Vol. 714
6. Interstellar and circumstellar matter

Deciphering the Green Monster in Cassiopeia A: Puncturing and sculpting a heterogeneous circumstellar shell

by S. Orlando, H.-T. Janka, D. Milisavljevic, I. De Looze, T. Temim, R. Fesen, B.-C. Koo, M. Miceli, F. Bocchino 2026, A&A, 714, A328

In an extensive set of simulations of a supernova and its environment, Orlando and collaborators have followed the evolution of the Cas A event from explosion to the present day and beyond. In this latest result, they examine the effect of the shock striking out into a preexisting circumstellar shell. The resulting fine structure induced by the individual knots of the ejecta that precede the main forward shock as it punches through this medium is compared with recent JWST observations of "hole-ring" structures, revealing a very heterogeneous circumstellar medium. This work has fundamental implications for many transients, developing a new way of understanding how fragmented shocks affect observables in cases as diverse as classical novae, wind-blown Wolf-Rayet bubbles, and fliers in H II regions and planetary nebulae.

Vol. 714
10. Planets, planetary systems, and small bodies

JWST detection of methane emission from a young brown dwarf

by K. L. Luhman, C. Alves de Oliveira, J. K. Faherty, B. C. Smith 2026, A&A, 714, L15

When spectroscopically observing young star-forming regions, researchers frequently detect emission lines from stellar members such as Herbig Ae/Be and T Tauri stars. These signatures —most notably atomic lines of hydrogen, helium, and calcium— stem from circumstellar disc accretion, chromospheric activity, or a combination of both. This same phenomenon is observed when scaling down in mass to investigate million-year-old brown dwarfs (objects below the hydrogen-burning mass limit), and it even persists below the deuterium-burning limit (~13 MJup). However, the planetary-mass regime introduces unique anomalies. In a recent study, Luhman et al. used the James Webb Space Telescope to detect robust methane emission from the upper atmosphere of LRL 11001, a ~6-MJup object in the ~5 Myr-old IC 348 star-forming region. This marks the first discovery of methane emission in a brown dwarf of any mass. The authors suggest its origin may be due to either an accretion shock or external radiation from neighboring cluster members.

Vol. 714
4. Extragalactic astronomy

A first GLIMPSE into star-cluster populations across cosmic time

by A. Claeyssens, A. Adamo, V. Kokorev,et al. 2026, A&A, 714, A148

Star clusters represent the smallest structures in galaxies, sharing a common origin and tracing the star formation history of their hosts. Due to their faint luminosities and compact sizes, detailed observations have historically been limited to nearby systems. In this recently accepted paper, strongly lensed regions (located behind massive galaxy clusters) were targeted with JWST/NIRCam to identify the most distant star clusters. The team studied a sample of 218 star clusters at unprecedented redshifts, with 75% of the sample located between z=0.5 and z=4. This work is part of the GLIMPSE collaboration, which targets strongly lensed regions to explore distant galaxies. All stellar clusters are highly magnified (mu>10), and the observations mostly indicate young ages, corresponding to a formation epoch at cosmic noon (11, revealing a power law with a slope around -2. The accompanying figure shows a color composite of the candidate star clusters (open circles) overlaid on the JWST/NIRCam images of the targeted galaxies, which are labeled by their redshift.

Vol. 713
5. Galactic structure, stellar clusters and populations

Expanding stellar associations as Galactic accelerometers

by Till Sawala, Núria Miret Roig 2026, A&A, 713, A261

The mass distribution of the Milky Way remains one of the largest variables that bears relevance to our understanding of galactic structure and formation. This study introduces a novel method to characterize and precisely quantify the Galactic gravitational potential by using the time evolution of stellar associations. These are ensembles of coeval and spatially coherent, albeit expanding, stars with on the order of tens of members, typically, and ages of a few tens of million years. By using statistical metrics (namely a matrix description of the the positional covariances) to describe the spatial interdistance of a given star group as test cases over time, the authors succeeded at placing constraints on the shape and overall mass of the underlying potential. This is particularly valuable in light of the upcoming fourth data release (DR4) of Gaia at the end of this year: at its superior astrometric and velocity precision, the authors demonstrate that a single association alone would be able to recover the Galaxy’s underlying halo mass to a precision at the 0.6x1012 Mo level, although parameter degeneracies are inescapable.

