Context. Long-period Wolf-Rayet (WR) star binaries produced by mass transfer are predicted to be abundant, but are observationally rare. This yields constraints on the evolution of initially wide O star binaries, including those potentially leading to the formation of gravitational-wave sources through the Common Envelope Channel. Aims. We investigate this issue in the light of a new type of orbital evolution for initially wide O star binaries, which is driven by mass ejection at periastron passage during the Luminous Blue Variable (LBV) phase. Methods. The assumption that the mass ejection occurs instantly at periastron passage allows us to analytically describe the orbital evolution. This approach is motivated by our understanding of an Eddington-limit driven LBV phase. We perform population synthesis calculations for the WR stars in the Small Magellanic Cloud (SMC), and compare them to the observed SMC WR star population. Results. Different from mass transfer, our mass ejection scenario leads to increased orbital periods and eccentricities. The Galactic system WR 140 (orbital period 2895 d, eccentricity 0.9) could be a typical result of this evolution scenario. Our models predict measurable binary space velocities, and allow for the disruption of the binary. Our SMC population synthesis model predicts statistically 5.3 close, 3.7 long-period, and further 2 runaway single WR stars. With largely increased orbital periods and eccentricities, such WR+O star binaries may not be ruled out by past radial-velocity searches. Applying our scenario to the Gaia BH1 and BH2 systems, we find that it provides viable progenitor evolution models. Conclusions. The mass-ejection-driven orbital evolution could explain why so few wide WR binaries are observed, and why some of the apparently single WR stars have high space velocities. We discuss implications for gravitational-wave sources.

Context. The early interaction of supernova blast waves with circumstellar material has the potential to accelerate particles to petaelectronvolt energies, although this has not yet been detected. Current models for this interaction assume that the blast wave expands into a smooth freely expanding stellar wind, although multiwavelength observations of many supernovae do not support this assumption. Aims. We extend previous work by considering blast waves expanding into complex density profiles consisting of smooth winds with dense circumstellar shells at various distances from the progenitor star. We aim to predict the gamma-ray and multiwavelength signatures of circumstellar interaction. Methods. We used the code PION to model the circumstellar medium around luminous blue variables including a brief episode of enhanced mass-loss and to simulate the formation of photoionization-confined shells around red supergiants. Consequently, we used the time-dependent acceleration code RATPaC to study the acceleration of cosmic rays in supernovae expanding into these media and to evaluate the emitted radiation (both thermal and nonthermal) across the whole electromagnetic spectrum. Results. We find that the interaction with the circumstellar shells can significantly boost the gamma-ray emission of a remnant, with the emission peaking weeks to years after the explosion when γγabsorption has reduced to negligible levels. The peak luminosity for Type IIP and Type IIn remnants can exceed the luminosity expected for smooth winds by several orders of magnitude. For Type IIP explosions, the light-curve peak is only reached years after the explosion, when the blast wave reaches the circumstellar shell. We evaluated the multiwavelength signatures expected from the interaction of the blast wave with a dense circumstellar shell from radio to optical and thermal X-rays. Conclusions. High-cadence optical surveys and continuous monitoring of nearby supernovae in radio and millimeter wavelengths are the best-suited strategies for identifying targets. They should be followed-up by gamma-ray observatories. We predict that gamma-rays from interaction with dense circumstellar shells may be detectable out to a few megaparsec for late interaction and out to tens of megaparsec for an early interaction.

Context. Long-period Wolf-Rayet (WR) star binaries produced by mass transfer are predicted to be abundant, but they are observationally rare. This yields constraints on the evolution of initially wide O star binaries, including those potentially leading to the formation of gravitational-wave sources through the common envelope channel. Aims. We investigated this issue in the light of a new type of orbital evolution for initially wide O star binaries, which is driven by mass ejection at periastron passage during the luminous blue variable (LBV) phase. Methods. The assumption that the mass ejection occurs instantly at periastron passage allowed us to analytically describe the orbital evolution. This approach is motivated by our understanding of an Eddington limit-driven LBV phase. We performed population synthesis calculations for the WR stars in the Small Magellanic Cloud (SMC) and compared them to the observed SMC WR star population. Results. Different from mass transfer, our mass ejection scenario leads to increased orbital periods and eccentricities. The Galactic system WR 140 (orbital period 2895 d, eccentricity 0.9) could be a typical result of this evolution scenario. Our models predict measurable binary space velocities and allow for the disruption of the binary. Our SMC population synthesis model statistically predicts 5.3 close, 3.7 long-period, and 2 further runaway single WR stars. With largely increased orbital periods and eccentricities, such WR+O star binaries cannot be ruled out based on past radial-velocity searches. When applying our scenario to the Gaia BH1 and BH2 systems, we found that it provides viable progenitor evolution models. Conclusions. The mass-ejection-driven orbital evolution could explain why so few wide WR binaries are observed and why some of the apparently single WR stars have high space velocities. We discuss the implications for gravitational-wave sources as well.

