Vol. 677
6. Interstellar and circumstellar matter

The extremely sharp transition between molecular and ionized gas in the Horsehead nebula

Massive stars erode and photo-evaporate their surrounding cloud with their strong UV radiation field, which can either trigger or quench the formation of new stars. Understanding this star-cloud interaction requires studying in detail the structure of a cloud exposed to a UV radiation field, which is expected to contain an intermediate zone of neutral atomic gas between the molecular cloud and the surrounding ionized component. To study this transition zone, Claudio Hernández-Vera and collaborators observed, with a resolution of ~200 au, the iconic Horsehead Nebula, which is viewed nearly edge-on with respect to the illuminating source σ Ori. The ALMA observations presented by these authors reveal a very sharp transition between the molecular and ionized components, which suggests that the atomic layer in the nebula is very thin (< 650 au). The new ALMA observations also reveal a web of molecular gas filaments and the presence of a steep density gradient at the cloud edge that can be explained by a stationary isobaric photo-dissociation model, although dynamical effects cannot be completely ruled out. Even higher angular observations will be needed to unveil their role.

Vol. 673
3. Cosmology (including clusters of galaxies)

A state-of-the-art strong-lensing model of MACS J0416.1-2403 with the largest sample of spectroscopic multiple images

The paper presents a new strong lensing model for the cluster MACS J0416.1-2403 using new data coming from HST multiband imaging and new VLT MUSE spectroscopy, as well as Chandra X-ray data on the intracluster hot-gas distribution and stellar kinematic information for 64 cluster galaxies. In particular, the new model exploits as many as 237 multiple images with spectroscopic redshifts, and this is the largest sample of multiply imaged sources available in any cluster to date. The new model obtained is better suited to accurately reproduce the positions, shapes, and fluxes of the observed multiple images, for example it can reproduce the position with an amazing accuracy of 0.4 arcsec +. The authors are making their model available to the community through the newly developed Strong Lensing Online Tool (SLOT), which allows users to obtain magnification values at any position in the field.

Vol. 677
4. Extragalactic astronomy

JADES NIRSpec Spectroscopy of GN-z11: Lyman-alpha emission and possible enhanced nitrogen abundance in a z = 10.60 luminous galaxy

This paper presents NIRSpec JWST spectroscopic observations of GN-z11, one of the most distant and luminous confirmed galaxies at z>10. At these high redshifts, the intergalactic medium is expected to be highly neutral, and as such it should prevent the transmission of the Ly alpha emission line from any galaxy. However, spatially extended Ly alpha emission is observed, and the authors argue that the visibility of the line might be due to a powerful outflow present in this source. The NIRSpec spectrum of GN-z11 is remarkably rich with emission lines, enabling the study of interstellar medium properties at z > 10. One possible explanation of the observed high ionization lines is that the object is powered by an active galactic nucleus. Alternatively, if this galaxy is indeed powered by star formation, the Balmer emission lines and blue UV continuum suggest a current star formation rate of ∼ 30 M_sun/year and low dust attenuation. Overall, this work is a remarkable example of how JWST is enabling detailed physical and chemical properties to be derived for galaxies formed within the first few hundred million years after the Big Bang.

Vol. 673
4. Extragalactic astronomy

Origin of the diffuse 4-8 keV emission in M82

The starburst galaxy M82 is the poster child for stellar feedback in a hyperactive galactic nucleus. This study uses hard diffuse line and continuum X-ray emission to tease out the details of the interactions between the different actors in this complex environment. They find that contributions from nonthermal inverse Compton emission is a significant fraction of the X-ray emission, along with hot thermal gas. An important result is that the strongest supernova remnants are embedded within giant molecular clouds, and hence the future is bright. Another notable result is that this hot gas is connected with the chimney outflow from the supernova-heated starburst zone.

Vol. 677
10. Planets and planetary systems

Do all gaps in protoplanetary discs host planets?

The Atacama Large Millimeter/submillimeter Array (ALMA) has unveiled numerous protoplanetary disks with a multi-ringed structure, indicating the presence of gaps. Traditionally, these gaps were thought to host young, growing planets that interact with the disk.