Vol. 713
10. Planets, planetary systems, and small bodies

The JWST weather report: Unravelling the atmospheric variability of isolated worlds using principal component analysis

by Merle A. Schrader, Johanna M. Vos, Evert Nasedkin, Jennifer Kestell, et al. 2026, A&A, 713, A192

SIMP J013656.5+093347.3 (SIMP 0136) is a young, T2.5-type brown dwarf at the planetary-mass boundary, located at a distance of only 6 pc. In addition, it is one of the most photometrically variable field brown dwarfs known, if not the most variable. It was observed with JWST/NIRSpec PRISM over a rotation period of just 2.4 h. The authors analyzed the principal components of the low-resolution 1--5 mum spectroscopic time series and found that most of the brown dwarf's variability can be explained by changes in just two dominant components: temperature and cloud vertical structure. This allowed them to describe the brown dwarf's spectral variability as a linear combination of three distinct atmospheric states.

Vol. 713
4. Extragalactic astronomy

A large population of overmassive black hole quasars at z=0.3-0.8 revealed by eROSITA

by Johannes Buchner, Isabelle Gauger, Qiaoya Wu, Hattie Starck, Catarina Aydar, et al. 2026, A&A, 713, A11

In the standard framework of galaxy formation and evolution, the two main components -- dark matter and baryonic (gas and stellar) mass -- are complemented by the presence of a central (super)massive black hole (BH). This BH is expected to shape galaxy evolution, sometimes substantially, by regulating the transformation of gas into stars. The mass fraction contributed by the BH is typically about one percent of the host galaxy’s stellar mass. However, recent observations have identified exceptions where the BH mass is much larger. Such overmassive systems (with BH-to-stellar mass ratios exceeding 5\%) have been observed in low-redshift dwarf galaxies and also at very high redshifts. This new A&A publication is based on a large-scale eROSITA X-ray survey, complemented by NUV/optical/IR photometric and spectroscopic data. The authors studied a sample of quasars that are undergoing rapid mass growth, uncovering a significant population (10–20 per cubic Gpc) of galaxies with overmassive central BHs at intermediate redshifts (between z=0.3 and 0.8). The authors suggest a rapid, exponential growth phase of the central BH decoupled from host galaxy development. This growth mode does not require super-Eddington rates and is not included in current galaxy evolution models. In the context of high redshift overmassive BHs, the newly discovered galaxies may suggest an alternative growth channel at cosmic dawn.

The figure shows a montage with the optical images of the X-ray quasars overlaid on the BH versus stellar mass plane. The orange line and shaded region delimit the standard scaling relation (plus scatter) for nearby elliptical galaxies and bulges, with the corresponding trend for star-forming galaxies in blue. The systems at the top-left correspond to galaxies with overmassive central BHs.

Vol. 713
3. Cosmology (including clusters of galaxies)

Hidden Vela supercluster revealed by the first hybrid redshift and peculiar velocity reconstruction

by A.M. Hollinger, H.M. Courtois, R.C. Kraan-Korteweg, and al. 2026, A&A, 713, A4

The large-scale distribution of the Universe is characterized by an intricate web of galaxies with low and high density regions. The structure is remarkably variable over all scales, from groups of a few galaxies to the largest possible distributions. Superclusters represent the highest densities in this cosmic web over vast scales, up to 0.1 Gpc. The study of these structures is hampered by a number of pitfalls, most notably foreground dust, which selectively obscures regions and hinders a clean selection of galaxies over the apparent scales expected in these structures. In a study accepted for publication in Astronomy & Astrophysics, researchers constructed a catalog of about eight thousand new galaxy redshifts in the Zone of Avoidance (ZoA) near the Galactic plane. The methodology follows a hybrid approach that reconstructs the density contrast and peculiar velocity fields. The team investigated nine superclusters with a special emphasis on the effect of these new ZoA redshifts. The results reveal that the mass and density contrast of the Vela supercluster are considerably larger than previously thought, and a double-core internal structure is claimed. By contrast, the remaining superclusters showed relatively minor structural changes.