Context. The supernova remnant (SNR) candidate G310.7-5.4 was identified as a faint shell in the MGPS-2 survey. We followed up on it with multi-wavelength observations, confirming it to be an SNR at a high Galactic latitude that shows spatially coinciding γ-ray emission. Aims. We conducted the first detailed investigation of the radio emission from the G310.7-5.4 region, aiming to characterise the radio structure, polarisation measurements, and the coinciding GeV emission. Methods. We used recent radio continuum observations at 943.5 MHz from the EMU and POSSUM surveys with ASKAP, as well as 16.5 years of Fermi-LAT observations. We furthermore considered the multi-wavelength context of the object by investigating observations previously conducted with other instruments, such as IR and X-ray surveys. Results. We confirm that G310.7-5.4 is indeed an SNR, which we have dubbed Abeona. We detect the presence of a faint, extended (30′ diameter) bilateral radio shell with an ASKAP radio flux density of 1.5−0.1+1.5 Jy with no obvious IR counterparts. With a radio surface brightness of about 2.4−0.1+2.4 × 10−22 W m−2 Hz−1 sr−1, it is one of the faintest radio SNRs known. The northern part of the shell shows linearly polarised radio emission, which is characteristic of synchrotron emission in SNRs. The physical size of the SNR is estimated to be around 42−21+42 pc, which would correspond to a distance of around 4.9−2.5+4.9 kpc. Furthermore, the spatially coincident γ-ray source 4FGL J1413.9-6705 shows an energy flux of (1.26 ± 0.35) × 10−6 MeV cm−2 s−1 with a significance of 5.7 σ between 100 MeV and 100 GeV. We placed Abeona in context with other known high-latitude SNRs with γ-ray counterparts and compared their observational properties. Conclusions. New radio surveys are discovering a population of faint high-Galactic-latitude SNRs with GeV counterparts in isolated environments. More sensitive observations with the CTAO in the next few years should provide valuable insights into particle acceleration and escape in these important cosmic ray accelerators.

G310.7-5.4 is a supernova remnant (SNR) candidate identified as a faint shell in the second epoch Molonglo Galactic Plane Survey (MGPS-2), but this has not been followed up with multi-wavelength observations until now. It is an example of an SNR at high Galactic latitude showing spatially coinciding γ-ray emission. Here, we make the first detailed investigation of the radio emission from the G310.7-5.4 region, aiming to characterise the radio structure, polarisation measurements and the coinciding GeV emission. We used recent radio continuum observations at 943.5 MHz from the EMU and the POSSUM surveys with ASKAP, as well as 16.5 years of Fermi-LAT observations. We furthermore considered the multiwavelength context of the object by investigating observations previously conducted with other instruments, such as infrared and X-ray surveys. We confirm the SNR candidate as a new supernova remnant, dubbed Abeona. We detect the presence of a faint, extended, bilateral radio shell of the size of around 30' diameter and ASKAP radio flux density of 1.5^+1.5_-0.1 Jy with no obvious infrared counterparts. With a radio surface brightness of about 2.4^+2.4_-0.1×10^-22 W m^-2 Hz^-1 sr^-1, this SNR is one of the faintest radio SNRs known. The northern part of the shell shows linearly polarised radio emission, characteristic of synchrotron emission in SNRs. The physical size of the SNR is estimated to be around 42^+42_-21 pc, which would give a distance of around 4.9^+4.9_-2.5 kpc. Furthermore, the spatially coincident γ-ray source 4FGL J1413.9-6705 shows an energy flux of 1.26±0.35× 10^-6 MeV cm^-2 s^-1 with a significance of 5.7 σ between 100 MeV and 100 GeV. The SNR is also put in context with known high-latitude SNRs with γ-ray counterparts and compared with their observational properties.

