Vol. 664
10. Planets and planetary systems

A scaled-up planetary system around a supernova progenitor

by V. Squicciarini, R. Gratton, M. Janson, et al. 2022, A&A, 664, A9 alt

The vast majority of known exoplanets were found around stars with a mass < 2.3 M_sun, and the sharp decrease in the occurrence frequency above 2 M_sun is interpreted — within the core accretion model in which planets form through core growth on timescales of a few million years — as due to the more rapid dispersal of the protoplanetary disk for heavier stellar masses. By targeting 85 B-stars (mass > 2.4 M_sun) in the young (5-20) Scorpus-Centaurus association, using SPHERE/VLT high-contrast imaging, the BEAST project specifically addresses the presence of planets around intermediate and massive stars. After the discovery by Janson et al. (2021) of a 10.9 M_J planet around the stellar binary b Cen with a total mass of 6-10 M_sun, Squicciarini et al. here report the detection of a planetary system around the most massive star to date. μ2 Sco, which the authors establish to be 20 ± 4 Myr old and have a 9.1 ± 0.3 M_sun mass (therefore likely ending its life as a supernova in the future), appears to harbor a 14.4 M_Jup companion at a projected separation and 290 au, and probably a second similar object (18.5 M_Jup) at 21 au. Although the companions are slightly above the planet/brown dwarf limit at 13 M_J, their mass ratio to the star (1.5-1.9 x 10-3) compares well with that of Jupiter (0.95 x 10-3), making the system arguably a scaled-up version of the Solar System. With already two or three planet-like objects detected around the first 25 stars observed within BEAST, the population of planets around massive stars seems richer than anticipated, but such a conclusion will have to be confirmed upon completion of the program. Yet, the formation of such objects remains uncertain. Their mass ratio and location at separations consistent with the expected size of the protoplanetary disks would argue for a bottom-up, planet-like, core accretion formation mode, but this mechanism faces a severe timescale problem. On the other hand, rapid formation (~10^4 year) from top-down, stellar-like, gravitational instability is expected to lead to much higher mass ratios (~0.01-0.1) than observed for the b Cen and μ2 Sco companions, which would imply an unusually low conversion efficiency of disk mass into companion mass.

Vol. 657
6. Interstellar and circumstellar matter

The EDIBLES survey. V. Line profile variations in the lambdalambda5797, 6379, and 6614 diffuse interstellar bands as a tool to constrain carrier sizes

by H. MacIsaac, J. Cami, N. L. J. Cox, et al. 2022, A&A, 661, A24

This study of the high resolution profile correlations among the three strong optical diffuse interstellar bands (DIBs) along well specified sight lines is an exciting application of standard laboratory spectroscopic structural techniques to an ongoing mystery, that is to say identifying the molecular carrier species of these features. The authors find that the three DIBs have similar structures along each of the sight lines and they argue that profile changes between sources are due to varying excitation conditions. They find no obvious correlation with molecular hydrogen or C2 excitation. With this as the starting point, and assuming the profiles are rotational bands of spherical top or linear molecules, which is a simplifying but reasonable decision, the authors derived excitation temperature ladders and rotational constants. Considering acetylenic species, cyanopolyynes, fullerenes, and planar PAH molecules as paradigms, they derived approximate compositions for the carriers of the three bands, ranging in size from the smallest for 6614Å and the largest for 6379Å .

Vol. 664
5. Galactic structure, stellar clusters and populations

TRAPUM discovery of 13 new pulsars in NGC 1851 using MeerKAT

by A. Ridolfi, P. C. C. Freire, T. Gautam, et al. 2022, A&A, 664, A27 alt

The paper presents a deep pulsar candidate survey of the core-collapsed globular cluster NGC 1851, for which only a single pulsar had previously been known. In this study, 64 MeerKAT antennas were used in two frequency bands (centered at 124 and 816 MHz). The result is a wealth of millisecond pulsars, 14 in all, with periods between about 2 and 32 msec. Among these are six isolated sources, one of which is actually 100% linearly polarized, and eight binary systems, of which three are highly eccentric and at least one has a neutron star companion. All are located very close to the cluster core. Polarization and flux density variations are presented for the sample; none appear due to ablation effects of the pulsar on its companion for the binaries. This is a harbinger of things to come, with the continuation of timing studies for this cluster and the enlargement of the sample for population studies and probes of the internal medium of the clusters.