Vol. 712
2. Astrophysical processes

The exponential growth of infinitesimal perturbations in the long-term evolution of simulated galaxies

by T. Asano, S. Portegies Zwart 2026, A&A, 712, A233

It is well known that self-gravitating systems of N particles are chaotic. In this work, Asano and Portegies Zwart investigated the consequence of this chaos on galaxy stability and morphology. They carried out calculations using a softened gravitational N-body tree-code, with varying numbers of particles, N, up to 40 million. They ran several simulations, each with different copies of the initial conditions with small perturbations, and determined the Lyapunov timescale, after which the chaos settled. This timescale varies as the square root of N but is independent of the time step. Surprisingly, the timescale for bar instability (1.8 Gyr) is insensitive to these infinitesimal perturbations. Only the bar strength and its further evolution sensitively depend on such perturbations. All the bars eventually buckle, but this occurs over a timescale ranging from approximately 2.8 Gyr to 4.2 Gyr. Smoothing the gravitational force over a larger softening length tends to suppress chaos. Softening prevents the system from behaving as chaotically as a granular system, since the dominant driver of chaos occurs on a scale shorter than the softening length. For a purely stellar galaxy disk of Milky Way size without softening, the chaos timescale is estimated to be shorter than 0.1 Myr.

Vol. 712
6. Interstellar and circumstellar matter

The outer rings of SN,1987A from year 1994 to 2024. Morphology, light curves, and optical to mid-infrared spectra

by S. Rosu, E. Gerville-Reache, S. Thomas, et al. 2026, A&A, 712, A188

The remnant and circumstellar environment of Supernova 1987A in the Large Magellanic Cloud has been the target of intense study since its first moments. The Olympics-like emission nebula that was photoionized by the event consists of two distinct regions. The innermost, equatorial circumstellar rings were hit by the expanding ejecta a few years ago, but the outermost portions -- those from the earlier mass loss stages of the pre-supernova progenitor -- have been more elusive. This paper presents the comprehensive narrow filter photometric evolution of the structure supplemented by recent multi-object JWST near-infrared spectroscopy tracing through recombination analyses of the density and temperature structure of the gas. The ejecta have not yet impacted any portion of the rather homogeneous gas. A remarkable atlas of the images covering the interval from about 3000 to almost 13000 days since the outburst is included.

Vol. 711
10. Planets, planetary systems, and small bodies

Planetary formation tracks on the Hertzsprung--Russell diagram. Visualising the processes of giant planet growth

by Benedikt Gottstein, Gabriel-Dominique Marleau, Christoph Mordasini 2026, A&A, 711, A173

For over a century, the Hertzsprung–Russell (HR) diagram, a representation of the (observational or theoretical) luminosity-temperature relation in stars, has been a cornerstone in understanding stellar evolution, from the formation of stars to their final stages. Motivated by the emerging observational constraints on young forming or recently formed planets, and exploiting the global Bern planet formation and evolution model, Gottstein et al. here extend the HR diagram framework to visualize planet formation and evolution, investigating how gas and solid accretion, cooling and contraction, and orbital migration altogether shape evolutionary tracks under different formation scenarios. The planetary HR diagram shows three branches, each associated with a distinct formation and/or evolution phase. In the “ascending branch,” during which the planet is still deeply embedded in the protoplanetary disk, it gains almost all its new mass and luminosity from solid accretion; this phase is heavily influenced by the accretion mode (planetesimal vs. pebble) and planet migration. In the second phase, the “planetary horizontal branch,” gas accretion becomes disk-limited; the planet detaches from the disk, subsequently contracts, and its surface temperature increases dramatically. Finally, in the “descending branch,” the planet further contracts and cools at constant mass and with only small changes in radius, leading to diagonal tracks with L proportional to T^4. While comparisons with directly imaged planets in this third phase indicate broad agreement with predictions, observationally populating the first two, short-lived branches is more challenging. Further theoretical work is needed to refine the model in the early phases of the evolution (e.g., by including the effect of accretion-shock emission and circumplanetary-disk reprocessing) and enhance its predictive power. Still, this work represents a novel and promising framework for interpreting planetary formation and evolution.

