Quentin Andrew Parker

Aims. K 1-6 has long been classified as a planetary nebula (PN) hosting a binary central star; however, it has remained poorly studied due to its faintness. The central star exhibits pronounced photometric variability whose origin is still unclear. We present a comprehensive characterisation of the K 1-6 system, including the physical properties of its stellar components and the nature of the surrounding nebulosity. Methods. We conducted a multi-wavelength analysis combining optical and UV spectroscopy obtained with the Gran Telescopio Canarias, the Telescopio Nazionale Galileo, the Nordic Optical Telescope, and the Hubble Space Telescope. We also present long-term multi-band ground- and space-based photometry, including high-cadence data from the Transiting Exoplanet Survey Satellite, narrowband imaging, and the latest astrometric constraints from Gaia . Results. Our results show that the nebula is not a remnant PN, but instead consists of interstellar medium photoionised by a hot white dwarf, which is relatively evolved. It has a cooling age of 1–2 Myr, implying that any original PN has long since dissipated. We further find that the central object is a hierarchical triple system, comprising an inner binary with an orbital period likely of the order of thousands of days and a distant tertiary companion orbiting on a timescale of tens of thousands of years. The optically dominant cool component of the inner binary is an inflated K-type star displaying extreme magnetic activity, including large-amplitude variability and flaring. Its properties resemble those of BY Dra-type binaries and Abell 35-type systems, and are difficult to reconcile with single-star evolution, pointing instead to a history of binary interaction.

We reveal post-VLTP stellar recovery, dust formation, obscuration and evolution of born-again planetary nebula HuBi 1 by combining archival plates, optical photometry, and infrared observations from the 1890s to 2025. These revealed a century long, linear brightening interpreted as the post-VLTP recovery phase where central star contraction and reheating shifts emergent flux toward shorter wavelengths, while enhanced nebular ionization strengthened blue-band emission. The subsequent optical decline from 1986, provides a tight constraint for dust obscuration and formation onset. The infrared SED shows that HuBi 1 remained dust-emitting throughout and that it cannot come from a single dust component but requires at least two dust-emitting regions associated with the compact inner shell and extended outer nebula. A two-shell Cloudy model favors amorphous carbon as the dominant dust species. In our model the infrared dust evolution is driven by the compact inner shell, whose dust temperature declines from 587 to 462 K between 1986 and 2025, while inferred dust mass increases from 1.412 × 10 ⁻⁶ to 1.023 × 10 ⁻⁵ M _⊙ . We interpret this as continued evolution of the dust reservoir in the born-again ejecta, rather than uniform dust formation under the shell-averaged modeled conditions. These results provide the clearest observational link between preobscuration brightening, dust formation/obscuration onset, and subsequent infrared dust evolution in HuBi 1 and indeed for any PN. They yield tight observational constraints for theoretical models for a brief stellar evolutionary stage, making HuBi 1 a valuable laboratory and benchmark for testing models of late helium-shell flashes, post-VLTP reheating, and transition toward a compact hot central star.

This study was motivated by the search for planetary nebulae (PNe) and open cluster (OCs) associations. Only five PNe are currently in Galactic OCs among the previous ∼4800 OCs known. They are valuable because as cluster members their properties can be linked to their progenitor stars, something no possible for general field PNe. Since the GAIA astrometric satellite first data release, thousands of new OC candidates have been identified. This offers fresh motivation to search them as hosts for known PNe. Statistically we expect 30 OC-PN pairs to be present. We searched these GAIA candidate OCs, found by E. L. Hunt & S. Reffert, for known central stars of PNe (CSPNe) as a practical proxy for the PNe themselves and one suited to automated searches. We identified 1 potential CSPN (IRAS 15127-5811) of “likely” PN [GKF2010] MN18 in the HASH database, as a claimed member of the putative cluster HSC 2686. A second cluster (Lynga 3) with similar parameters and in very close angular proximity could also be a potential host. Here we reclassify CSPN IRAS 15127-5811 as a blue supergiant with a surrounding bipolar nebular. Nevertheless, we collected all available information on proposed member stars for both these clusters. We conclude that neither claimed cluster is real. Our results shed considerable doubt on the veracity of many newly identified OCs that have modest numbers of stellar members.

Stellar evolution studies of low-to-intermediate mass stars are hindered by the lack of a solid link that connects their initial and final properties. This problem can be resolved for stars that are members of star clusters and at the final stages of their evolution. In such cases, we can estimate their final properties from direct measurements and obtain the necessary information for the properties of their progenitors from independent cluster studies. Here, we focus on the study of planetary nebulae that are confirmed members of Galactic open star clusters. These rare instances provide us with a new additional sample of data, which contributes to the fundamental initial-to-final mass relation (IFMR). We examine the quality of this contribution and assess the efficacy of our method, showing that our approach is very promising with already significant initial results. Our data generally confirm the latest IFMR trends, filling important gaps in the process.