Vol. 662
1. Letters to the Editor

The origin of chaos in the Solar System through computer algebra

by F. Mogavero and J. Laskar 2022, A&A, 662, L3

We have known for more than 30 years that the Solar System is chaotic. This Letter goes further in identifying the origin of chaos. For that, the authors expand the equations that rule the motion of the planets using their own algebraic manipulator, TRIP. These expansions, which consist of tens of thousands of terms, permit them to isolate the different contributions in the equations, and in particular the resonances between the fundamental frequencies of the system. Then these different contributions are confronted with a chaos indicator, the finite-time maximum Lyapunov exponent (FT-MLE). This numerical study emphasizes the secular resonances that are responsible for chaos, offering different possible orbital configurations for the planets of the inner Solar System over the ages.

Vol. 663
4. Extragalactic astronomy

Extragalactic fast X-ray transient candidates discovered by Chandra (2000–2014)

by J. Quirola-Vasquez, F.E. Bauer, P.G. Jonker, et al. 2022, A&A, 663, A168

Thanks to the continuously improving sensitivity of X-ray instruments, unexplained short X-ray flares, lasting from a few seconds to hours, have recently been discovered. These fast extragalactic X-ray transients (FEXTs) are a particularly understudied population. Two events were discovered in nearby galaxies with an underlying quiescent emission. Another two were discovered in the Chandra Deep Field South data, and a fifth FEXT was discovered during a Chandra calibration observation of the cluster Abell 1795. Quirola-Vasquez and collaborators carried out one of the first systematic searches for FXRTs in the Chandra Source Catalog. They detected 14 FXRT candidates. The nature of FEXTs remains a mystery. The increased sample allows the authors to derive the event rate. This rate is consistent with the volumetric rate of long and short gamma-ray bursts and tidal disruption events, but well below the rate of supernovae. This shows the rarity of this type of event.

Vol. 662
10. Planets and planetary systems

Nucleation and growth of iron pebbles explains the formation of iron-rich planets akin to Mercury

by Anders Johansen and Caroline Dorn 2022, A&A, 662, A19 alt

Mercury has a higher iron content than any other planet in the inner Solar System. The common explanation is that Mercury originally had a metal-silicate ratio similar to that of chondrite meteorites, but that early in its history it underwent one or several giant impacts that stripped away much of its crust and mantle, leaving the iron core as a major component. A difficulty with this "evaporation model" is that the relatively high abundance of moderately volatile elements in Mercury's mantle (e.g., K) precludes the occurrence of temperatures much higher than ~ 1000 K during its history. Johansen and Dorn propose here a completely different interpretation of Mercury's high iron content that invokes the separation of iron and silicates following the cooling of a gas with a solar composition in the inner part of the protosolar disk. They show that due to its high surface tension, Fe is prone to homogeneous nucleation under very high supersaturation (S ~ 10^5), leading to the formation of large iron pebbles on a sparse population of nucleated iron nanoparticles. In contrast, silicates (in the form of MgSiO3) reach smaller pebble sizes due to a much higher abundance of nucleated particles. As a consequence, the Stokes number of iron particles may be a factor of ~10 higher than that of silicates, which favors the preferential incorporation of iron in planetesimals through streaming instability filaments. The separation of Fe from the oxygen-bearing silicates may also explain why Mercury’s mantle has the lowest FeO fraction of all inner Solar System bodies, and the model, applied to the condensation of corundrum (Al2O3), can also account for the low Al/Si ratio in Mercury's crust. These ideas may apply as well to the growing population of iron-rich exoplanets, which are then not required to have experienced mantle-stripping impacts, but whose formation may be a natural consequence of temperature fluctuations in protoplanetary disks and the ensuing chemical !

Vol. 657
2. Astrophysical processes

A study of natural frequencies in a dynamic corona - disk system

by A. Mastichiadis, M. Petropoulou, and N. Kylafis 2022, A&A, 662, A118 alt

Quasi-periodic oscillations (QPOs) in X-ray binaries were discovered long ago, with the EXOSAT satellite back in 1985. As the name states, they are not coherent periodicities, but flux oscillations at a characteristic frequency. Quasi-periodic oscillations in X-ray binaries have been classified into three types (A, B, and C). Type-C QPOs occur in the range of a few tens of millihertz to a few tens of hertz, and their frequency increases with the source luminosity. Several mechanisms have been put forward to explain type-C QPOs. In this paper, Mastichiadis and collaborators show that these QPOs can arise naturally from the interaction of a hot corona with the accretion disk. The hot electrons of the corona can be viewed as the prey of the soft disk photons, which act as predators. This behavior establishes a typical pray-predator pattern, generating the QPO.