Vol. 711
4. Extragalactic astronomy

A quiescent galaxy in a gas-rich cosmic web node at z∼3

by Weichen Wang, Sebastiano Cantalupo, Marta Galbiati, Andrea Travascio, Antonio Pensabene, et al. 2026, A&A, 711, A84

A massive (Mstar ~ 10^11 solar masses) galaxy is discovered within a large-scale structure (MQN01) at z ≃3.25, containing one of the largest overdensities of galaxies and active galactic nuclei (AGNe). This galaxy has a very low star formation rate (4+6−2 M⊙/yr), placing it more than 1 dex below the star-forming main sequence, and it has no detectable molecular gas. The galaxy is located at the center of a large cool gas reservoir, as traced by bright Lyα and Hα emission, and in a gas reservoir that is kept cool and in a dynamically hot, turbulent state, by an external mechanism that reduces the ability of the gas to condense into a dynamically cold, efficiently star-forming disk. The suggested mechanism can be due to kinetic energy injected by an AGN jet in a nearby galaxy. Star formation rates of high-redshift galaxies could be substantially reduced and maintained at a low level even within gas-rich and overdense environments.

Vol. 711
4. Extragalactic astronomy

Extremely ultraviolet-bright starbursts at the end of cosmic reionization

by R. Marques-Chaves, D. Schaerer, M. Dessauges-Zavadsky, et al. 2026, A&A, 711, A112

J0217–0208 at z = 6.204 (MUV =−23.4) was observed with JWST/NIRCam and NIRSpec. Its spectral energy distribution indicates a young starburst (∼5 Myr) with a stellar mass of a billion solar masses and a high specific star formation rate (∼100 Gyr−1). Its very compact size (r_eff ~ 260 pc) corresponds to stellar mass and star formation rate surface densities ≈100 times higher than those of typical galaxies at comparable redshifts. There is strong nebular emission (high electron density, n_e ≃1000/cm3) and a super-solar oxygen-to-nitrogen ratio (log(N/O)≃−0.30). J0217–0208 shows broad components in several rest-optical emission lines, which indicates powerful ionized outflows. They appear to be heavily obscured (E(B−V)out ≃0.6) despite the nearly dust-free stellar continuum (E(B−V)⋆ = 0.01±0.01). ALMA observations show a massive (Mdust ≃2 × 108 M⊙), extended (∼1.4 kpc), and cold (Tdust ≃25 K) dust reservoir. These findings point to dusty, feedback-driven outflows carrying and pushing dust well beyond the stellar core and likely boosting the observed UV luminosity.

Vol. 710
4. Extragalactic astronomy

You can’t see me: Super-Eddington growth hindering X-ray detection in high-z broad-line active galactic nuclei

by Alessandro Trinca, Alessandro Lupi, Francesco Haardt, Piero Madau 2026, A&A, 710, A299

Massive black holes (MBHs) play an important role in the evolution of galaxies, as active galactic nucleus (AGN) activity impacts the formation of their stellar mass component. Consequently, understanding MBH growth is essential to extragalactic astrophysics. High redshift data from JWST and ALMA have recently sparked interest in MBHs, as several detected systems exhibit masses that are challenging to reconcile with the young age of the early Universe. While a well-defined correlation exists between stellar mass and central black hole mass in lower redshift galaxies, high-redshift AGNs frequently display unusually high (BH-to-stellar) mass ratios. Furthermore, the weak or absent X-ray emission in these systems challenges standard, lower-redshift AGN growth models. This paper revisits the data with alternative models of MBH evolution, finding that lower mass MBHs accreting at super-Eddington ratios would better reconcile the well-established trends between black hole mass and stellar mass. In this scenario, weak X-ray emission is explained by a confined, over-cooled corona within a narrow super-Eddington funnel. These findings suggest that the growth mode of many AGNs at high redshift is markedly different from their lower redshift counterparts.

The figure illustrates that the low BH mass solution (filled red squares) brings these galaxies into closer alignment with the low-redshift scaling relation, whereas traditional interpretations (filled circles) render them as outliers. Moreover, the secondary solution of the models (open red squares) predicts even higher masses, placing them further from the expected relation.