This study challenges this assumption by simulating the evolution of such systems over 100 million years. The authors employed N-body simulations, initially assuming the presence of three to seven planets. They took into account pebble and gas accretion, migration, damping of eccentricities and inclinations, disk-planet interactions, and disk evolution, while exploring different values of viscosity.

Interestingly, the results obtained from these simulations are inconsistent with the observed planetary systems. Systems initially containing more than four young planets became unstable, while those with three or four planets ended up with final eccentricities much smaller than what was observed. Additionally, these simulations show the presence of giant planets that orbit too far from the central star.

Consequently, this leads to the conclusion that alternative explanations for the observed gaps are required. They could include pebble growth near condensation zones, zonal flows, self-induced dust traps, or gravitational instabilities.

Vol. 674
1. Letters to the Editor

Vacuum polarization alters the spectra of accreting X-ray pulsars

The X-ray spectrum of high mass X-ray binaries (HMXBs) sometimes shows a feature around an energy of 10 keV. Absorption features in the HMXB spectra are often due to cyclotron absorption in the high neutron star magnetic field. However, in addition to the "10 keV" feature, in many of these sources we do already see the cyclotron feature at higher energies, ruling out this explanation. Other authors suggested that this 10 keV feature might be caused by calibration issues in the X-ray detectors. It is known that the high neutron star magnetic field alters the propagation of X-ray photons, making the vacuum birefringent. At variance with common belief, in this Letter, Sokolova-Lapa and collaborators show that the effects of vacuum polarization can play a role in the propagation of X-rays within the hot plasma accreting onto the neutron star, suggesting an intriguing new explanation for the 10 keV feature.

Vol. 677
8. Stellar atmospheres

Does the i-process operate at nearly solar metallicity?

This paper presents a high-resolution spectroscopic analysis of 15 s-process rich candidates with close-to-solar metallicity. The authors found that two stars of their sample are enriched in both r- and s-process elements, with an abundance pattern compatible with i-process nucleosynthesis which is efficient in polluting asymptotic giant branch (AGB) star companions. If the i process is indeed active in objects such as AGB stars with a close-to-solar metallicity, an important impact on nucleosynthesis and galactic chemical evolution can be expected. In particular, the i-process contribution to our Solar System would need to be revisited.

Vol. 674
9. The Sun and the Heliosphere

Accounting for differential rotation in calculations of the Sun’s angular momentum-loss rate

The rotation rate of solar-type stars provides a clock for estimating their age and that of any cluster in which they reside, separate from other nuclear evolutionary chronometers. However, this requires careful calibration because the rate could depend on the differential rotation of the star and its magnetic dynamo cycle. This study aims to provide a sensitivity study of the effect of these assumptions on the spindown torque for a particular, rather nearby star for which the braking torque on the solid body rotation shifts in latitude during the magnetic cycle. The authors used a semi-empirical approach for the solar wind, reconstructing the topology of the field from 15 years of surface magnetograms and computing the braking variation over a complete solar cycle. They find that, for at least this slow rotator, the open field line outflows in the solar wind are not strongly connected to or influenced by the differential rotation. The implications for spindown of sola! r-type and low-mass stars, and possible modifications of the Skumanich law, are extremely interesting and will have a resonance in the "Sun as a star" community.

Vol. 676
6. Insterstallar and circumstellar matter

A detailed look at the thermal and nonthermal X-ray emission from the Vela supernova remnant with SRG/eROSITA

The Vela supernova remnant, along with its embedded pulsar (and associated nebula), is fundamental to our understanding of how supernovae affect their surroundings. This study exploits of the broad field of view of eROSITA -- the authors produced a 15 deg x 15 deg mosaicked field for this study -- and its high energy resolution -- about 5% at 1.5 keV -- to dissect the abundance distribution of matter within and surrounding the remnant; at a distance of only 0.3 kpc, 0.1 arcmin binning yields a spatial resolution of about 0.04 pc in the range 0.2 - 2.3 keV. Individual regions as small as 0.1 deg are spectrally resolved, permitting the abundance variation for N, O, Ne, Mg, Si, and Fe to be mapped. This high resolution also allows the thermal and chemical structure of individual high-speed fragments, called shrapnel, and the diffuse, shocked matter within the remnant to be determined. The authors compare several emission models, adopting a two-temperature thermal plasma (around 0.2 and 0.6 keV) and a nonthermal source. The authors also provide an extensive comparison with multiwavelength observations and include discussions of the pulsar wind nebula and the complex environment in which the supernova remnant is situated. This study provides a sort of "prolegomena," a benchmark dataset for any modeling of the interaction of supernova ejecta with the surrounding interstellar medium.

