Vol. 690
4. Extragalactic astronomy

Multi-wavelength spectroscopic analysis of the ULX Holmberg II X-1 and its nebula suggests the presence of a heavy black hole accreting from a B-type donor

UltraLuminous X-ray sources (ULXs) are extragalactic X-ray sources brighter than ~5x1039 erg s-1, so that the radiation pressure, in normal conditions, can stop accretion unless the central object has a mass larger than ~40 M⊙. ULXs were discovered in the first X-ray images obtained by the Einstein satellite, back in the 1980s. These sources were indeed interpreted as the result of accretion onto intermediate-mass black holes in binary systems. This framework was revolutionised in 2014 by the discovery of coherent pulsations from one of the ULXs in the star-forming galaxy M82. Since then, the presence of intermediate-mass black holes in ULXs has always been regarded with caution. Beamed emission or a high magnetic field neutron star were often invoked to explain the overwhelming luminosity. In this paper, Reyero Serantes and collaborators carried out a multiwavelength spectral analysis of one of the brightest ULXs, Holmberg II X-1, and its circumstellar nebula. Studying the nebula, which acts as a calorimeter, and the ULX source itself, the authors were not able to find evidence of beaming. All the observational properties were accounted for by a close binary consisting of ≳ 66 M⊙ black hole accreting matter from a ~22 M⊙ B-supergiant companion. Holmberg II X-1 provides one of the first pieces of evidence for the progenitors of gravitational wave black hole binary mergers.

Vol. 687
1. Letter-to-the-Editor

Resonant sub-Neptunes are puffier

Exoplanets whose masses have been measured through transit timing variations (TTVs) have systematically lower densities than those whose masses have been measured with radial velocities (RVs). This discrepancy was initially feared to reflect shortcomings in the TTV mass measurement methods. It was later attributed to either a measurement bias, from RVs struggling to measure masses as low as TTVs can for planetary systems close to a dynamical resonance, or to a true difference in the properties of resonant and nonresonant planets. Leleu et al establish through a careful statistical analysis that the latter is true: that resonant planets, including those measured only through RVs, do have lower densities, and that the density of a planet increases with its distance from a resonance. They show that these observational trends have counterparts in synthetic planet populations, in which the nonresonant planets went through giant collisions that both increased their densities and expanded their orbits.

Vol. 690
9. The Sun and the Heliosphere

The solar beryllium abundance revisited with 3D non-LTE models

The solar chemical composition is a key reference in astronomy, serving as a yardstick against which all other cosmic objects may be compared, and as a test of stellar evolution models. In this study, the authors revisit the beryllium abundance in the solar atmosphere by analyzing the Be II 313.107 nm resonance line in the solar spectrum. This is the first work to employ 3D non-LTE models together with a comprehensive calibration technique to account for missing line and continuous opacity. The authors find an abundance that is 0.17 dex lower than the value commonly adopted since 2004. It is also 0.11 dex lower than the initial beryllium abundance in the protosun, as inferred from the CI chondrite class of meteorites, which suggests that material from the solar convective zone has been mixed into the radiative interior, where beryllium ignites at temperatures of around 3.5 million K. This result places stringent constraints on the, as of yet unknown, physical process responsible for the observed depletion, which in turn opens new avenues for detailed studies of the dynamical processes acting in the solar interior and their impact on the evolution of the Sun and solar twins.

Vol. 686
10. Planets and planetary systems

Into the thick of it: ALMA 0.45 mm observations of HL Tau at a resolution of 2 au

The disk orbiting the pre-main-sequence star HL Tau presents a series of striking bright and dark rings that have been mapped with the ALMA telescope in an already iconic image. The origin of these rings is still under debate, but a favored interpretation is that they originate from the interaction between the disk and its nascent planets. To further characterize the dust in the HL Tau disk, Osmar Guerra-Alvarado and collaborators present new ALMA images with a higher resolution of 2 au at a wavelength of 0.45 mm. At this higher frequency, the emission from the dust is optically thick and therefore highly sensitive to the temperature structure of the dust. These new images reveal an asymmetry in the emission coming from the first ring that is interpreted as a combination of very optically thick emission, a moderately inclined disk, and a large dust scale height. The images, in addition, show an increase in brightness temperature inside the estimated water snow line. This could indicate a pileup of small particles due to a decrease in the drift velocity of the small grains after crossing the water snow line.