Vol. 661
7. Stellar structure and evolution

The crucial role of surface magnetic fields for stellar dynamos: epsilon Eridani, 61 Cygni A, and the Sun

byS. V. Jeffers, R. H. Cameron, S. C. Marsden, et al. 2022, A&A, 661, A152

The solar magnetic activity has a period of about 11 years, and its spatial-temporal structure has been well established and can be explained by the solar dynamo model. Whether the dynamo model can explain the magnetic activities of other stars remains unclear. In other words, are solar and stellar dynamos similar? With a Zeeman-Doppler imaging (ZDI) technique, the authors have reconstructed the evolution of large-scale magnetic field geometry over several decades for two nearby, active, and low-mass K dwarfs: 61 Cygni A and Epsilon Eridani. The derived magnetic butterfly diagram is then compared to the two stars' chromospheric activity (S-index). They found that the dynamo of 61 Cygni A is quite similar to that of the Sun, while Epsilon Eridani represents a more extreme level of the solar dynamo. They conclude that the surface magnetic field plays a crucial role in the dynamos of the three stars, 61 Cyni, Epsilon Eridani, and the Sun.

Vol. 657
10. Planets and planetary systems

Early planet formation in embedded protostellar disks. Setting the stage for the first generation of planetesimals

by A. J. Cridland, G. P. Rosotti, B. Tabone, L. Tychoniec, M. McClure, P. Nazari, and E. F. van Dishoeck 2022, A&A, 662, A90 alt

A classical model of planetary system formation is that planets form from the protoplanetary disk that surrounds a star. In this paper, the authors show that planetesimals may form before the formation of the star is completed. They do this by numerically simulating the collapse of a dusty proto-stellar cloud in which streaming instability is triggered; this is itself believed to produce the first planetesimals. In particular, they find a sweet spot for planetesimal formation for grain sizes of a few tens of microns. This study could change our view of planet formation, which may start earlier in the evolution of the planetary system than currently thought.

Vol. 661
9. The Sun and the Heliosphere

Observational constraints on the origin of the elements. IV. Standard composition of the Sun

by E. Magg, M. Bergemann, A. Serenelli, et al. 2022, A&A, 661, A140 alt

For over two decades, stellar astrophysics has faced the so-called solar abundance crisis: solar structure models computed using the observed elemental abundances for the Sun were in conflict with the solar interior properties independently determined through helioseismology. Magg et al. analyzed the solar chemical composition again, coupling new non-local thermodynamical equilibrium (LTE) atomic models with three-dimensional simulations of solar convection to better represent the solar atmosphere.They also used two independent descriptions of the atomic properties of neutral oxygen — the most critical element for helioseismic predictions. The corresponding two estimates of the oxygen abundance agree well, thus supporting their revised solar oxygen abundance, and their abundances for refractory elements agree with the composition of the CI chondrites — the primitive meteorites that still carry the chemical fingerprint of the early Solar System. Excitingly, the new abundances resolve the previous puzzling mismatch between the solar interior structures inferred from helioseismology and from solar evolution models. This suggests that the “solar problem” stemmed from slightly incorrect solar abundances, and that no non-standard physics — such as accretion of metal-poor material, energy transport by dark matter particles, or a revision of the opacities — needs to be invoked any longer.

Vol. 662
1. Letters to the Editor

Asteroseismology of the multiple stellar populations in the globular cluster M4

by M. Tailo, E. Corsaro, A. Miglio, J. Montalbán, K. Brogaard, A. P. Milone, A. Stokholm, G. Casali, A. Bragaglia 2022, A&A, 662, L7 alt

The detection of solar-like oscillations in stellar clusters has an enormous potential, although it is a challenging task, particularly for globular clusters that are far away and affected by crowding. Among the old Galactic globular clusters, only M4 is currently accessible to this kind of research. The authors have taken advantage of observations with the NASA Kepler satellite to detect oscillations with a power excess in 37 cluster members, the largest sample of solar-like oscillators in a globular cluster to date, which includes both red giant branch and red horizontal branch stars.

For the first time, an asteroseismic characterization of the multiple stellar populations hosted in this cluster has been possible; the masses and radii estimated for these stars are in very good agreement with the values obtained with more classical methods, and the authors confirm that the difference in mass among stars belonging to the cluster multiple populations is quite small. Moreover, the characterization of a good number of red horizontal branch stars has provided solid confirmation that these objects lose a fairly high amount of mass, and that stellar mass loss in globular clusters seems to be a different phenomenon compared to that in younger stellar associations.