Vol. 673
6. Interstellar and circumstellar matter

A spectacular jet from the bright 244-440 Orion proplyd: The MUSE NFM view

The life of a protostar is brief and dynamic. It is embedded in a disk and drives outflows that impact its nascent surrounding cloud. This study presents a detailed unfolding of the very extended jet emerging from a low mass protostellar disk in the Orion nebula. Although the jet has been studied in the past as a one-sided structure, the authors have succeeded in finding the oppositely oriented component. With this, using spatially resolved spectroscopy, they determine the density and velocity structure of the outflow, from which they place limits on the rate of mass loss and momentum transfer. Most notable is their discovery and analysis of its corkscrew structure, which they interpret as arising from the orbital motion of a component in a low mass binary system, the first such system uncovered from its jet.

Vol. 675
7. Stellar structure and evolution

Wide binaries demonstrate the consistency of rotational evolution between open cluster and field stars

Gyrochronology represents a valuable technique for dating the age of main sequence stars by capitalizing on the observed reduction in stellar rotation over time. By utilizing rotation periods and a mass proxy such as color, gyrochronology enables the estimation of ages for late-type main sequence stars. While its successful application in open clusters is well established, establishing a connection to field stars has posed a persistent challenge.

In their study, the researchers delved into the rotation rates of wide binaries, which represent the smallest possible open clusters and thus serve as informative intermediaries between clusters and field stars. Remarkably, despite the wide binaries' diverse nature, they conform to the same rotational decline pattern observed in open clusters. Moreover, the authors determined that age estimates based on rotation yielded consistent results for both components of the wide binaries. These findings strongly indicate that cluster and field stars undergo the same rotational spindown process, thereby establishing gyrochronology as a viable method for determining the ages of field stars.

Vol. 673
14. Catalogs and data systems

VISIONS: The VISTA Star Formation Atlas. Papers I and II

VISIONS is a large ESO public survey of nearby (d < 500 pc), star-forming molecular cloud complexes in the constellations of Chamaeleon, Corona Australis, Lupus, Ophiuchus, and Orion. This is another large impact southern survey carried out with VISTA/VIRCAM in the near-infrared. It consists of a shallow wide sub-survey comprising six epochs, plus a set of dedicated deep observations of the areas with the largest amounts of dust extinction, and a control sub-survey with observations of low dust extinction regions. VISIONS measures proper motions with a precision of <1 mas/yr, which is important to securely identify embedded and low-mass objects in these dusty regions that are beyond Gaia’s reach. VISIONS is a treasure trove to improve our understanding of the process of star formation. This survey would enable different studies of the formation and evolution of embedded clusters, the kinematics of embedded objects down to substellar masses, the characterization of YSOs, the initial mass function, the 3D dust maps to characterize the dynamics of the local interstellar medium, as well as the dust properties and the Galactic reddening law.

Vol. 675
6. Interstellar and circumstellar matter

The dark neutral medium is (mostly) molecular hydrogen

The transition regime between the atomic and molecular phases of the interstellar medium is not well traced by either HI or CO. It is often referred to as the dark neutral medium (DNM), and its composition has so far remained poorly constrained. To investigate the DNM composition, Liszt and Gerin have combined ALMA absorption HCO⁺(1--0) observations with IRAM 30m CO(1--0) emission data toward about 30 compact extragalactic continuum sources seen with low extinction toward the Galactic anticenter. Using the HCO⁺ absorption intensities as a proxy for the H₂ column densities and prior determinations of the column densities of the DNM, these authors find that the mean DNM and molecular gas column densities are comparable and vary compatibly by factors of 50-100 over the observed sample, while the column density of HI varies by only factors of 2-3. A detailed analysis of the different gas components suggests that although the studied sightlines sample primarily atomic gas with a mean H₂ fraction ≈ 1/3, the DNM is almost entirely molecular.

