Peter A. M. van Hoof

Sakurai’s object provides a rare opportunity to observe stellar evolution on human time-scales. Since its born-again event and detection in 1996, its evolution has been extensively monitored, and recent optical spectroscopy has suggested the emergence of [WR]-type emission features. In this letter, we present a secure spectroscopic identification of its central star based on new VLT/FORS2 observations and NLTE expanding atmosphere models. Several observed emission lines arise in a [WR]-type stellar wind, establishing the central star as a [WR] object. The strongest stellar features are due to C ii–iii and He i. We derive the stellar and wind parameters of Sakurai’s object and discuss its evolutionary status. The [WR] star is much cooler ($T_\mathrm{eff} \sim$ 30.5 kK) than the born-again star V605 Aql ($T_\mathrm{eff} \sim$ 95 kK), which erupted about 80 yr earlier. The inferred temperature is lower than predicted by models suppressing convective mixing efficiency during the VLTP, but is consistent with higher time-resolution calculations involving slightly lower-mass ($\lesssim 0.6$ M$_\odot$) remnants. Continued spectroscopic monitoring and atmospheric modelling will be crucial for tracing its reheating curve and constraining the physics of this rapid evolutionary phase.

Planetary nebulae are sites where ejected stellar material evolves into complex molecules, but the precise physical conditions and chemical routes that govern these processes are unclear. The presence of abundant carbon-rich molecules in O-rich environments poses particular challenges. Here we report the first detection of methyl cation (CH 3 + ) in any planetary nebula, observed in the O-rich nebula NGC 6302 using JWST MIRI/Medium Resolution Spectrometer observations. CH 3 + is a key driver of organic chemistry in UV-irradiated environments. Spatially resolved observations reveal that CH 3 + is colocated with ¹² CO, H ₂ , H ii , HCO ⁺ , and polycyclic aromatic hydrocarbons. LTE modeling of the CH 3 + emission yields excitation temperatures of 500–800 K in the inner bubble and torus, rising to 1000–2000 K in the outer bubble of NGC 6302, with column densities ranging from ∼10 ¹¹ to 10 ¹³ cm ⁻² . This detection suggests that hydrocarbon radical chemistry must be incorporated into planetary nebulae chemical models. Further near-IR observations are crucial to map different chemical networks operating in these environments.

With the launch of the XRISM microcalorimeter mission, space-based X-ray observations will achieve a record spectral resolving power of R ≡ E/ΔE ∼ 1200. With this resolving power, emission features associated with fine-structure energy levels of some species will be resolved, sometimes for the first time. The plasma code, Cloudy, was not originally designed for high-resolution X-ray spectroscopy and throughout its history did not resolve fine-structure components of Lyman lines. Here, we expand Cloudy to resolve these fine-structure energy levels and obtain predicted X-ray spectra that match the resolution of new microcalorimeter observations. We show how the Lyman lines can be used as column density indicators in the hot X-ray emitting gas in a cluster of galaxies such as Perseus, and examine their sensitivity to external radiation fields and turbulence.

The planetary nebula NGC 6720, also known as the “Ring Nebula,” is one of the most iconic examples of nearby planetary nebulae whose morphologies present a challenge to our theoretical understanding of the processes that govern the deaths of most stars in the Universe that evolve on a Hubble time. We present new imaging with JWST of the central star of this planetary nebula (CSPN) and its close vicinity, in the near-to-mid-IR wavelength range. We find the presence of a dust cloud around the CSPN, both from the spectral energy distribution at wavelengths ≳5 μm as well as from radially extended emission in the 7.7, 10, and 11.3 μm images. From the modeling of these data, we infer that the CSPN has a luminosity of 310 L _⊙ and is surrounded by a dust cloud with a size of ∼2600 au, consisting of relatively small amorphous silicate dust grains (radius ∼0.01 μm) with a total mass of 1.9 × 10⁻⁶ M _⊕. However, our best-fit model shows a significant lack of extended emission at 7.7 μm—we show that such emission can arise from a smaller (7.3 × 10⁻⁷ M _⊕) but uncertain mass of (stochastically heated) ionized polycyclic aromatic hydrocarbon (PAHs). However, the same energetic radiation also rapidly destroys PAH molecules, suggesting that these are most likely being continuously replenished, via the outgassing of cometary bodies and/or the collisional grinding of planetesimals. We also find significant photometric variability of the central source that could be due to the presence of a close dwarf companion of mass ≤0.1 M _⊙.