Vol. 690
7. Stellar structure and evolution

Asymmetries in asymptotic giant branch stars and their winds. I. From 3D RHD models to synthetic observables

Between JWST and ALMA, the ability to spatially resolve the environments of evolved stars has become almost routine. This paper illustrates the wealth of possible structures that can result from asymptotic giant branch evolution. Combining state-of-the-art hydrodynamic modeling and a suite of radiative transfer models, this study presents synthetic spectral energy distributions and monochromatic images for a range of dust properties and viewing angles for realistic observing constraints of distance and angular resolution. However, these dramatic simulations also demonstrate the requirements for the proper interpretation of observed structures: channel maps and multiwavelength comparisons are essential.

Vol. 686
5. Galactic structure, stellar clusters and populations

22Ne distillation and the cooling sequence of the old metal-rich open cluster NGC 6791

There is a significant correlation between the luminosity of white dwarfs (WDs) and their cooling age, rendering WDs invaluable as cosmic timekeepers. However, the accuracy of this temporal gauge hinges upon precise WD models, particularly the processes governing their energy reservoirs. Recent simulations have unveiled a notable phenomenon: the efficient transport of 22Ne within a CO WD toward its core, resulting in the release of gravitational energy and consequent cooling delays spanning several gigayears. This paper presents pioneering calculations that integrate CO crystallization, phase separation, 22Ne diffusion in the liquid phase, and distillation during CO crystallization. The novel model successfully replicates the faint end of the luminosity function observed in WDs within the old, metal-rich open cluster NGC 6791. A forthcoming comparison with the color-magnitude diagram of NGC 6791 observed by the James Webb Space Telescope (JWST) holds promise for validating the models and refining our understanding of neon distillation efficiency.

Vol. 689
14. Catalogs and data

The VISTA Variables in the Vía Láctea eXtended (VVVX) ESO public survey: Completion of the observations and legacy

The low-latitude regions of the Milky Way are a hostile environment for optical astronomers, owing to the high extinction obscuring the targets of interest. Therefore, it is imperative to resort to large-scale, infrared surveys to unravel the diverse populations and astrophysics that make up the central parts of our Galaxy. With the final release of its vast dataset (several tens of Tb), the VISTA Variables in the Vía Láctea eXtended (VVVX) survey lays a new foundation for studying stellar populations at low latitudes in the infrared JHKs filters. Combining the earlier VVV and its extension (VVVX), 4% of the sky was sampled at a time coverage of 4200 hours over 15 years. This all came to fruition by the release of data for over 1.5 billion point sources, including their proper motions and distances, using well-calibrated distance indicators such as red clump or variable RR Lyrae stars. The resulting unique map allows for a vast range of science returns, from the discovery and characterization of hidden star clusters, to the study of low-mass stars and exoplanets, monitoring of microlensing events, and, last but not least, the eponymous variable stars.

Vol. 686
10. Planets and planetary systems

A 3D picture of moist-convection inhibition in hydrogen-rich atmospheres: Implications for K2-18b

The issue of convective inhibition has received increased attention from a diverse range of planetary research groups around the world as its centrality to understanding the atmospheric dynamics and thermal evolution of the Solar System and exoplanets has become increasingly apparent. So far, researchers have relied primarily on linear stability analysis to draw conclusions, and the applicability of the involved assumptions to real systems has been suspect. This work addresses the major shortcoming of prior works by constructing highly detailed, physically realistic, 3D, cloud-resolving simulations of model planetary atmospheres to (computationally) demonstrate the phenomenon empirically. The insights from these high-fidelity simulations are used to construct self-consistent 1D models, which are then compared against analytical theory. This paper represents a giant leap forward in our understanding of the fundamental fluid dynamics of planetary atmospheres in our Solar System and beyond, both lending credibility to existing studies and elucidating new phenomena.