Vol. 672
6. Insterstellar and circumstellar matter

Galactic population synthesis of radioactive nucleosynthesis ejecta

The projected (line-of-sight) distributions of the short-lived radioactive nuclei ²⁶Al and ⁶⁰Fe have been detected in the Galactic interstellar medium with gamma-ray satellite observations from COMPTEL and INTEGRAL/SPI. This paper is a clever global simulation of those maps, using a range of formal representations for the spatial distribution of star formation (combinations of radial exponential profiles and spiral structure imposed on a disk) and with careful rendering of the underlying sources and their stochastic variability. While this is not ab initio modeling, the details of the in-out processes have been individually evaluated before being combined with a population synthesis at the Galactic level. A particularly attractive feature of this approach is that the lifetimes of the two species are so short that large-scale redistribution from macroscale dynamics within the interstellar medium is not a major concern. Therefore, unlike pre-solar grains, the simulations can exploit the "snapshot" approach to obtain a statistically consistent sky distribution. The authors find that a spiral-dominated disk (with a weak exponential radial dependence) with a scale height of about 0.7 kpc and a moderate star formation rate, about 8 M_sun yr, provides the best fit. This approach should encourage more detailed modeling and communication between the different specialties regarding Galactic chemical and population evolution.

Vol. 675
1. Letters

The mass of gas giant planets: Is Saturn a failed gas giant?

Standard core-accretion models of giant planets invoke an initial formation of a heavy-element core, followed by gradual solid and gas accretion that continues until runaway gas accretion starts. Historically, these models have been challenged by timescale issues (given the need for massive gas accretion before disk gas dissipation occurs after ~ 3 Myr), but the problem is now alleviated by the possibility that core formation is accelerated by pebble accretion. In addition, in these traditional models, runaway accretion occurs at a typical planetary mass of ∼ 30 M⊕. Recent results on the composition and internal structure of Jupiter and Saturn, particularly the existence of "fuzzy" cores, point to an intermediate phase of heavy-element accretion; this has been shown to delay runaway gas accretion by prolonging planetary cooling. Based on these considerations, Helled proposes that for a typical heavy-element accretion rate of 10^-5 M ⊕/yr lasting a couple of megayears, the runaway phase may start only at ~100 M⊕. This scenario, in which Saturn appears as a "failed giant planet," naturally explains its much smaller mass compared to Jupiter's (95 M⊕ vs 318 M⊕), without needing to resort to a fine-tuning of the timing of the disappearance of the gas disk, as well as its greater enrichment in heavy elements. It also provides a plausible interpretation of the mass versus radius relation observed in giant exoplanets, which presents a transition near a mass of ~100 M⊕. In this view, this transition would delimit failed giants from actual gas giants. Many questions remain unresolved, such as the origin (pebbles, planetesimals, or both?) and precise value of the heavy-element accretion rate, or the existence of a diversity of metallicities in exoplanets not correlated with their mass, but the proposed concept provides a new look on the fundamental topic of giant (exo)planet formation.

Vol. 672
3. Cosmology

JWST’s PEARLS: A new lens model for ACT-CL J0102-4915, “El Gordo,” and the first red supergiant star at cosmological distances discovered by JWST

This paper presents new strong-lensing mass modeling of the high-z (z=0.87) El Gordo massive cluster using recently obtained JWST imaging data. In addition, it introduces new candidates for caustic-crossing events that demonstrate the power of the JWST.

The number of lenses is enhanced by the presence of an overdensity of background galaxies (at a redshift of z=4.3). The large number of new multiple image candidates revealed by the JWST data helps improve the mass modeling of this cluster. This cluster is known for its high mass, which is difficult to explain in the framework of Lambda cold dark matter cosmology so early in cosmic time.

The most likely explanation for one of the caustic-crossing events is a red supergiant star that the authors nicknamed "Quyllur," the Quechua term for star. If confirmed at the proposed redshift of z=2.18, this would be the first red supergiant found at cosmological distances.

