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

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

by Maurizio Salaris, Simon Blouin, Santi Cassisi, Luigi R. Bedin 2024, A&A, 686, A153 alt

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. 681
10. Planets and planetary systems

Formation of flattened planetesimals by gravitational collapse of rotating pebble clouds

by Sebastian Lorek, Anders Johansen 2024, A&A, 681, A38

In their study, Lorek & Johansen delve into the fascinating process of how a kilometer-sized planetesimal might form from a rotating cloud of pebbles. They used a Monte Carlo method to simulate this complex process and find that when pebble clouds that rotate slowly collapse under their own gravity, they can form shapes such as flattened ellipsoids. This finding provides insights into the unique shapes of celestial bodies, such as the lobes of the contact binary Arrokoth and the bilobed comet 67P/Churyumov-Gerasimenko. The authors further explore how the initial spin of these pebble clouds influences the final shape of the planetesimal. With less angular momentum, the resulting planetesimal is almost spherical. But as the spin increases, the shape becomes more flattened and disk-like. This research suggests that the varied shapes of minor bodies in our Solar System – particularly those in the cold classical Kuiper belt and comets – can reveal a lot about the early shapes of planetesimals formed from these pebble clouds, providing a glimpse into the early stages of our Solar System's formation.

Vol. 686
10. Planets and planetary systems

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

by Jérémy Leconte, Aymeric Spiga, Noé Clément, et al. 2024, A&A, 686, A131

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. 682
6. Interstellar and circumstellar matter

eROSITA studies of the Carina nebula

by Manami Sasaki, Jan Robrade, G.H. Martin Krause, R. Jonathan Knies, Kisetsu Tsuge, Gerd Pühlhofer, Andrew Strong 2024, A&A, 682, A172

Whatever might be wanted to study feedback processes and the symbiosis of stars and interstellar clouds can be found in the Carina nebula. This star-forming region harbors a broad range of massive stars, including the ever-popular luminous blue variable (and multiple interacting wind system) eta Carina. The region and its core clusters have been subject to continuing multiwavelength observations for a few decades, but this paper presents the deepest, spatially broadest X-ray coverage to date by the eROSITA consortium. The results include photometry and imaging of eta Car, spectroscopy of the diffuse gas and the massive stellar members (Wolf-Rayet and OB stars), and a phenomenological model for the internal superbubble generated by the outflow from the OB stars and eta Car. This is a benchmark dataset, comparable to that of the Orion complex, and is certain to remain an essential resource.

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

by A. Batrakov, H. Hu, N. Wex, P.C.C. Freire, V. Venkatraman Krishnan, M. Kramer, Y. J. Guo, L. Guillemot, J. W. McKee, I. Cognard, G. Theureau 2024, A&A, 686, A101 alt

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. 682
7. Stellar structure and evolution

PSR J0210+5845: Ultra-wide binary pulsar with a B6 V main sequence star companion

by E. van der Wateren, C. G. Bassa, G. H. Janssen, I. V. Yanes-Rizo, J. Casares, G. Nelemans, W. B. Stappers, C. M. Tan 2024, A&A, 682, A178

A neutron star forms in the explosion of a massive star as a supernova. When the exploding star is part of a binary system, the binary usually gets disrupted. In favorable conditions (e.g., asymmetries in the supernova explosion), however, the binary system can survive, leaving a newborn neutron star orbiting a massive companion. If the light beam of the radio pulsar associated with the neutron star intercepts our line of sight, we can detect the system. Until recently, only six such binaries had been known, a testament to their rarity. Van der Wateren and colleagues report here on the seventh such system. The orbital period is loosely constrained to ~47 years, with a moderate orbital eccentricity of ~0.5. At variance with all the other known binary systems, the mass of the companion star is low, ~3.6 solar masses, requiring more stringent constraints on the asymmetry of the supernova explosion. The next periastron passage will occur in 2030-2034, but the neutron star will still be too far away from the companion to expect any X-ray emission or perturbation of the radio signal.