Vol. 689
5. Galactic structure, stellar clusters and populations

eROSITA narrowband maps at the energies of soft X-ray emission lines

The promise of the eROSITA mission is demonstrated in dramatic fashion with the publication of these soft X-ray emission line half-sky maps. Elucidating the hot phase of the interstellar medium requires observations in the soft X-ray band, below about 2 keV. The mission's most important innovation is its spectral resolution, which permits spectrophotometric mapping of the sky in the emission lines that dominate the cooling of the medium, principally during high-ionization stages of oxygen, in addition to the broadband continuum emission. This paper presents maps of the third and fourth Galactic quadrants, which show strong variations in the O VII/O VI line ratio (a proxy measure of temperature) along the Galactic plane and an asymmetry on the order of ten percent between the northern and southern hemispheres on scales of tens of degrees. Such structuring will be essential information for modeling the effects of stellar feedback, especially superbubbles, and the imbedding of molecular clouds within this medium.

Vol. 686
11. Celestial mechanics and astrometry

A new pulsar timing model for scalar-tensor gravity with applications to PSR J2222-0137 and pulsar-black hole binaries

General relativity has been tested in great detail thanks to relativistic binary pulsars. The double binary pulsar PSR J0737-3039 allowed us to reach a 99.99% accuracy. Despite this, tests of alternative theories of gravity continue (and must continue for the good of science). In this paper, Batrakov and collaborators put forward a new formalism to enable the testing of scalar-tensor theories of gravity (STG). In this class of theories, an additional scalar field is coupled to matter such that the law of gravitation is dependent on velocity and/or location. The authors then apply this new formalism (called DDSTG) to the binary pulsar PSR J2222-0137, slightly improving the existing limits of STG theories. They also discuss the limits that can be reached with an as yet undiscovered neutron star-black hole binary system.

Vol. 688
4. Extragalactic astronomy

MHONGOOSE: A MeerKAT nearby galaxy survey

The timescales for gas consumption by star formation for the inner regions of nearby spiral galaxies are much shorter than a Hubble time, so if these galaxies are to continue forming stars at their current rate, they need an external supply of gas. A possible source of this fresh material is the accretion of cold gas from the galactic halo, a process that could be observed through its HI emission signature. To search for this signature, the MHONGOOSE (MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters) survey is using the MeerKAT interferometer in South Africa to perform ultra-deep HI observations of 30 nearby gas-rich spiral and dwarf galaxies. The goal of the survey is to detect and characterize any low-column density, potentially infalling atomic gas and to probe its link to star formation. While the survey is still ongoing, Erwin de Block and collaborators present an overview of the survey that includes the sample selection, survey design, and observation and reduction procedures. They also present initial results based on the full-depth data of a limited number of galaxies and single-track observations of the complete sample.

Vol. 686
10. Planets and planetary systems

Atmospheric entry and fragmentation of the small asteroid 2024 BX1: Bolide trajectory, orbit, dynamics, light curve, and spectrum

The asteroid 2024 BX1 collided with Earth on January 21, 2024, a few hours after its discovery. In this paper, the authors analyze the fireball associated with the atmospheric entry and fragmentation of this asteroid. They utilize data recorded by the European Fireball Network, particularly its Czech component, and the AllSky7 network. The majority of the data consist of optical, that is to say photographic, and video recordings, though the authors also utilized radiometric and spectral records to determine asteroid properties. Leveraging these datasets and employing a method developed by one of the authors, they successfully pinpointed the trajectory of the bolide and its fragments with a sufficient accuracy to permit the recovery of meteorites. Specifically, they anticipate four fragments with a mass exceeding 100 grams. The asteroid is estimated to have had an initial mass of 140 kilograms and is classified as an aubrite, characterized by its low iron content.

Vol. 688
4. Extragalactic astronomy

A JVLA, LOFAR, e-Merlin, VLBA, and EVN study of RBS 797: can binary supermassive black holes explain the outburst history of the central radio galaxy?

