic hydrocarbon (PAH) emission. If the aromatic emission arose from nominal PAHs, the 3.3 µm feature – unavailable in previous observations – together with the mid-infrared aromatic features, would constrain them to be large (≳200 carbon atoms), compact, and at least partially ionized. These properties predict a strong 8.6 µm feature. However, the 8.6 µm feature is weak or absent in all four sources, ruling out PAHs as the source of the aromatic emission. Instead, the emission must arise from the same carriers as the aliphatic emission. The position of the 6.9 µm feature and the broadened 11–14 µm out-of-plane emission independently support this. The CH ₂ and CH ₃ feature strengths vary across the sample, with the class D2 sources showing higher CH ₃ content. These carriers are far more aliphatic than the PAHs of planetary nebulae and the interstellar medium. However, they retain the aromatic skeleton that produces their aromatic features.

The infrared spectra of many carbon-rich postasymptotic giant branch stars are dominated by emission features from aromatic and aliphatic hydrocarbons, but the chemical structure of the carriers remains unidentified. Class D sources show unusual emission profiles with strong aliphatic emission, providing a stringent test of carrier candidates. Here we investigate whether hydrogenated carbonaceous molecular particles can be the carriers of these features. Rather than assuming candidate geometries we generate the initial structures using ab initio molecular dynamics simulations and optimize them using density functional theory. We then compare the computed emission spectra to JWST NIRSpec and MIRI/MRS observations of Class D sources. The structures providing the best agreements contain sizeable domains with defect-bearing aromatics and hydrogenated aromatics connected by aliphatic bridges, in contrast to the “arophatic” cluster model of isolated two- to three-ring aromatics connected by aliphatic and olefinic bridges. As a representative example, we discuss in detail the molecular particle C ₁₃₀ H ₉₆ (radius of 5.5 Å) and compare its computed spectrum to the JWST spectra of IRAS 05110-6616, a Class D2 source with apparent shifts of the aromatic C–H out-of-plane bands. The calculated spectra are in qualitative agreement with the observed 3 μ m profile, the aliphatic C–H bending bands, the broad 8 μ m feature, the 11–14 μ m C–H out-of-plane region, and the bands at 16 and 21 μ m. We hypothesize that UV photodriven fragmentation of such molecular particles releases aromatics linked by aliphatics, followed by the formation of polycyclic aromatic hydrocarbons and fullerenes as these objects evolve into planetary nebulae.

Polycyclic aromatic hydrocarbons (PAHs) and carbonaceous dust have been observed in oxygen-rich planetary nebulae (PNe), yet their formation and evolution in these environments is an open question. We aim to study the formation and evolution of the PAHs in the O-rich PN NGC 6302 (the Butterfly Nebula), by characterizing the PAH emission throughout this object. We obtained James Webb Space Telescope Mid-Infrared Instrument (MIRI) Integral Field Unit (IFU) spectroscopic observations of NGC 6302 pointed at its central source. From these, we extract four template spectra that are representative of the variation in PAH emission throughout NGC 6302. We detect the major PAH features at 6.2, 7.7, 8.6, 11.2, and 12.7 μm, and several minor PAH features. The template spectra vary in terms of their PAH feature widths, peak wavelengths, and relative intensities. The 11.2 μm PAH feature is class B, typical for PNe. The 6–9 μm PAH features are class A, which is less common for PNe. We estimate the PAH charge to be 75 per cent cations on average, based on the 7.7/11.2 PAH feature ratio. The relative strengths of the 11–15 μm PAH features indicate a range of PAH edge structures. To ensure consistency in our observations and explain variable PAH feature widths, we propose a variation in the size of the PAHs throughout NGC 6302, and suggest variation in the shielding of these PAHs. The PAHs range from large and non-compact, to small and compact. We find that NGC 6302 formed PAHs in situ in the current PN phase, likely from small hydrocarbons formed by the photodissociation of CO.