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

by P. Spurny, J. Borovicka , L. Shrbeny, M. Hankey, R. Neubert 2024, A&A, 686, A67 alt

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. 682
12. Atomic, molecular, and nuclear data

The UMIST database for astrochemistry 2022

by T. J. Millar, C. Walsh, M. Van de Sande, A. J. Markwick 2024, A&A, 682, A109

Detailed astrochemical models are necessary for interpreting the observations of interstellar and circumstellar molecules. They depend on the values of the rate coefficients of individual chemical reactions, and these need to be continuously updated to reflect advances in experimental and theoretical estimates. In this volume, Tom Millar and collaborators present the sixth release of the UMIST Database for Astrochemistry, one of most widely used databases of gas-phase chemical reactions. Compared to the previous release from ten years ago, the new update increases the number of reactions by over 40% and the number of species by over 55%. Making use of it, Millar et al. investigate the gas-phase chemistries appropriate to the dark cloud TMC-1 and the circumstellar envelope IRC+10216, and identify the successes and failures of gas-phase-only models.

Vol. 685
7. Stellar structure and evolution

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

by Gaia Collaboration: P. Panuzzo, T. Mazeh, F. Arenou, B. Holl, E. Caffau, A. Jorissen, C. Babusiaux, P. Gavras, J. Sahlmann, U. Bastian, Ł. Wyrzykowski, L. Eyer, et al. 2024, A&A, 685, L2 alt

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. 682
10. Planets and planetary systems

Secular change in the spin states of asteroids due to radiation and gravitation torques. New detections and updates of the YORP effect

by J. Durech, D. Vokrouhlicky, P. Pravec, et al. 2024, A&A, 682, A93

The motion of near-Earth asteroids is significantly influenced by non-gravitational forces, specifically thermal torques resulting from the proximity to the Sun. These effects are known as the Yarkovsky effect for orbital dynamics and the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect when impacting the object's rotation. The latter effect is stronger when the object's shape is irregular, a common characteristic of small bodies. Detecting the YORP effect involves identifying a secular acceleration in rotation, a phenomenon that has been observed in fewer than ten objects. In this study, the authors detect the YORP effect in two new asteroids, refine measurements for two others, and highlight an asteroid that exhibits a deceleration in rotation, the opposite of what is usually seen. To achieve this, they analyzed the light curves of these objects, obtained through photometric observations from various observatories. Since these light curves depend on the rotation and shape of the objects, their analysis requires combining rotational and shape models. These new data could mark a significant advancement in our understanding of the YORP effect, which remains largely unconstrained.

Vol. 685
4. Extragalactic astronomy

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

by Federica Loiacono, Roberto Decarli, Marco Mignoli, Emanuele Paolo Farina, et al. 2024, A&A, 684, A121

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. 681
2. Astrophysical processes

A MeerKAT view of the double pulsar eclipses. Geodetic precession of pulsar B and system geometry

by M.E. Lower, M. Kramer, R.M. Shannon, R.P. Breton, N. Wex, et al. 2024, A&A, 681, A26

PSR J0737−3039A/B is a unique double neutron star binary system in which both neutron stars are detected as radio pulsars. In addition, the system is viewed almost edge on (90.5 deg), allowing us to study the interaction of a radio signal with the neutron star magnetosphere. A variety of general relativity tests could be carried out on this system. In this paper, Lower and collaborators exploit MeerKAT radio observations of this binary pulsar. Thanks to the improved timing precision, they were able to model the emission geometry of the pulsar, which is now out of our line of sight, predicting that we will start reobserving pulsar B in early 2035.