The study of precessing jets has been a small industry since the first radio imaging studies of SS 433. Inspired by that work, the authors present a new take on the dynamics and evolution using a multiwavelength approach, combining X-ray and radio data, to tease out the history of precessing jets launched from a possible binary supermassive black hole. The structures cover an enormous span of time and length scales, indicating different phases of nuclear activity. The outermost structures are some 30-50 Myr old, while the inner kiloparsec shows what the authors interpret as a precessing structure of variable intensity and alignment that they link to the activity of the central engine on timescales as short as months. The radio and X-ray morphologies provide a new signature of the presence of a binary central beast.

Vol. 685
7. Stellar structure and evolution

Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry

Detections of gravitational waves from merging black holes demonstrate that pairs of black holes more massive than 30 solar masses are relatively common in the nearby universe. Yet, the most massive black hole known to date in our Galaxy is 20 solar mass Cyg X1, with all others below or around 10 solar masses. As part of the validation effort for the fourth Gaia data release (DR4), the Gaia consortium identified a 33 solar mass black hole, Gaia BH3, through the astrometric apparent motion of its companion. Radial velocity measurements, from both Gaia and ground-based observatories, confirm the orbit. This detection fills the gap between the masses of previously known Galactic black holes and detections of mergers through gravitational waves. Analysis of an UVES spectrum of the companion shows that its metallicity is [Fe/H]=-2.6, consistent with the hypothesis that a low stellar metallicity helps form more massive black holes. This exciting discovery is no doubt only a harbinger of the detections to be expected in DR4, which will help characterize Galactic black holes as a population.

Vol. 688
8. Stellar atmospheres

3D non-local thermodynamic equilibrium magnesium abundances reveal a distinct halo population

Chemical abundances of stars are the fossil record of their formation environment and can be used to disentangle stellar populations with different origins, especially when they are measured with high precision. One example is the discovery of two major populations in the Milky Way halo, high-Mg stars and low-Mg stars, which are respectively considered to have formed in the Milky Way and in dwarf galaxies, disrupted by and accreted onto our Galaxy.

The authors have employed state-of-the-art modeling of stellar spectra in 3D stellar atmospheres under nonlocal thermodynamic equilibrium (non-LTE) for precise and accurate Mg abundance measurements. They find that the 3D non-LTE analysis significantly improves the consistency of the Mg abundances derived from Mg I and Mg II lines. When the 3D non-LTE analysis is applied to the Mg abundance of Milky Way halo stars in the solar neighborhood, the authors find further subpopulations among the low-Mg stars that also show different kinematics and hence have different origins. This work demonstrates the importance of 3D non-LTE analysis for studying stellar populations in the Milky Way using the chemical abundances of stars.

Vol. 685
4. Extragalactic astronomy

A quasar-galaxy merger at z~6.2: Black hole mass and quasar properties from the NIRSpec spectrum

The authors used JWST NIRSpec to obtain high resolution integral field spectroscopy of the spectacular system composed of a quasar host interacting with two companion galaxies, namely PJ308-21 at z=6.23. The quasar has an inferred black hole mass of 2-3x10^9 Msol and an Eddington ratio of 0.67, which is in line with the population of the most distant quasars. Most notably, the new data reveal the presence of significant outflowing ionized gas, which was detected thanks to an accurate modeling of the [OIII] emission which is blue-shifted by almost 2000 km/s. Such high velocity outflows in early galaxies are predicted by models to play a significant role in regulating black hole and galaxy growth, but we have not been able to detect them unambiguously so far. This study might in fact be the first example of such strong outflows in the epoch of reionization and it confirms the unique capabilities of NIRSpec in allowing detailed studies of quasars at very early times even with modest integration times.