Aims. We present a simple yet powerful column density diagnostic for plasmas enabled by X-ray microcalorimeter observations. Methods. With the recent developments of the spectral simulation code CLOUDY, inspired by the high spectral resolution of the X-Ray Imaging and Spectroscopy Mission (XRISM) and the Advanced Telescope for High Energy Astrophysics (Athena), we make predictions for the intensity ratio of the resolved fine-structure lines Lyα₁ and Lyα₂ of H-like ions. Results. We show that this ratio can be observationally constrained and used as a plasma column density indicator. We demonstrate this with an XRISM observation of the high-mass X-ray binary Centaurus X-3. Conclusions. This diagnostic is useful for a wide range of X-ray emitting plasmas that are either collisionally or radiatively ionized.

Context. The very late thermal pulse (VLTP) affects the evolution of ∼20% of 1–8 M_⊙ stars, repeating the last red giant phases within a few years and leading to the formation of a new, but hydrogen-poor, nebula within the old planetary nebula. The strong dust formation in the latter obscures the optical and near-infrared radiation of the star. Aims. We aimed to determine the reheating timescale of the central star in Sakurai’s Object, which is an important constraint for the poorly understood VLTP evolution. Methods. We observed the radio continuum emission of Sakurai’s Object for almost 20 years, from 2004 to 2023. Continuous, multi-frequency observations proved to be essential for distinguishing between phases dominated by photoionization and shock ionization. Results. The flux density fluctuates by more than a factor of 40 within months to years. The spectral index remained negative between 2006 and 2017 and has been close to zero since 2019. The emission region has been only barely resolved since 2021. Conclusions. Non-thermal radio emission observed from 2004 to 2017 traces shocks induced by wind interactions due to discrete mass-loss events. Thermal emission dominates from 2019 to 2023 and may indicate photoionization of the nebula by the central star.

We announce the C23.01 update of cloudy. This corrects a simple coding error, present since ∼1990, in one routine that required a conversion from the line-center to the mean normalization of the Lyα optical depth. This affects the destruction of H i Lyα by background opacities. Its largest effect is upon the Lyα intensity in high-ionization dusty clouds, where the predicted intensity is now up to three times stronger. Other properties that depend on Lyα destruction, such as grain infrared emission, change in response.

Over the past decades, libraries of stellar spectra have been used in a large variety of science cases, including as sources of reference spectra for a given object or a given spectral type. Despite the existence of large libraries and the increasing number of projects of large-scale spectral surveys, there is to date only one very high-resolution spectral library offering spectra from a few hundred objects from the southern hemisphere (UVES-POP) . We aim to extend the sample, offering a finer coverage of effective temperatures and surface gravity with a uniform collection of spectra obtained in the northern hemisphere. Between 2010 and 2020, we acquired several thousand echelle spectra of bright stars with the Mercator-HERMES spectrograph located in the Roque de Los Muchachos Observatory in La Palma, whose pipeline offers high-quality data reduction products. We have also developed methods to correct for the instrumental response in order to approach the true shape of the spectral continuum. Additionally, we have devised a normalisation process to provide a homogeneous normalisation of the full spectral range for most of the objects. We present a new spectral library consisting of 3256 spectra covering 2043 stars. It combines high signal-to-noise and high spectral resolution over the entire range of effective temperatures and luminosity classes. The spectra are presented in four versions: raw, corrected from the instrumental response, with and without correction from the atmospheric molecular absorption, and normalised (including the telluric correction).

We describe the 2017 release of the spectral synthesis code Cloudy. A major development since the previous release has been exporting the atomic data into external data files. This greatly simplifies updates and maintenance of the data. Many large datasets have been incorporated with the result that we can now predict well over an order of magnitude more emission lines when all databases are fully used. The use of such large datasets is not realistic for most calculations due to the time and memory needs, and we describe the limited subset of data we use by default. Despite the fact that we now predict significantly more lines than the previous Cloudy release, this version is faster because of optimization of memory access patterns and other tuning. The size and use of the databases can easily be adjusted in the command-line interface. We give examples of the accuracy limits using small models, and the performance requirements of large complete models. We summarize several advances in the H- and He-like iso-electronic sequences. We use our complete collisional-radiative models of the ionization of these one and two-electron ions to establish the highest density for which the coronal or interstellar medium (ISM) approximation works, and the lowest density where Saha or local thermodynamic equilibrium can be assumed. The coronal approximation fails at surprisingly low densities for collisional ionization equilibrium but is valid to higher densities for photoionized gas clouds. Many other improvements to the physics have been made and are described. These include the treatment of isotropic continuum sources such as the cosmic microwave background (CMB) in the reported output, and the ability to follow the evolution of cooling non-equilibrium clouds.