Jeremy R. Walsh

Aims. The bright-end cutoff of the planetary nebula luminosity function (PNLF), defined by the [O III ] λ 5007 emission line magnitude as M _* = −4.53, serves as a standard candle for extragalactic distance measurements. However, the physical properties of planetary nebulae (PNe) at the PNLF cutoff have been studied only in nearby galaxies, where the PNe appear mostly as point sources. The Galactic PNLF is further poorly constrained, primarily due to uncertainties in Galactic PN distances. Following a recent Galactic PNLF survey based on Gaia distances, we aim to characterise PN candidates at the Galactic PNLF cutoff. This is the first step towards comparing the extragalactic PNLF cutoff against its Galactic counterpart. Methods. We observed three PN candidates at the Galactic PNLF cutoff using the Potsdam Multi-Aperture Spectrophotometer (PMAS). We determined their chemical abundances and constructed photoionisation models to derive the central star luminosity ( L ) and effective temperature ( T _eff ). Based on their positions in the Hertzsprung–Russell (HR) diagram and kinematical ages, we constrained the central star masses and most likely distances consistent with the prediction of stellar evolution models. Using these distances, we determined the absolute magnitude in [O III ] λ 5007 emission line ( M ₅₀₀₇ ). The PMAS data also allowed us to distinguish the origin of emission lines between the central star and the nebula. Results. The PNe of our sample are located within the top 1 mag of the Galactic PNLF (−4.20 ≲ M ₅₀₀₇ ≲ −3.60) and have similar nebular properties and a narrow progenitor mass range (1.30 − 1.75 M _⊙ ). The measured extinction is mostly dominated by the foreground Galactic extinction. The trend between the circum-nebular extinction and central star mass of our PNe is in agreement with those derived in the Large Magellanic Cloud (LMC) and M31. All PNe in our sample exhibit weak-emission line star ( wels ) features in their spectra, but we confirmed that some of these lines originate from the nebula. Conclusions. The PNe around the Galactic PNLF cutoff are similar in terms of their nebular properties and evolutionary stage. Their characteristics are also consistent with the most [O III ] λ 5007-luminous PNe in the LMC and M31. We demonstrate the importance of circum-nebular extinction and the initial-final mass relation (IFMR) for understanding the universality of PNLF as a standard candle.

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.

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.

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.

Context. Optical integral field spectroscopy of planetary nebulae (PNe) offers a unique tool to explore the spatial relationships between the complex mixture of the many components (neutral, low- and high-ionisation gas, dust, and the central star) and their underlying physical conditions. Aims. The optical line and continuum emission in the very-high-ionisation Galactic PN, NGC 4361, were mapped to study the distribution of ionisation, extinction, electron temperature, and density. Methods. Based on commissioning data, MUSE Wide Field (60×60″) normal-mode (4750–9300 Å) observations of NGC 4361 were reduced. The PN is larger than a single MUSE field and only the central 1 arcmin² of the PN was observed in good conditions. Emission images in recombination and collisionally excited lines were extracted and the line ratios provided the dust extinction, electron density and temperature, and ionic abundances using standard techniques. A family of compact low-ionisation knots (dubbed ‘freckles’) was discovered and techniques developed to measure their spectra, independently of the extended high-ionisation medium. Results. The nebula is confirmed as optically thin in the H-ionising continuum, based on its very low He I emission, even to the edges of the field. The electron temperature, T_e, is shown to have a large-scale spatially coherent structure, as indicated by a previous long-slit spectrum. Prior to this study, no low-ionisation emission had been positively detected, although MUSE revealed both weak extended [N II] and [O II] and >100 spatially unresolved knots. There are several linear associations of these knots, but none of them point convincingly back to the central star. They have low-to-moderate ionisation with T_e ~ 11 000 K, N_e ~ 1500 cm⁻³ and generally exhibit a higher extinction than the extended high-ionisation nebula. Within the MUSE field, a low-redshift emission-line galaxy was serendipitously found to be hiding behind NGC 4361. The spectrum of this dwarf galaxy was carefully extracted from the bright foreground nebular emission and the galaxy’s line and continuum properties were then determined. Conclusions. NGC 4361 is not completely optically thin, as indicated by several extended regions and many compact features of lower ionisation emission. The low-ionisation ’freckles’ identified here do not clearly appear to differ in (He, N, O, S) abundance with respect to the extended high-ionisation gas. The spatial distribution and radial velocities of these features suggest that they belong to a thick disk oriented perpendicular to the large-scale nebular gas, which may perhaps be remnants of an earlier structure. The low-luminosity disk galaxy at ~87 Mpc has bright H II regions with metallicity 12+log(O/H) ≅ 8.4 and is suggested to be a Magellanic irregular or low-mass spiral.