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

Observed kinematics of the Milky Way nuclear stellar disk region

by M. Zoccali, A. Rojas-Arriagada, E. Valenti, R. Contreras Ramos, A. Valenzuela-Navarro, C. Salvo-Guajardo 2024, A&A, 684, A214

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. 682
9. The Sun and the Heliosphere

A comparative study of two X2.2 and X9.3 solar flares observed with HARPS-N. Reconciling Sun-as-a-star spectroscopy and high-spatial resolution solar observations in the context of the solar-stellar connection

by A.G.M. Pietrow, M. Cretignier, M.K. Druett, et al. 2024, A&A, 682, A46

By virtue of its proximity, the Sun has always been observed and treated differently than the more distant exemplars. However, with advances in detectors and spectrographs, it has become increasingly more feasible to obtain a similar level of information for more remote systems. The "Sun as a star" approach of this study treated observations of two X-class flares (those with fluences of Nx1031 erg) observed with the same instrumentation that has been used for TESS and Kepler spectroscopy for which the flares have been up to 100 times the dissecting total energy of the solar cases. The HARPS-N spectra were sampled with a five-minute cadence at a resolution of 105, precisely as for the stellar observations. The authors used a range of line probes of the events, dissecting the line profile and intensity variations to create similar indices to the stellar case for, for instance, the Balmer, He I (no variations detected), Na In (line asymmetries noted but otherwise no detected variability), and Ca II (distinct flare-strength-dependent variations with a different duration) lines and radial velocity measurements of the profile to a stability of better than 10 m/s. The authors discuss other features of the events that can be targeted for this sort of comparative measurement. The paper is a pioneering illustration of a new approach to unraveling the activity-related phenomena on solar-type stars.

Vol. 684
10. Planets and planetary systems

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

by Jérémie Mathurin, Laure Bejach, Emmanuel Dartois, et al. 2024, A&A, 684, A198 alt

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.

Vol. 681
6. Interstellar and circumstellar matter

S 308 and other X-ray emitting bubbles around Wolf-Rayet stars

by Francesco Camilloni, Werner Becker, and Manami Sasaki 2024, A&A, 681, A122 alt

The role of massive stars in shaping their local environment is a continuing theme in the study of galactic star formation. Including stellar feedback in galactic models through the combined effects of wind mass loss and photoionization is physically challenging and, still, hypothetical, or at best loosely based on phenomenological prescriptions. Supernovae also bespeak the need of understanding the circumstellar medium to interpret the development of the energetics and spectra of some stripped systems. The eROSITA study of S 308, one of the Wolf-Rayet stars that have been targeted for X-ray and UV observations over the years, is the focus of this paper, but the sample includes other wind-shaped circumstellar environments (e.g., wind-blown bubbles). The energetics and dynamics are modeled using high-quality X-ray multiband imaging. This is a paradigmatic study and a harbinger of what is to come from the current sky-surveying high-energy satellites combined with ground-based data. Modelers of galactic evolution, supernovae, and gravitational wave progenitors will find much of interest in this paper.

Vol. 684
10. Planets and planetary systems

Thermal tides in neutrally stratified atmospheres: Revisiting the Earth's Precambrian rotational equilibrium

by Mohammad Farhat, Pierre Auclair-Desrotour, Gwenaël Boué, Russell Deitrick, Jacques Laskar 2024, A&A, 684, A49

The climatic history of the Earth is scarcely constrained. A key element to model is the length of the day, which was likely shorter in the past than it is now. The variation in the length of the day is a consequence of the Earth's spin variation, resulting from different torques involving the Earth, the Sun, the Moon, the atmosphere, among others.

In this study, the authors revisit the evolution of the Earth's length of day since the Precambrian, challenging the hypothesis that the length of the day could have stabilized at 21 hours during that time. This study follows a previous one by the same team in which they revisited the evolution of the Earth-Moon system. Here, the authors use their previous results in addition to a new ab initio model of atmospheric tides to simulate the variation of the length of the day over the ages. This tidal model, which is suitable for rocky planets, describes a neutrally stratified atmosphere and considers dissipative processes with Newtonian cooling and diffusive processes in the planetary boundary layer.

The Lamb resonance in the atmosphere, a frequency overlap between atmospheric free oscillations and the semidiurnal forcing, was initially thought to be responsible for the stabilization of the length of the day during the Precambrian. In this study, the authors show that this resonance might have occurred but later in the Phanerozoic, and with insufficient strength to stabilize the length of the day.

In addition to its application to Earth, the authors propose a new model that could be applied to rocky exoplanets.