The computational expense of solving non-equilibrium chemistry equations in astrophysical simulations poses a significant challenge, particularly in high-resolution, large-scale cosmological models. In this work, we explore the potential of machine learning, specifically Neural Operators, to emulate the Grackle chemistry solver, which is widely used in cosmological hydrodynamical simulations. Neural Operators offer a mesh-free, data-driven approach to approximate solutions to coupled ordinary differential equations governing chemical evolution, gas cooling, and heating. We construct and train multiple Neural Operator architectures (DeepONet variants) using a dataset derived from cosmological simulations to optimize accuracy and efficiency. Our results demonstrate that the trained models accurately reproduce Grackle's outputs with an average error of less than 0.6 dex in most cases, though deviations increase in highly dynamic chemical environments. Compared to Grackle, the machine learning models provide computational speedups of up to a factor of six in large-scale simulations, highlighting their potential for reducing computational bottlenecks in astrophysical modeling. However, challenges remain, particularly in iterative applications where accumulated errors can lead to numerical instability. Additionally, the performance of these machine learning models is constrained by their need for well-represented training datasets and the limited extrapolation capabilities of deep learning methods. While promising, further development is required for Neural Operator-based emulators to be fully integrated into astrophysical simulations. Future work should focus on improving stability over iterative timesteps and optimizing implementations for hardware acceleration. This study provides an initial step toward the broader adoption of machine learning approaches in astrophysical chemistry solvers.

We present the serendipitous radio-continuum discovery of a likely Galactic supernova remnant (SNR) G305.4–2.2. This object displays a remarkable circular symmetry in shape, making it one of the most circular Galactic SNRs known. Nicknamed Teleios due to its symmetry, it was detected in the new Australian Square Kilometre Array Pathfinder (ASKAP) Evolutionary Map of the Universe (EMU) radio–continuum images with an angular size of 1 320 $^{\prime\prime}$ $\times$ 1 260 $^{\prime\prime}$ and PA = 0 $^\circ$ . While there is a hint of possible H $\alpha$ and gamma-ray emission, Teleios is exclusively seen at radio–continuum frequencies. Interestingly, Teleios is not only almost perfectly symmetric, but it also has one of the lowest surface brightnesses discovered among Galactic SNRs and a steep spectral index of $\alpha$ =–0.6 $\pm$ 0.3. Our best estimates from H i studies and the $\Sigma$ –D relation place Teleios as a type Ia SNR at a distance of either $\sim$ 2.2 kpc (near-side) or $\sim$ 7.7 kpc (far-side). This indicates two possible scenarios, either a young (under 1 000 yr) or a somewhat older SNR (over 10 000 yr). With a corresponding diameter of 14/48 pc, our evolutionary studies place Teleios at the either early or late Sedov phase, depending on the distance/diameter estimate. However, our modelling also predicts X-ray emission, which we do not see in the present generation of eROSITA images. We also explored a type Iax explosion scenario that would point to a much closer distance of $\lt$ 1 kpc and Teleios size of only $\sim$ 3.3 pc, which would be similar to the only known type Iax remnant SN1181. Unfortunately, all examined scenarios have their challenges, and no definitive Supernova (SN) origin type can be established at this stage. Remarkably, Teleios has retained its symmetrical shape as it aged even to such a diameter, suggesting expansion into a rarefied and isotropic ambient medium. The low radio surface brightness and the lack of pronounced polarisation can be explained by a high level of ambient rotation measure (RM), with the largest RM being observed at Teleios’s centre.

While many astrophysical plasmas can be modelled successfully assuming ionisation and thermal equilibrium, in some cases this is not appropriate and a non-equilibrium approach is required. In nebulae around evolved stars, the local elemental abundances may also strongly vary in space and time. Here we present a non-equilibrium multi-ion module developed for the fluid-dynamics code pion describing the physical processes included and demonstrating its capabilities with some test calculations. A non-equilibrium ionisation solver is developed that allows arbitrary elemental abundances for neutral and ionised (but not molecular) gas, for the elements H, He, C, N, O, Ne, Si, S, and Fe. Collisional ionisation and recombination, photoionisation and charge-exchange reactions are included, and ion-by-ion non-equilibrium radiative cooling is calculated based on the instantaneous ion fractions of each element. Element and ion mass-fractions are advected using passive scalars, operator-split from the microphysical processes. The module is validated by comparing with equilibrium and non-equilibrium calculations in the literature. Effects of charge exchange on ion abundances in cooling plasmas are discussed. Application to modelling shocks and photo-ionised H ii regions is demonstrated. The time-dependent expansion of a WR nebula is studied, including photoionisation and collisional processes, and spectral-line luminosities calculated for non-equilibrium and equilibrium plasma states. The multi-ion module enables simulation of ionised plasmas with spatially varying elemental abundances using self-consistent ion abundances and thermal evolution. This allows prediction of spectral lines in UV, optical, IR, and X-ray even in cases where the plasma is out of ionisation equilibrium.