While there has been significant progress in our understanding of the origin and evolution of planetary nebulae in the last 50 years, there remain several unsolved problems. These include the true 3D morphological structure of the nebulae, origin of multipolar nebulae, the dust and molecular distribution relative to the optical nebulosity, large-scale structures outside of the main nebulae, the relevance of binarity to planetary nebulae evolution, and a precise definition of the planetary nebula phenomenon. The long-standing problem of elemental abundance discrepancy still remains unsolved. In this paper, we summarize current observations related to these problems and present possible future directions to tackle them.

Planetary nebulae (PNs) studies are essential for understanding the late stellar evolution of low-to-intermediate mass stars. PNs in open clusters (OC) are rare but valuable since their study directly links their properties to those of their progenitors, something that cannot be achieved for field PNs. Here, we report the identification of one more OC-PN association to add to the small sample of, now, five pairs. The physical properties of the host cluster, PN, and its central star (CSPN) have been explored using high and intermediate-resolution spectral and deep photometric data. The close agreement of the radial velocities of the PN and host cluster, together with concordance of reddening and distance, shows that the PN PHR J1724-3859 is highly likely to be physically associated with the OC Trumpler 25. Deep photometric data allowed clear identification of the CSPN. We find a CSPN effective temperature of around 250 kK and a nebular kinematic age of 23 kyr, both at the extreme end, like the other members of this small class. The progenitor and final CSPN masses have been estimated to be 5.12 − 0.15 + 0.16 M _⊙ and 0.95 ± 0.12 M _⊙ respectively. These latest results agree with the emerging trend for our other OC-PNs, falling below, but approximately parallel to, the latest initial-to-final-mass relation estimates derived from cluster white dwarfs, and have important implications for stellar evolution models. All OC-PNs also possess some other common properties (e.g., all are Type I PNs and bipolars) to be explored in future studies.

Some planetary nebulae (PNs) host X-ray-emitting hot bubbles shaped by stellar wind interactions and/or harbor X-ray-emitting central stars due to accretion, shocks within their fast stellar winds, or even chromospheric emission from binary companions. In both cases, the properties of the X-ray emission critically probe late stages of stellar evolution for such low- and intermediate-mass stars. While extant Chandra and XMM-Newton observations have detected X-ray emission in PNs, the numbers known remain very small (∼40) compared to the overall Galactic PNs population (∼4000). We have initiated a project aimed at increasing the sample of known PNs with X-ray emission using both current and new space-based X-ray telescopes, such as the Einstein probe. To further investigate their X-ray properties to elucidate what drives current X-ray PN detections, we have cross-searched the SRG eROSITA-DE eRASS1 source catalog and the Hong Kong (HASH) PNs Database. Five known X-ray PNs have been detected (A30, NGC 2392, NGC 3242, NGC 5315, and LoTr 5), two new X-ray PNs are revealed (IC 1297 and NGC 2867), one (K 1–27) is removed from previous X-ray compilations, and another 11 previously detected X-ray emitting PNs are not recovered. A comparison of the X-ray flux of detected and undetected X-ray PNs reveals that eROSITA eRASS1 is sensitive to PNs with X-ray fluxes larger than ≈2 × 10⁻¹⁴ erg cm⁻² s⁻¹. The frequency of occurrence is ≃0.5% among the 1430 HASH True PNs in the eRASS1 footprint.

The rate of stellar evolution can rarely be measured in real time. The fastest evolution (excluding event-driven evolution), where stars may evolve measurably over decades, is during the post-AGB phase. In this paper we provide direct evidence for such a case. A secular, linear, factor of ~2.5 increase is found in the strength of the [O III] lines relative to H-beta over an 130 year period in the young, well-known, low excitation planetary nebula IC 418. The increase is caused by the rising temperature of the central star. We use photo-ionization models to derive a model dependent heating rate for the central star in the range 15-42 K\/yr. These derived heating rates are very sensitive to the stellar mass, and yield a central-star mass of 0.560-0.583 solar masses. Initial-final mass relations based on the Miller-Bertolami models give a progenitor main-sequence mass of 1.25-1.55 solar masses. IC 418 is a carbon rich planetary nebula and its central star, HD 35914, has evolved from an AGB carbon star. This result shows that carbon star formation at solar metallicity extends to these low masses. This is lower than commonly assumed and suggests that post-AGB evolution may be slower than recent post-AGB models predict.