Context. Conspicuous excess emission is present in the near-infrared (NIR) region in various objects, including reflection nebulae, planetary nebulae, and nearby galaxies. However, the spatial distribution and spectral shape of the excess emission remain poorly understood. Aims. We studied the NIR continuum emission spectroscopically and obtained its spatial distribution relative to the aromatic infrared band (AIB) at 3.3 µm in the Orion Bar prototypical photodissociation region (PDR). We aim to characterize its spectral shape and discuss its origin. Methods. We employed 3D spectroscopic data of the Orion Bar taken with the integrated field unit of NIRSpec on JWST from the Early Release Science program “PDRs4All.” Contribution from the foreground ionized gas was estimated using the Cloudy code and subtracted. The observed regions were divided into nine physically distinct regions and an average spectrum was derived for each region. Results. The nine regions, including the ionized gas, atomic PDR, and molecular PDR, clearly show remaining continuum in the region 1–4.5 µm. The continuum at wavelengths longer than 2.7 µm shows good correlations with the 3.3 µm AIB, while the correlation of the continuum at 1.2 µm is not significant. We further find that the NIR continuum in the Orion Bar can be approximated by a summation of two blackbodies. The low-temperature component correlates with the AIB well, while the high-temperature component does not. The average spectra also show absorption features at 3.0 and 4.27 µm, which are attributed to the presence in the spectra of water ice and CO ₂ ice. Conclusions. We discuss possible origins of the NIR continuum, among which recurrent fluorescence from carbon clusters better explains the observed low-temperature component. The presence of ice species suggests a contribution from a deeper layer of the PDR along the line of sight producing characteristic ice absorption features.

We report the first detection of the neutral methyl radical CH ₃ in a highly evolved C-rich object, SMP LMC 011, using the Mid-Infrared Instrument (MIRI) Medium-Resolution Spectrometer (MRS) on JWST. CH ₃ is well fitted by an excitation temperature of T _ex ≃ 190 K and a column density of N _tot (CH ₃ ) ≃ 5.6 × 10 ¹⁷ cm ⁻² . We also report the nondetection of ethane (C ₂ H ₆ ), consistent with CH ₃ reacting preferentially with unsaturated radicals rather than self-recombination, which may be inefficient in SMP LMC 011. Combined with the exceptionally large benzene column density of SMP LMC 011, this supports a methyl-addition route from benzene toward alkyl-substituted aromatics. We propose that the high CH ₃ abundance is driven by the erosion of hydrogenated amorphous carbon (HAC) dust grains in the dense warm torus by UV photons from the central star and/or by shocks, which release CH ₃ directly into the gas phase. These results establish CH ₃ as a key reactive intermediate in the formation of complex hydrocarbons in C-rich circumstellar environments. Chemical models that incorporate methyl-addition reactions are needed for a better understanding of polycyclic aromatic hydrocarbon (PAH) formation pathways in evolved stars. The potential detection of alkyl-substituted aromatics such as toluene (C ₇ H ₈ ) and ethylbenzene (C ₈ H ₁₀ ) in C-rich evolved stars would provide direct confirmation of CH ₃ -driven aromatic growth in circumstellar environments.

Aims . The planetary nebula Tc 1 (PN G345.2–08.8), one of the rare group of Galactic PNe showing fullerene emission in the infrared, is spatially extended with high a surface brightness. Optical imaging spectroscopy enables the line and continuum structure and diagnostics to be examined, and permits a search for any correlations to the presence of fullerene dust. Methods . Tc 1 was observed with MUSE wide field mode with adaptive optics, wavelength range 4700–9300 Å, at three exposure levels to ensure unsaturated emission lines. Extinction, electron temperature ( T _e ), and density ( N _e ) diagnostic images are presented from collisionally excited and recombination line ratios. Results . The nebula has a high surface brightness core, with a diameter 12″, an elliptical ring of major axis 2.8″ around the central star and some low ionization knots, and an extended halo of 55″ in diameter; between the core and halo is an annulus with intermediate properties, including a higher T _e and a lower N _e than in the core. The spectrum of the central star was extracted and fitted by a 31 000 K model atmosphere and is of type O7.5I(f). The image of optical extinction from H Balmer line ratios is highly structured, and shows an annulus, which is adjacent to the core, of low extinction, with values lower than the line-of-sight interstellar extinction. Instrumental effects to account for this anomalously low extinction area were investigated and intrinsic effects from the scattering properties of the nebular dust; neither can entirely explain the low-extinction region and the most likely cause is a local non-standard dust reddening law. This low extinction region additionally shows an anomalously high He I 7281/6678 Å line ratio, possibly caused by a contaminating line, but none have been conclusively identified; its origin remains unresolved. Spectra over extended regions were also analysed and distinct enhancement of the continuum above the expected nebular continuum (as also seen in some other PNe observed with MUSE) was found. Conclusions . The annulus of low extinction occurs outside the region of strongest fullerene emission, in the zone where N _e declines and T _e rises. A change in dust properties linked to conditions in this transition region between the higher density core nebula and lower density halo is deduced.