Fast Radio Bursts (FRBs) are highly energetic, extremely short-lived bursts of radio flashes. Despite extensive research, the exact cause of these outbursts remains speculative. The high luminosity, short duration, and high dispersion measure of these events suggest they result from extreme, high-energy extragalactic sources, such as highly magnetized and rapidly spinning neutron stars known as magnetars. The number of detected FRBs, including repeating ones, has grown rapidly in recent years. Except for FRB 20200428D, and FRB-like radio burst that is associated to Galactic magnetar SGR 1935+2154, no multi-wavelength counterpart to any FRB has been detected yet. The High Energy Stereoscopic System (H.E.S.S.) telescope has developed a program to follow up FRBs searching for their gamma-ray counterparts, helping to uncover the nature of FRBs and FRB sources. This paper provides an overview of the searches for FRB sources conducted by H.E.S.S., including follow-up observations and simultaneous multi-wavelength campaigns with radio and X-ray observatories. Among the FRB sources observed by H.E.S.S., nine are localized with redshifts ranging between 0.11 and 0.492 from 2015 to 2022. No significant very high energy (VHE) emission was detected during these observations. We report constraints on the VHE luminosity ranging from 10^44 erg s^-1 and 10^48 erg s^-1, placing limits on the FRB's region persistent VHE emission and potential FRB afterglow emission across timescales from hours to years.

Context. X-ray emission from wind-driven bow shocks is both difficult to measure and predict, but may give important insights into the energy budget of the hot phase of the interstellar medium (ISM) by quantifying mixing at the interface between hot and warm gas phases. Aims. We investigate the effect of magnetic fields and numerical resolution on predicted X-ray emission and other observable properties of bow shocks, to study convergence properties and assess robustness of predicted observables from simulations. Methods. A suite of 2D and 3D hydrodynamic and magnetohydrodynamic simulations of bow shocks were run and analysed to generate synthetic emission maps and light curves in X-ray and infrared emission. Results. Resolving the Kelvin-Helmholtz (KH) instability at the wind-ISM contact discontinuity is crucial for obtaining converged results and for predicting X-ray emission and the properties of the hot shocked wind. When sufficient spatial resolution is used, we measure time variation of X-ray emission of at least an order of magnitude on a timescale comparable to the advection timescale of the wake downstream from the bow shock. Good correspondence is found between 2D and 3D simulations with comparable resolution, and 3D simulations can achieve the required resolution with reasonable computing resources. Development of the KH instability is inhibited for shear flows parallel to the ISM magnetic field, compared with what is seen in the perpendicular direction, resulting in synthetic IR emission maps of bow shocks that are smooth when seen from one perspective but show strong distortions from another. Conclusions. Measuring the X-ray morphology and luminosity in bow shocks may be useful for constraining mixing and energytransfer rates between hot and warm gas phases of the ISM. Dynamical instabilities at the wind-ISM interface are a crucial ingredient in determining the properties of the hot-gas phase in stellar bow-shocks, in particular to capture its time dependence.

Supernova (SN) explosions interacting with dense circumstellar medium are considered to be very promising sites for efficient cosmic-ray (CR) acceleration and subsequent emission of neutral-pion-decay gamma rays. These environments share similarities with already detected gamma-ray novae, but with much greater available energy content, so it is important to characterize their emission in the very-high-energy range. We present the results of H.E.S.S. observations of one such candidate source - SN 2024ggi, located in NGC 3621 at a distance of 7.24 Mpc. A total of 30 hours of data, gathered throughout a month of post-explosion observations, provide flux upper limits that are used to constrain source parameters, offering meaningful insights for theoretical predictions. We exclude bright gamma-ray emission in the first day after explosion, and later upper limits are consistent with wind densities derived from optical observations.