Vol. 710
6. Interstellar and circumstellar matter

Is the nitrogen-rich source PN K4-47 a true planetary nebula?

by T. Steinmetz, T. Kamiński, D. Jones, et al. 2026, A&A, 710, A198

Faced with an object that resembles a planetary nebula but with unusual abundances, the authors present a paradigmatically comprehensive panchromatic attempt -- from centimeter radio through the far UV -- to unravel the nature of K4-47. The star possesses both an extended circumstellar environment and an atomic molecular jet. The authors show the position-velocity plots for several ions and CO (2-1), revealing a strong velocity gradient along the jet. [S II] and H_2 lines, typically indicating shocks, are present only in the extended structure and appear consistent with a working surface of the jet. No central UV source is observed, probably due to enshrouding; however, the ionization of the nebula is consistent with photoionization from a central star (or system), likely a hot white dwarf. The authors find a vast array of molecules, similar to those in luminous red novae (which are likely mergers), with an abundance pattern resembling that of classical novae. Ultimately, the authors' own assessment is a Scottish verdict: they tentatively identify the system as a likely planetary nebulae but point to its singular nature, warranting further investigation at higher angular and spectral resolution.

Vol. 710
10. Planets, planetary systems, and small bodies

Hourly radio variability of PD,70c from time-differential photometry

by Simon Casassus, Miguel Cárcamo, Oriana Domínguez-Jamett, Yuhiko Aoyama, Gabriel-Dominique Marleau, Ondřej Chrenko, Hauyu Baobab Liu, Barbara Ercolano 2026, A&A, 710, L10

Galaxies accrete, stars accrete, and most formation scenarios predict that planets accrete during their first stages as well. PDS 70 is a keystone ~5 Myr-old T Tauri star in Centaurus with a wide gap in its protoplanetary disk. Inside this gap, there are at least two young massive planets that are actively growing: PDS 70b, the first confirmed protoplanet to be directly imaged, and PDS 70c, which has its own "proto-moon" disk and is the subject of this Letter. The team analyzed ALMA observations at ~343 GHz (Band 7) obtained during the last decade and find, for the first time, a significant indication of radio variability of PDS 70c at several timescales (the illustration shows a sudden radio flux rise on 6 December 2017). The authors discuss the potential origin of the radio variability being the interplanetary environment and, especially, the surface shock from planetary accretion, in particular H I free-free, their favored explanations.

Vol. 709
6. Interstellar and circumstellar matter

Planet-forming disks and their environment across regions and time from the full NIR census

by A. Garufi, C. Ginski, M. Benisty, et al. 2026, A&A, 709, A269

Extreme adaptive optics observations in the near infrared (NIR) over the past 15 years have provided high angular resolution images of planet-forming disks that reveal their diverse structure and offer unique clues as to their evolution history. Using this rich dataset, Antonio Garufi and collaborators have compiled the most complete list so far of disk-bearing young stars with NIR high-contrast images, consisting of 268 sources and including 51 targets with no prior publication. Thanks to the large size of their sample, these authors have been able to study subsamples from individual star-forming regions (mainly Taurus, Ophiuchus, Chamaeleon, Lupus, Upper Sco, and Orion), as well as from systems with a different geometry and age. Their analysis reveals very diverse disk and ambient morphologies. Disks in Lupus are bright, in Chamaeleon they are faint, and in Corona Australis and Taurus they are frequently surrounded by ambient emission. In general, disks experience an abrupt increase in IR brightness between 2 and 5 Myr. The emerging picture from this study is one of disk diversity and evolution driven by the connection between the disk and a stochastically evolving local interstellar medium via late infall and interaction with other stars.