Vol. 681
3. Cosmology (including clusters of galaxies)

ACT-DR5 Sunyaev-Zel’dovich clusters: Weak lensing mass calibration with KiDS

by N. C. Robertson, C. Sifon, M. Asgari et al. 2024, A&A, 681, A87 alt

Galaxy clusters trace the large-scale matter distribution in the Universe. It is fundamental to know precisely their number density, masses, baryon content, and evolution with redshift, to reveal the growth of structure and constrain the amplitude of matter fluctuations, the matter density, and the dark energy equation of state. They are important diagnostics of any cosmological model, and can test the standard LCDM model. The present work has measured the mass of 157 clusters within the Kilo Degree Survey (KiDS), with weak gravitational lensing (WL). These clusters were also clearly detected by the thermal Sunyaev-Zel’dovich (SZ) effect with the Atacama Cosmology Telescope (ACT). It is expected that the WL masses are higher than the SZ ones, since they are sensitive to the total mass, independent of the cluster dynamical state. The SZ cluster mass estimate MSZ, calibrated using X-ray observations, is biased with MSZ/MWL = (1 - bSZ) = 0.65 ± 0.05. This bias value bSZ does not depend on the cluster mass. This bias value is compatible with the predictions from the Planck cosmic microwave background LCDM cosmological model. This agreement is in contrast with previous measurements, as shown in the figure.

Vol. 683
2. Astrophysical processes

The nature of the X-ray filaments around bow shock pulsar wind nebulae

by Barbara Olmi, Elena Amato, Rino Bandiera, Pasquale Blasi 2024, A&A, 683, L1

When a pulsar moves across the interstellar medium, it is likely to release energetic leptons. In some cases, they become visible as a linear, filamentary structure emitting X-rays. These structures could be as long as 15 pc, as in the Lighthouse Nebula, but with a very small cross section (1-10% of the length). In this Letter, Olmi and collaborators suggest that the X-ray emission of these filaments can be explained by the excitation of a non-resonant instability. For this to happen, tight conditions on the collimation of the released leptons must be satisfied, providing one of the best diagnostics to study the escape of particles from evolved pulsar wind nebulae. This is key to understanding cosmic ray transport around sources and it may link these systems to TeV halos.

Vol. 680
4. Extragalactic astronomy

An almost dark galaxy with the mass of the Small Magellanic Cloud

by M. Montes, I. Trujillo, A. Karunakaran, R. Infante-Sainz, K. Spekkens, G. Golini, M. Beasley, M. Cebrián, N. Chamba, M. D'Onofrio, L. Kelvin, J. Román 2023, A&A, 680, A15 alt

Some low surface brightness galaxies are so weak that they elude detection in the Sloan Digital Sky Survey. These “almost dark galaxies” have a red surface brightness at the center of more than 26 mag/arcsec2, corresponding to a stellar surface density of less than a few solar masses per pc^2. This small amount of stars implies that no stellar feedback has modified its dark matter halo distribution. These objects thus offer an interesting possibility to constrain the microphysical properties of dark matter. The authors report on the serendipitous discovery with deep imaging of Nube, an almost dark galaxy located at a distance of 107 Mpc. From observations with the 100m Green Bank Telescope, they derive a dark halo mass of 2.6 10^{10} Msun. Ultra-deep multi-band observations with the 10.4m Gran Telescopio Canarias favor an age of ∼ 10 Gyr and a metallicity of [Fe/H]∼ −1.1. With a stellar mass of ∼ 4×10^8 Msun and a half-mass radius of Re = 6.9 kpc, Nube is the most massive and extended object of its kind discovered so far. The galaxy is ten times fainter and has an effective radius three times larger than typical ultra-diffuse galaxies with similar stellar masses. Current cosmological simulations within the cold dark matter scenario, including baryonic feedback, do not reproduce the structural properties of Nube. They are, however, compatible with a fuzzy dark matter scenario in which the dark matter particles are ultra-light axions with a mass of 0.8 10^{-23} eV, in agreement with other astrophysical constraints.