Carbocations are key species in interstellar chemistry, providing entry points for building larger hydrocarbons. CH+ , and more recently, CH_3^+, have been detected. Other carbocations await detection to provide a comprehensive view of the astrochemical network that is at work in the interstellar medium. We search for CH_2^+ in objects in which CH_3^+ was detected and evaluate the most favorable conditions for detecting the elusive ̧htwo reactive cation. We calculated the CH_2^+ rotational and rovibrational transitions expected to contribute in the mid- to far-infrared, focusing on the lower-energy rovibrational levels. We then calculated CH_2^+ infrared emission spectra at different excitation temperatures and compared them to JWST spectra of the externally irradiated disk d203-506 in Orion, where rm CH^+ and rm CH_3^+ have already been detected. We used thermochemical models to predict the abundance and spatial morphology of CH_2^+ to better understand its nondetection. The comparison to JWST spectra allowed us to provide excitation-temperature-dependent upper limits to the excited column density. These are several times lower than those detected for CH+$ and CH_3^+ in their excited states. Based on model calculations for photodissociation regions and assuming similar excitation temperatures, the upper limit derived from observations and CH$_2^+ model spectrum is either slightly above or below the column density expected from models of photodissociation regions. We provide a list of tabulated transitions to allow the community to search for this carbocation in future observations as CH_2^+ is key in providing observational constraints on astrochemical models.

We report the detection of a set of new near-infrared emission features between 3.5 and 5.2 μ m in JWST/NIRSpec observations of Tc 1, the planetary nebula known for displaying the cleanest and most prominent mid-infrared cosmic fullerene spectrum. These broad features share the same spatial distribution as the well-known C ₆₀ and C ₇₀ mid-infrared emission bands, peaking in an asymmetric ring approximately 5″–6″ from the central star. Through comparison with new anharmonic quantum chemical calculations, we demonstrate that these features arise from C ₆₀ combination bands, marking their first detection in an astrophysical environment. The total energy radiated in the combination bands amounts to ∼17% of the total energy emitted from all C ₆₀ modes, with direct implications for fullerene cooling models. These near-infrared combination bands offer a promising new window for identifying and studying the molecular astrophysics of C ₆₀ in sources where mid-infrared spectra are more complex.

Aims. We characterize the mid-infrared spectrum of the outer regions of the Red Rectangle nebula to probe the carbonaceous dust and molecular content beyond the circumbinary disk. Methods. We present JWST MIRI-MRS observations of the southwest whisker that were extracted from three distinct environments: the biconical outflow, the whisker itself, and the shadow region outside the outflow. We compare them with an archival ISO-SWS observation of the inner nebula. Results. The JWST spectra display only classical aromatic infrared band (AIB) emission on a weak dust continuum, with no signatures of the oxygen-rich circumbinary disk mineralogy nor of the rich molecular emission seen at optical wavelengths. The AIBs are predominantly Class 𝒜, which is in marked contrast to the exclusively Class ℬ profiles previously reported for the inner regions; there are systematic differences between the outflow and shadow regions that point to environmentally driven polycyclic aromatic hydrocarbon processing.

Using JWST/MIRI observations, we report the detection of CO ₂ ice in the dusty torus of the planetary nebula NGC 6302, an environment generally considered hostile to fragile molecular species and ices due to intense UV irradiation. This detection accompanies cold (20–50 K) gas-phase CO ₂ along the same sightlines. The ice absorption profile exhibits a double-peak profile, which is characteristic of pure crystalline CO ₂ ice. The CO ₂ gas-to-ice ratio is higher by more than an order of magnitude than in young stellar objects, which indicates distinct ice formation or processing mechanisms in evolved stellar environments. This discovery demonstrates that the dusty torus provides sufficient shielding to harbor ice chemistry, and that ice-mediated surface reactions must be incorporated into chemical models of planetary nebulae.