Vol. 709
1. Letters to the Editor

First measurement of the Hubble constant from a combined weak-lensing and gravitational-wave standard siren analysis

by Felipe Andrade-Oliveira, David Sanchez-Cid, Danny Laghi, Marcelle Soares-Santos 2026, A&A, 709, L14

The Hubble constant (H₀) quantifies the current rate of cosmic expansion. Although various probes have significantly reduced measurement uncertainty over the years, a conflict remains between results derived from different cosmic scales. New estimates based on alternative methods are therefore critical. In this Letter, the authors present the first combination of cosmological constraints from standard DESI results—incorporating density and shear fields from two-point correlation analyses—with gravitational-wave (GW) constraints from 142 binary coalescence events (sourced from the LVK transient catalog v4.0). Their analysis yields an H₀ estimate of 67.94 km/s/Mpc with 6.4% precision (potentially widening to ~10% when accounting for model uncertainties). Integrating GW data provides independent information for the standard cosmological model, helping to clarify the "Hubble tension." Furthermore, combining weak lensing and GW data improves constraints on matter density (Ω_m), serving as a pilot study for datasets that will only increase in quality and volume.

Vol. 708
7. Stellar structure and evolution

Radio timing constraints on the orbital orientation and component masses of PSR J1455-3330

by D.S. Pillay, V. Venkatraman Krishnan, David J. Champion, et al. 2026, A&A, 708, A213

PSR J1456–3330 is a millisecond pulsar with a long orbital period of about t 2 1/2 months and a continuous observational record spanning several decades. Since it is also included in the Pulsar Timing Array sample, it benefits from a particularly rich archive of multisite, multifrequency, high-timing-resolution observations. PSR J1456–3330’s orbit is so nearly circular that only upper limits have been obtained for apsidal precession and period change. However, in this work, through a detailed solution of the pulsar’s polarization properties and spin down, its geometric parameters were obtained. The viewing effect of proper motion on the projected semimajor axis was also obtained, along with the relativistic time delay known as the Shapiro effect. The combined orbital and rotational solutions produce a superb determination of the orbital inclination and therefore provide masses for both the neutron star and its white dwarf companion through the orbital solution and the change in projected separation. The pulsar’s parallax was directly obtained and is about 1/3 larger than the distance inferred from the dispersion measure, a result that will prove useful for interstellar medium studies. The proper motion yields a relatively low transverse velocity of about 35 km/s. This study presents a step-by-step analysis, notably including parameters that remain below detectability—such as orbital period change, annual orbital parallax, and possible dynamical orbital variations—which are expected to become attainable through continuing scrutiny.

Vol. 707
7. Stellar structure and evolution

A candidate proton cyclotron feature in the ultraluminous X-ray source NGC 4656 ULX-1

by Nelson Cruz-Sanchez, Enzo A. Saavedra, Federico A. Fogantini, Federico García, Jorge A. Combi, Matteo Bachetti, Matteo Imbrogno, Lara Sidoli, Alessio Marino 2026, A&A, 707, L20

UltraLuminous X-ray (ULX) sources are bright X-ray sources in the outskirts of local galaxies, with luminosities in excess of the Eddington rate for an ~10 solar mass black hole. Their nature is puzzling, and initially, they were considered intermediate (100-1000 solar mass) black holes. Even more puzzling was the discovery of X-ray pulsations in several ULXs, indicating the presence of a neutron star. To explain this, beaming and high-magnetic-field models were proposed, which are not necessarily mutually exclusive. In this Letter, Cruz-Sanchez et al. report signs of a high magnetic field B∼(6–7) ×10^14 G in ULX-1 in the barred galaxy NGC4657. This high magnetic field is inferred thanks to a narrow absorption feature in the X-ray spectrum around 3 keV, whose statistical significance has been robustly assessed. This absorption feature is then interpreted as a proton cyclotron resonant scattering feature. This is the second neutron star ULX in which such a feature is found, supporting the connection between magnetars and ULXs.

Vol. 707
9. The Sun and the Heliosphere

Three-dimensional mapping of coronal magnetic field and plasma parameters in a solar flare

by Tatyana Kaltman, Sijie Yu, Gregory D. Fleishman, Daniel F. Ryan 2026, A&A, 707, A158

The study presents reconstructed 3D maps of the magnetic field strength, Alfvén speed, and plasma beta in a solar flare volume based on imaging microwave spectroscopy and stereoscopic soft X-ray data. They reveal a magnetically dominated environment. This diagnostic provides important constraints for modeling magnetic reconnection, which drives flare dynamics, and represents a step toward a realistic 3D characterization of energy release in solar energetic phenomena.