William J. Henney

We present a spatially contiguous spectroscopic analysis of the physical and chemical structure of the Helix Nebula (NGC 7293, PNG 036.1-57.1) using integral-field observations from the SDSS-V Local Volume Mapper (LVM). The nearly complete nebular coverage allows us to map extinction, electron density and temperature, ionisation structure, and chemical abundances within a common dataset. We reconstruct calibrated datacubes from the LVM row-stacked spectra and measure 41 optical emission lines from hydrogen, helium, and collisionally excited ions. The Helix is globally a low-density nebula, with characteristic electron densities of ∼102 cm−3, but shows a strongly structured thermal distribution. Te([O III]) exhibits a U-shaped radial profile, decreasing from the central cavity to intermediate radii and rising toward the outer nebula, while Te([N II]) increases outward and Te([O II]) decreases. These contrasting behaviours show that different diagnostics trace distinct ionisation zones. The spatially resolved abundance analysis yields a near-solar oxygen abundance, 12 + log (O/H) ≃ 8.7, consistent with an approximately chemically homogeneous nebula once ionisation structure is considered. Simulated long-slit extractions show that inferred O/H depends systematically on slit orientation and can remain biased after ionisation correction, demonstrating the importance of spatial coverage in strongly stratified nebulae. We also find sulfur systematically deficient relative to other α-elements, consistent with the planetary-nebula sulfur anomaly. These results highlight the power of wide-field integral-field spectroscopy for linking the thermal, ionisation, and chemical structure of extended planetary nebulae.

We study the effects of using the optical properties of irregular hexahedral grains in photoionization models of circumstellar nebulae around evolved stars. Dust opacities for the irregular grains were obtained from the scattering properties available in the TAMUdust2020 database and these were implemented in the spectral synthesis code cloudy. A sample of photoionization models that use opacities from both spherical and irregular hexahedral grains across a standard MRN size distribution (0.005-0.25 μm) was produced. We consider the optical properties of graphite, amorphous carbon and silicate dust grains and find that differences between the model nebula continua calculated using spherical and irregular dust grains increase with the grain size, especially for graphite. In particular, we find that the luminosities at the infrared peak for the hexahedral grain models can be up to 60% higher than those from the equivalent spherical grain models for the largest grains. This result suggests that traditional spherical grain assumptions may lead to an overestimate of the dust mass in photoionized nebulae.

We present new deep imaging and high-resolution spectroscopy of the extreme-abundance-discrepancy planetary nebula Ou 5, together with photoionization modelling aimed at probing its unusual thermal and chemical structure. The nebula exhibits a nested bipolar morphology, including inner and outer shells, faint outer lobes, and polar knots. Remarkably, all these components share a dynamical age of order 10,000 yr. Thermal broadening of the H alpha line relative to heavier ions implies a hydrogen temperature 3000 K to 6000 K, in contrast to the ~ 10,000 K derived from collisionally excited line diagnostics. This provides independent support for the presence of at least two distinct temperature/metallicity phases, as previously proposed to explain extreme abundance discrepancies. Photoionization models with sinusoidally varying metallicity successfully reproduce the observed nebular spectrum and morphology. A mixture of fluctuations with both extreme and moderate metallicity contrasts is required to simultaneously fit the O II and the [O III] observations. The nebular He II emission demands a hotter and more luminous central star than previously inferred, consistent with a ~ 0.58 solar mass post-AGB progenitor evolving toward a CO white dwarf. Ou 5 thus reinforces the link between close-binary nuclei and extreme abundance discrepancies, and provides a valuable benchmark for understanding how common-envelope ejections give rise to the thermal and abundance inhomogeneities observed in planetary nebulae.