Vol. 657
4. Extragalactif astronomy

Impact of non-thermal particles on the spectral and structural properties of M87

by C. M. Fromm, A. Cruz-Osorio, Y. Mizuno, A. Nathanail, Z. Younsi, O. Porth, H. Olivares, J. Davelaar, H. Falcke, M. Kramer, L. Rezzolla 2022, A&A, 660, A107

The M87 jet, the multiwavelength protoype of active galactic nucleus outflows, was modeled in full general relativistic magnetohydrodynamics including wind and disk contributions. Spectral energy distributions were computed with approximate distribution functions for the charged particles. The images for a wide range of model parameters of a Kerr central engine are shown.

Vol. 662
10. Planets and planetary systems

High D/H ratios in water and alkanes in comet 67P/Churyumov-Gerasimenko measured with Rosetta/ROSINA DFMS

by D. R. Müller, K. Altwegg, J.J. Berthelier, et al. 2022, A&A, 662, A69 alt

Measurements of isotopic abundances in comets bear key information regarding the early Solar System’s composition and physical conditions. Thanks to its sensitivity and mass resolution, the mass spectrometer ROSINA/DFMS onboard Rosetta had the capability to detect tens of new atomic and molecular species in comet 67P/Churyumov-Gerasimenko and to measure isotopic ratios in many of them. Here, Müller et al. present two sets of new results. First, they show that the D/H ratio in water, which is enhanced by a factor 3.2 over the terrestrial value, is independent of heliocentric distance, the level of cometary activity, and the distance to the nucleus; the same is true for the 16O/17O ratio. More remarkably, they also measured for the first time in comets the D/H ratios in a suite of alkanes (CH4, C2H6, C3H8, and C4H10). The inferred values are 4-5 times enhanced over the D/H ratio in water, but comparable to its value in several other carriers, the most noteworthy one being in the organic refractory component of 67P dust. This suggests that these organic molecules may be inherited from the presolar molecular cloud, where ionized radiation in cold environments favored D/H fractionation, both in water and (differentially) in organics. In contrast, the 13C/12C ratio in 67P's alkanes is found to be consistent with the 13C/12C ratio on Earth and other planets, adding further evidence for a "universal" Solar System 13C/12C value.

Vol. 659
14. Catalogs and data

An expanded ultraluminous X-ray source catalogue

by M. C. i Bernadich, A. D. Schwope, K. Kovlakas, et al. 2022, A&A, 659, A188

Ultraluminous X-ray sources (ULXs) are X-ray sources in external galaxies with X-ray luminosities higher than 5x10^39 erg s^-1, such that they overcome the Eddington luminosity of even a ~30 solar mass black hole. In this paper, Bernadich and collaborators scrutinize the XMM-Newton-based catalog 4XMM-DR9 to identify 779 ULX candidates (brighter than 10^{39} erg s^-1) with 30 extremely bright objects (>1×10^41 erg s^−1). These extreme ULXs might indeed host intermediate-mass black holes. Starting from an existing catalog (4XMM-DR9), limited additional spectral analysis is performed on the single ULXs. The large sample allowed the authors to show that ULXs are most likely hosted in spiral galaxies. Ultraluminous X-ray sources also exist in elliptical and lenticular galaxies, where they are spectrally harder and more variable.

Vol. 662
2. Astrophysical processes

Partially accreted crusts of neutron stars

by L. Suleiman, J. L. Zdunik, P. Haensel, M. Fortin, 2022, A&A, 662, A63

Accretion of matter onto a neutron star heats its surface. Transient X-ray binaries experience sporadic outbursts, lasting weeks to months, during which matter falls at a high rate onto the neutron star surface. A puzzle arises when studying the following cooling of the neutron star: the surface is too hot for the amount of material that has fallen onto it during the outburst. A new, unknown source of heat is usually postulated so as to make observations agree with theory. In this study, Suleiman and collaborators investigate, for the first time, the impact of a crust made by freshly accreted matter, as well as the original crust compressed by the new material (known as hybrid crust). From this preliminary study, the authors show that the amount of heat due to compression of the original crust is of the same order of the one due to the accreted crust, paving the way for a solution to this long-standing problem.

Vol. 659
4. Extragalactic astronomy

Making Faranoff-Riley I radio sources. III. The effects of the magnetic field on relativistic jets’ propagation and source morphologies

by S. Massaglia, G. Bodo, P. Rossi, S. Capetti, A. Mignone, 2022, A&A, 659, A139

The authors present a remarkable series of magnetohydrodynamic (MHD) simulations of the propagation of mildly relativistic jets on the tens of kiloparsec scales covering the power range of both Fanaroff-Riley types. Their three-dimensional models are compared with observed jet morphologies and length scales with considerable success. Perhaps the most notable aspect of the visualization is how the inferred jet properties depend on the viewing angle and how they change over time as the core continues to power the flow (be sure to watch the animations). A further result, which should serve as inspiration to observers, is that despite being imbedded in a spherically symmetric environment, one set of models shows small fluctuations that morph into large-scale bending. This disrupts the flow, generates turbulent structures, and the jets transition between morphological classes.