Vol. 687
5. Galactic structure, stellar clusters and populations

Chronology of our Galaxy from Gaia colour--magnitude diagram fitting (ChronoGal). I. The formation and evolution of the thin disc from the Gaia Catalogue of Nearby Stars

Studying the star formation history (SFH) of the Milky Way on a star-to-star basis is still an expensive endeavor. To address this, the authors introduce a new tool for deriving this SFH by fitting color-magnitude diagrams (CMDs) based on Gaia data. After being used to obtain the metallicity and age distributions of the solar neighborhood, the new method was applied to the complete catalog of stars within 100 pc of the Sun, comprising mostly (but not only) thin-disk stars. As a result, the authors find that stars at the solar radius predominantly formed at solar metallicities early on (~11 Gyr ago) until a break occurred and three stellar populations emerged from ``some dramatic event’’ in the Milky Way’s life. This period was then followed by more significant star formation that occurred in a bursty manner. Intriguingly, thick-disk stars were also found in the sample. The results could be independently confirmed with high-precision spectroscopy: the presented CMD fitting technique achieved an accuracy and precision better than 10%, thereby providing an important proof-of-concept for detailed, future studies of the formation history of the Milky Way disk(s).

Vol. 684
5. Galactic structure, stellar clusters and populations

Observed kinematics of the Milky Way nuclear stellar disk region

Zooming into the central regions of the Milky Way unravels a great deal of structures, though often obscured by large extinction. These comprise the Galactic bulge at the kiloparsec scale, down to stellar and gaseous components with a 100 pc extent, as also seen in other galaxies. In past works, a flat, stellar disk with a 150 pc radius was discovered, it appears to cohabit with the dense molecular disk in the inner regions, and it is believed to rotate faster than its surroundings. For the present work, the authors combined the kinematics of red clump stars (to alleviate reddening issues) from the infrared APOGEE and VVV surveys to search for rotation in fields in and outside of the purported stellar disk. A rotational signal — in the sense of eastward and westward motion — of stars of different magnitudes (read, distances) of the nuclear region was indeed detected as expected. However, the team also found a kinematic structure with only one velocity direction present, which they interpret as the missing component being vignetted by the molecular zone, blocking stars behind itself. Thus there appears to be no clear signal of the nuclear stellar disk as a distinct kinematic component, which highlights the need for also a careful analysis of comparison fields, ultimately shedding more light on the mystery of the Galactic central regions.

Vol. 687
6. Interstellar and circumstellar matter

Narrow absorption lines from intervening material in supernovae. I. Measurements and temporal evolution

It has often been noted by historians and sociologists of science that negative or null results remain in desk drawers. This paper stands as a striking counterexample. The authors present a meticulous study of the absorption lines from presumed circumstellar and galactic environments in early supernova spectra. The result is a caution regarding the interpretation of these lines and their variability, as well as a set of recipes for automated detection and measurement. In this age of big data and machine learning, such studies are needed to provide essential, real-world constraints.

Vol. 684
10. Planets and planetary systems

AFM-IR nanospectroscopy of nanoglobule-like particles in Ryugu samples returned by the Hayabusa2 mission

Vibrational spectroscopy is a powerful tool to investigate the chemical structure of organic matter and minerals in meteorites, micrometeorites, and return samples from space missions. "Classic" infrared microspectrocopy (µ-FTIR) is, however, diffraction limited to spatial resolutions of a few microns. Infrared photothermal nanospectrocopy (AFM-IR), which combines the chemical analysis power of infrared spectroscopy and the high spatial resolution of scanning probe microscopy, yields a hundred times better lateral spatial resolution (i.e., approximately a few 10 nm) than µ-FTIR does. This novel technique has already been successfully applied to the organic matter of some meteorites and micrometeorites. For this work, Mathurin et al. performed AFM-IR nanospectroscopy of organic material in samples of the asteroid Ryugu, which returned to Earth by the Hayabusa2 mission in December 2020. This material includes whole-rock particles and insoluble organic matter (IOM) residues extracted from the samples. The authors identified that organic matter in the whole-rock samples is present in two forms, a diffuse phase intermixed with the phyllosilicate matrix and individual organic nanoparticles. The latter ones were identified as nanoglobule-like inclusions, resembling nanoglobules in primitive meteorites. The high lateral resolution allowed infrared spectra of the organic nanoparticles to be recorded for the first time, revealing enhanced carbonyl (C=O) and CH contributions with respect to the diffuse organic component. The results are consistent with scenarios in which nanoglobules are formed from UV or ion irradiation of ices in the outer regions of the protoplanetary disk or in the protosolar cloud.