Vol. 675
3. Cosmology

Euclid preparation. XXVIII. Forecasts for ten different higher-order weak lensing statistics

Now that Euclid has been launched and while we are awaiting the first results, it is a good time to develop a practical sense of what insights the forthcoming data can provide. This paper presents a superb tutorial comparing the information derived from different tools, ranging from classical (e.g., two-point correlation functions and Gaussian statistics) to topological measures (e.g., Minkowski functionals and Betti numbers) using realistic cosmological models that are constrained by Planck data.

Vol. 672
4. Extragalactic astronomy

Diffusion of cosmic-ray electrons in M 51 observed with LOFAR at 54MHz

Cosmic rays are accelerated to relativistic energies in supernova remnants. Cosmic ray electrons (CREs) then emit synchrotron radiation while traveling and spiraling along magnetic field lines. The synchrotron radio continuum is therefore a tracer of star formation in galaxy disks, as confirmed by its tight correlation with far-infrared flux. In this work, the authors exploit this correlation to estimate the length that the CREs are transported during their lifetime in the nearby galaxy M 51, as well as for cosmic-ray energy between 1 and 10 GeV. They use new ultra-low frequency observations from the LOw Frequency ARay (LOFAR) at 54 MHz and ancillary data between 144 and 8350 MHz. Because of the CRE diffusion length, the radio maps are smoothed in comparison to the distribution of the star formation. The authors then convolve the map of the star formation rate (SFR) surface density to fit the radio map. The best-fitting convolution length yields a measure of the CRE transport length. They find that the CRE transport length increases at low frequencies, as is expected since the CREs have longer lifetimes. The phenomenon is similar to a diffusion, with a coefficient independent of the CRE energy in the range considered.

Vol. 674
10. Planets and planetary systems

Spin of protoplanets generated by pebble accretion: Influences of protoplanet-induced gas flow

A new mechanism to generate the spin of protoplanets’ pebble accretion consists of the accretion of centimeter- to meter-sized particles into planetesimals in a protoplanetary disk, aided by aerodynamic drag from the gas present in the disk. This mechanism has been studied for over a decade, as it is efficient enough to build protoplanets of 10 Earth masses. The present study, by Takaoka et al., addresses the issue of protoplanetary spin. To do so, the authors performed two types of numerical simulations. The first one involved hydrodynamical simulations aimed at modeling the gas flow around the planet. They then used this gas flow as input for simulations of the three-dimensional orbits of the pebbles accreting onto the protoplanet. These simulations show that the gas envelope surrounding the protoplanet drags the accreting pebbles, resulting in an acceleration of spin regardless of planetary mass, pebble size, and the headwind speed of the gas. However, spin increases with a thicker envelope and an initial prograde spin of the protoplanet. These results suggest that the formation of the Earth by pebble accretion could have facilitated the formation of the Moon through a giant impact on a fast-spinning proto-Earth.

Vol. 671
6. Interstellar and circumstellar matter

A plausible molecular mechanism to explain near-infrared continuum emission: Recurrent fluorescence

The authors modeled neutral carbon clusters ranging about 24 to 60 atoms, which would be expected in regions of star formation, especially in reflection nebulae. They used a variety of structures and sampled the spectra using a Monte Carlo procedure to simulate the sort of mix that might be encountered in interstellar environments. The electronic and vibrational states were strongly coupled, so interactive excitations have resulted in a broad emission spectrum of the repeated fluorescence. Irradiated by a 20,000 K blackbody spectrum, taken as a proxy representation for a star that is not too hot, the emitted spectrum from all clusters resembles a blackbody with a color temperature of about 750 K. The emission bands are shown for the 1 to 10 micron band, which will be important for JWST and ground-based spectrophotometry. They also show the computed electronic absorption spectra in the ultraviolet (UV), which will be important for further modeling of star-forming regions. To obtain the spectrum and surface brightness observed in NGC 7023 — their paradigmatic reflection nebula — requires around 0.1 to 1% of the carbon to be sequestered in neutral clusters with between 24 and 60 atoms. This connects the UV absorption, extended red emission, and the near-infrared continuum emission.