Context. It is important to determine which Galactic cosmic-ray sources can accelerate particles to the knee of the cosmic ray spectrum at a few PeV, and in particular whether supernova remnants may contribute. Current models for particle acceleration in very young remnants assume the circumstellar material consists of smooth, freely expanding winds. There is strong evidence that some supernovae expand into much denser circumstellar material including dense shells ejected by eruptions shortly before explosion. Aims. We investigate the effects of dense circumstellar shells on particle acceleration in supernova shocks during the first few years post-explosion, to quantify whether such interaction supernovae may act as PeVatrons. Methods. We used the pion code to model the circumstellar medium around Luminous Blue Variables after having a brief episode with a mass-loss rate of up to dM/dt = 2Msol/yr. Consequently, we performed spherically symmetric 1-D simulations using our time-dependent acceleration-code RATPaC in which we simultaneously solve the transport equations for cosmic-rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit. Results. We find that the interaction with the circumstellar shells can significantly boost the maximum energy by enhancing particle escape during the onset of the shock-shell interaction followed by the reacceleration of the shock propagating into a medium with a pre-amplified field. Early interactions boost the maximum energy to a greater degree and interactions within the first 5 months after explosion can increase Emax to more then 1 PeV.

Context. X-ray emission from wind-driven bow shocks is both difficult to measure and predict, but may give important insights into the energy budget of the hot phase of the interstellar medium (ISM) by quantifying mixing at the interface between hot and warm gas phases. Aims. We investigate the effect of magnetic fields and numerical resolution on predicted X-ray emission and other observable properties of bow shocks, to study convergence properties and assess robustness of predicted observables from simulations. Methods. A suite of 2D and 3D hydrodynamic and magnetohydrodynamic simulations of bow shocks were run and analysed to generate synthetic emission maps and light curves in X-ray and infrared emission. Results. Resolving the Kelvin-Helmholtz (KH) instability at the wind-ISM contact discontinuity is crucial for obtaining converged results and for predicting X-ray emission and the properties of the hot shocked wind. When sufficient spatial resolution is used, we measure time variation of X-ray emission of at least an order of magnitude on a timescale comparable to the advection timescale of the wake downstream from the bow shock. Good correspondence is found between 2D and 3D simulations with comparable resolution, and 3D simulations can achieve the required resolution with reasonable computing resources. Development of the KH instability is inhibited for shear flows parallel to the ISM magnetic field, compared with what is seen in the perpendicular direction, resulting in synthetic IR emission maps of bow shocks that are smooth when seen from one perspective but show strong distortions from another. Conclusions. Measuring the X-ray morphology and luminosity in bow shocks may be useful for constraining mixing and energytransfer rates between hot and warm gas phases of the ISM. Dynamical instabilities at the wind-ISM interface are a crucial ingredient in determining the properties of the hot-gas phase in stellar bow-shocks, in particular to capture its time dependence.

Bow shocks generated from the interaction of winds from massive runaway stars with the interstellar medium have been shown to be prominent particle accelerators through recent γ-ray and radio synchrotron observations. Here, we study particle acceleration from bow shocks by conducting 3D ideal cosmic ray magnetohydrodynamic simulations in the advection-diffusion limit. We use the Eulerian grid-based code FLASH, where stellar winds are injected through tabulated wind velocities and mass loss rates. We implement a gradient-based shock detection algorithm to resolve the shocked regions where the CRs are injected dynamically. Simulations are performed for different values of the CR diffusion coefficient and star velocities within an ISM-like environment up to 180 kyr to showcase the impact of dynamical CR injection on the early evolution of the wind-driven bow shock. With a simplified spectral model in post-processing, we calculate the expected upper limits of γ-ray and synchrotron emission and compare with those from current observations. We observe that variations of CR diffusion rates can strongly dictate the morphology of the bow shock and the overall γ-ray and radio synchrotron luminosity due to the balance between the CR injection efficiency and diffusion. Our results yield qualitatively comparable results with current observations, primarily attributed to the high-energy protons and electrons contributing to non-thermal emission from efficient acceleration at the forward shock through the approximations and assumptions in the injection algorithm. We conclude that CR acceleration, with varying CR diffusion rates, may substantially affect the morphology of wind-driven bow shocks and their non-thermal emission, if there is efficient particle acceleration in the forward shock. [abridged]