We present high-angular-resolution ($\sim 0_{.}1\,\mathrm{ arcsec}$) VLA Ku-band (12–18 GHz) observations of two explosive molecular outflows (EMOs), DR 21, and G5.89, in a search for runaway stars related to these explosive events. In DR 21, we identified 13 compact radio sources (CRS), 9 located in the DR 21core and near the CO streamer ejection region. The radio properties of the CRSs show that three are non-thermal radio emitters, likely magnetically active stars, while the nature of the remaining CRSs cannot be conclusively identified. All detected CRSs are good candidates for follow-up proper motion studies to confirm whether they are runaway stars. We also identify multiple ionized arc-shaped structures that can be fitted with parabolas whose symmetry axes converge to a position coincident with CRSs #11, raising the possibility that this source is the main ionizing star. A re-analysis of the 18 molecular outflow streamers refines the center of the explosive event, which aligns closely with the position indicated by the arcs convergence point, supporting a common stellar origin for the EMOs and the H ii-region. In G5.89, the observations reveal a shell with a square-like morphology. The strong extended emission from this H ii region prevents the detection of weak compact radio sources inside the shell; only two were identified well beyond the shell, and a single parabolic arc was fitted within this region. Overall, arc structures in ionized regions seem to be good tracers of the origin of the ionizing sources.

The Rosette Nebula is a well-known H II region shaped by the interaction of gas with the OB stars of the NGC 2244 stellar association. Located within the remnant of a giant molecular cloud, it exhibits a complex structure of ionized gas, molecular material, dust, and embedded clusters. In October 2023, the region was observed as part of the SDSS-V Local Volume Mapper (LVM) integral field spectroscopy survey. Covering a radius of approximately 1 degree, the dataset comprises 33,326 spectra with spatially resolved information spanning 390 - 980 nm. We present a structural analysis of the ionized, molecular, and dusty components using multi-wavelength observations: optical spectroscopy from SDSS-V LVM, 12CO emission from PMO/MWISP (sub-millimeter), and dust emission from WISE (12 micron) and Herschel (far-infrared). These datasets were complemented with the positions of ionizing stars to study emission structures traced by H alpha, H beta, [O III], [N II], and [S II], as well as the spatial distribution of line ratios (H alpha/H beta, [O III]/H beta, [N II]/H alpha, and [S II]/H alpha) relative to the surrounding molecular cloud. Our analysis reveals interaction zones between ionized and neutral gas, including filaments, globules, and dense regions with or without ongoing star formation. Radial and quadrant-based flux profiles further highlight morphological and ionization variations, supporting the scenario in which the Rosette Nebula evolved from a non-homogeneous molecular cloud with a thin, sheet-like structure.

External photoevaporation of protoplanetary discs, by massive O stars in stellar clusters, is thought to be a significant process in the evolution of a disc. It has been shown to result in significant mass loss and disc truncation, ultimately reducing the lifetime of the discs, and possibly affecting potential planet populations. It is a well-studied process in the Orion Nebula Cluster (ONC) where the cometary morphology of proplyds is spatially resolvable due to its proximity to Earth. However, we need to study external photoevaporation in additional stellar clusters to better understand its prevalence and significance more globally. Unfortunately, more massive stellar clusters where the majority of stars form are much farther away than the ONC. In these more distant clusters the proplyds are spatially unresolvable with current facilities, hence the cometary morphology is not a useful identification of external photoevaporation. Therefore, in order to identify and interpret external photoevaporation, the only observations we have are of spatially unresolved emission lines. To resolve this issue we have used the cloudy code to develop an approximate general model of the emission lines emanating from the hot ionized wind of a proplyd. We have used the model to determine which line ratios are most sensitive to the distance from an OB star, and found that the most sensitive line ratios vary by multiple orders of magnitude over an far-ultraviolet field of between 10$^3$ G$_0$ to 10$^6$ G$_0$. By identifying spatial gradients of line ratios in stellar clusters, we can identify regions of ongoing external photoevaporation.

Recent years have seen a surge of interest in the community studying the effect of ultraviolet radiation environment, predominantly set by OB stars, on protoplanetary disc evolution and planet formation. This is important because a significant fraction of planetary systems, potentially including our own, formed in close proximity to OB stars. This is a rapidly developing field, with a broad range of observations across many regions recently obtained or recently scheduled. In this paper, stimulated by a series of workshops on the topic, we take stock of the current and upcoming observations. We discuss how the community can build on this recent success with future observations to make progress in answering the big questions of the field, with the broad goal of disentangling how external photoevaporation contributes to shaping the observed (exo)planet population. Both existing and future instruments offer numerous opportunities to make progress towards this goal.