Vol. 662
4. Extragalactic astronomy

The complex time and energy evolution of quasi-periodic eruptions in eRO-QPE1

by R. Arcodia, G. Miniutti, G. Ponti 2022, A&A, 662, A49 alt

Quasi-periodic eruptions (QPEs) are a recent and puzzling recurrent phenomenon that occur in the nuclei of low-mass galaxies. They consist of bright X-ray bursts that last for about half an hour and repeat every several hours. The origin of these QPEs is still unknown, and several models have been proposed. In this paper, Arcodia and coauthors, investigating the shape of these QPEs, find isolated bursts as well as bursts with a structure made by the superpositions of events. This is the first time that complex shapes have been detected. In addition, the energy spectrum of the QPEs (either simple or complex) changes with time, with the decay of each eruption being softer than its rise. No explanations for either of these new observational instances have been found yet. The mystery deepens.

Vol. 659
6. Interstellar and circumstellar matter

A search for cool molecular gas in GK Persei and other classical novae

by T. Kaminski, H. Mazurek, K. Menten, and R. Tylenda, 2022, A&A, 659, A109

Null results, though too often consigned to a desk drawer, can sometimes provide basic insights, and this is one such example. The authors conducted a deep spectroscopic survey of CO and HCN emission in a sample of more than one hundred Galactic novae, covering about 350 years and spanning the various subclasses of outbursts. Their conclusion is that no emission intrinsic to the ejecta is detected, but interstellar lines were. The lack of CO emission adds support to chemical models for molecular formation and persistence in the ejecta. For those novae for which light echos were historically seen, such as the prototypical outburst of GK Per 1901, the interstellar matter may be associated with the scattering, and they discuss several cases.

Vol. 661
10. Planets and planetary systems

Jupiter’s inhomogeneous envelope

by Y. Miguel, M. Bazot, T. Guillot, et al. 2022, A&A, 662, A18 alt

While Jupiter's gaseous envelope is mainly composed of hydrogen and helium, the distribution of the other (heavy) elements is a key constraint to its formation. Thanks to accurate measurements of Jupiter's gravitational moments (J2 to J10), the Juno mission is yielding unique information on Jupiter's internal structure and deep atmospheric winds. Building upon preliminary analyses of the Juno results, Miguel et al. present a comprehensive modeling of the gravity data, complemented by measured values of the helium abundance and metallicity in the observable atmosphere. Using both a three-layer model and a model that includes a dilute core as well as a variety of equations of state, they find that the ensemble of data can be satisfied only by invoking a gradient of metallicity, namely a heavy-element enrichment in the interior relative to the outer envelope. They further set an upper limit of 7 Earth masses for Jupiter’s inner core. The non-homogeneity of Jupiter's envelope implies that Jupiter continued to accrete heavy elements while its H2-He envelope was still growing, and that Jupiter's interior and outer envelope never fully mixed. This result favors the scenario of planetesimal ablation throughout the gas accretion phase over the pebble accretion scenario, in which solid accretion stops at the pebble isolation mass, but hybrid planetesimal-pebbles scenarios are also possible. While efforts are underway to determine exoplanet atmospheres' composition and metallicity from spectroscopic observations, the present study suggests that results will not necessarily be representative of the planets' bulk metallicities.

Vol. 659
1. Letters to the Editor

CHEOPS geometric albedo of the hot Jupiter HD209458 b

by A. Brandeker, K. Heng, M. Lendl, et al. 2022, A&A, 659, L4 alt

The albedo of a planet determines how much starlight enters into its atmosphere and hence its global energy budget, and it also provides a key diagnostic of the physical and chemical properties of this atmosphere. Albedos have been measured for a few exoplanets through the depth of their occultation of their star, but up to now that has been limited to hot planets which orbit so close to their star that their thermal emission extends into the visible range. Their occultation depth, therefore, measures the sum of their reflected light and thermal emission, and evaluating their albedo requires a large and model-dependent correction for the latter. Using the exquisite photometric precision provided by the CHEOPS mission, Brandeker et al. measured the depth of the occultation of HD 209458 by its planet, which is cool enough so that thermal emission negligibly contributes to the occultation. This provides the cleanest measurement to date of an exoplanet albedo and shows that HD 209458b is very dark, with Ag = 0.096±0.016.