Early interaction of supernova blast waves with CSM has the potential to accelerate particles to PeV energies, although this has not yet been detected. Current models for this interaction assume the shock expands into a smooth stellar wind, although observations of many SNe do not support this assumption. We extend previous work by considering shocks expanding into complex density profiles consisting of smooth winds with dense CSM shells at various distances from the progenitor star. We aim to predict the gamma-ray and multiwavelength signatures of CSM interaction. We used the PION code to model the CSM around LBV including a brief episode of enhanced mass-loss and to simulate the formation of photoionization-confined shells around RSGs. Consequently, we used the time-dependent acceleration-code RATPaC to study the acceleration of cosmic rays in SNe expanding into these media and to evaluate the emitted radiation across the whole electromagnetic spectrum. We find that the interaction with the CSM shells can significantly boost the gamma-ray emission, with the emission peaking weeks to years after the explosion. The peak luminosity for Type-IIP and Type-IIn remnants can exceed the luminosity expected for smooth winds by orders of magnitude. For Type-IIP explosions, the light-curve peak is only reached years after the explosion. We evaluate the multiwavelength signatures expected from the interaction of the blast wave with a dense CSM shell from radio, over optical, to thermal X-rays. We identify high-cadence optical surveys and continuous monitoring of nearby SN in radio and mm wavelengths as the best-suited strategies for identifying targets that should be followed-up by gamma-ray observatories. We predict that gamma-rays from interaction with dense CSM shells may be detectable out to a few Mpc for late interaction, and tens of Mpc for early interaction.

Some carbon-rich Wolf-Rayet stars (WC stars) show an infrared excess from dust emission. Dust forms in the collision of the WC wind with a companion star's wind. As this dust is carried towards the ISM at close to the WCd wind speed and the binary continues through its orbit, a spiral structure forms around the system. The shape depends on the orbital eccentricity and period, as well as stellar parameters like mass-loss rates and terminal wind speeds. Imaging of the WCd binary WR 140 with JWST/MIRI revealed at least 17 concentric dust shells surrounding the binary. We present new JWST imaging of four additional WCd systems (WR 48a, WR 112, WR 125, and WR 137) that were imaged in 2024. In this analysis, we show that the dust is long-lived, surviving for at least 130 years, but more than 300 years in some systems. Longer duration measurements are limited by sensitivity. Regular spacing of dust features confirms the periodic nature of dust formation, consistent with a connection to binary motion. We use these images to estimate the proper motion of the dust, finding the dust to propagate out to the interstellar medium with motion comparable to the wind speed of the WC stars. In addition to these results, we observe unusual structures around WR 48a, which could represent dusty clumps shaped by photoevaporation and wind ablation like young proplyd objects. These results demonstrate that WC dust is indeed long-lived and should be accounted for in galactic dust budgets.

We present the serendipitous discovery of a new radio-continuum ring-like object nicknamed K & yacute;klos (J1802-3353), with MeerKAT UHF and L-band observations. The radio ring, which resembles the recently discovered odd radio circles (ORCs), has a diameter of similar to 80 '' and is located just similar to 6 degrees from the Galactic plane. However, K & yacute;klos exhibits an atypical thermal radio-continuum spectrum (alpha = -0.1 +/- 0.3), which led us to explore different possible formation scenarios. We concluded that a circumstellar shell around an evolved massive star, possibly a Wolf-Rayet, is the most convincing explanation with the present data.

We summarise observations and our current understanding of the interstellar medium (ISM) in galaxies, which mainly consists of three phases: cold atomic or molecular gas and clouds, warm neutral or ionised gas, and hot ionised gas. These three gas phases form thermally stable states, while disturbances are caused by gravitation and stellar feedback in form of photons and shocks in stellar winds and supernovae. Hot plasma is mainly found in stellar bubbles, superbubbles, and Galactic outflows/fountains and is often dynamically unstable and is over-pressurised. In addition, in galactic nuclear regions, accretion onto the supermassive black hole causes enhanced star formation, outflows, additional heating, and acceleration of cosmic rays.

Winds from massive stars expand supersonically into their surroundings, creating dynamic and fascinating nebulae that can give us insight into physical processes in interstellar plasma, and into the evolutionary history of the stars. Around single stars, parsec-scale bubbles such as bow shocks and ring nebulae are formed, whereas in colliding-wind binary (CWB) systems the high wind density produces intense time- and space-dependent emission across the electromagnetic spectrum from radio to gamma-rays. This contribution summarizes some recent results from 3D MHD modelling of bow shocks around runaway stars such as zeta Oph, and of the wind-collision zone of the CWB systems WR140 and WR21a. A resolution study of 3D simulations of bow shocks shows that X-ray emission from the shocked wind is time-variable and that converged results can be obtained once the Kelvin-Helmholtz instability at the contact discontinuity is resolved. Simulations of the CWB system WR140 show that inverse-Compton cooling of the shocked plasma can trigger runaway cooling when the orbit is near periastron, producing strong compression and dynamical instabilities. This sharply reduces the hard-X-ray emission around periastron, in agreement with observations. Scaling tests of the simulation software pion are also presented for a model of the CWB system WR21a run on up to 8192 cores using the HPC system Karolina.