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Before yesterdayAstronomy & Astrophysics (A&A)

VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis

Vol. 654
10. Planets and planetary systems

VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis

by P. Vernazza, M. Ferrais, L. Jorda, et al. 2021, A&A, 654, A56 alt

Until recently, detailed, kilometer-accurate, 3D shape information of asteroids has been limited to bodies visited by spacecraft missions. This situation has changed with the advent of imagers operating at the ~20 mas diffraction limit in the optical, such as SPHERE/ZIMPOL at the VLT. Vernazza et al. report the final results of an ESO Large Programme in which 42 large main belt asteroids have been imaged β€” out of which 20 of the 23 asteroids have a diameter larger than 200 km β€” and down to objects as small as ~ 100 km. In addition to remarkable results reported previously on several individual objects, including the evidence for craters that testify of family-forming catastrophic collisions, these observations provide a unique dataset on asteroid shapes and densities (combining the measured volumes with available mass estimates). In particular, they reveal that the departure from smooth shapes is inversely correlated with the objects' volume-equivalent radius; furthermore, there is a bimodality in shape ("spherical" versus "elongated" objects) associated with the rotation period, and a bimodality in density ( 2.7 g cm-3) reflecting volatile-rich and volatile-poor bodies. Most objects, and especially the volatile-rich ones, appear to be located close to the Maclaurin equilibrium sequence. Finally the low density of some of the C- and P/D-type asteroids may point to an origin in the transneptunian disk.

Strong dependence of the physical properties of cores on spatial resolution in observations and simulations

Vol. 653
6. Interstellar and circumstellar matter

Strong dependence of the physical properties of cores on spatial resolution in observations and simulations

by F. Louvet, P. Hennebelle, A. Men'shchikov, et al. 2021, A&A, 653, A157

Louvet and collaborators present a systematic study of the effect of spatial resolution on the measured properties, particularly the mass, of dense stellar cores, the smallest gas entities that are currently believed to collapse and form a star, or a multiple star system. The results are chilling. Using both observations and simulations, they find that extracting dense cores from the same molecular cloud at different resolutions (distances) does not produce consistent results. They also find that the mass spectrum of the extracted dense cores is a function of resolution, implying that characterizing the rate of conversion of dense gas into stars is currently out of reach. The authors point out that the much used Gaussian beam deconvolution of imaging data to infer dense core sizes produces highly biased results in this context. The results in this paper have direct consequences on our overall ability to characterize the initial conditions of star formation, providing a warni! ng that it is a more complex task than currently assumed.

The chemical make-up of the Sun: A 2020 vision

Vol. 653
Sect. 9. The Sun and the Heliosphere

The chemical make-up of the Sun: A 2020 vision

by M. Asplund, A. Amarsi, and N. Grevesse 2021, A&A, 653, A141 alt

Accurate knowledge of the solar elemental abundances is crucial for an extremely broad range of astrophysics research topics, as the chemical composition of essentially all cosmic objects is relative to the solar one. Asplund et al. reassessed the solar abundances of all 83 long-lived elements, using highly realistic solar modeling and state-of-the-art spectroscopic analysis techniques, coupled with the best available atomic data and observations. Their results confirm the persistence of the so-called solar modeling problem, that is a discrepancy between helioseismology and solar interior models calculated with solar metallicity determined from spectroscopy. The results of Asplund et al. strengthen the suggestion of shortcomings with the opacities and/or treatment of mixing below the convection zone in existing standard solar models.

The ALPINE-ALMA [CII] survey: The contribution of major mergers to the galaxy mass assembly at z~5

Vol. 653
4. Extragalactic astronomy

The ALPINE-ALMA [CII] survey: The contribution of major mergers to the galaxy mass assembly at z~5

by M. Romano, P. Cassata, L. Morselli, et al. 2021, A&A, 653, A111

The ALMA ALPINE survey is a recently completed ALMA large program that is providing new valuable insight into the nature of star-forming galaxies up to z=6. In this work, the authors combine the ALMA observations to multi-wavelength data to study the morphology and kinematics of the ALPINE galaxies with the aim of assessing the importance of major mergers in the process of galaxy mass assembly at early cosmological epochs. The obtained constraints reveal the presence of a significant merging activity in the early Universe. The exact contribution to the galaxy mass build-up at these redshifts and through the cosmic epochs is strongly dependent on the assumption of the merger timescale. If a rapidly evolving merger timescale such as TMM ∝ (1+z) ^βˆ’2 is assumed, the results agree well with state-of-the-art cosmological simulations, suggesting a considerable role of mergers in the build-up of galaxies at early times.

Refined physical parameters for Chariklo’s body and rings from stellar occultations observed between 2013 and 2020

Vol. 653
10. Planets and planetary systems

Refined physical parameters for Chariklo’s body and rings from stellar occultations observed between 2013 and 2020

by B. E. Morgado, B. Sicardy, F. Braga-Ribas, et al. 2021, A&A, 653, A141 alt

Chariklo was the first small body around which rings were discovered, from observations of a stellar occultation by the asteroid. Besides determining the projected shape of Chariklo, the resulting light curves revealed that additional structures occulted the stellar light and that they appeared to be a system of two rings. Morgado et al. (2021) present observations of an additional eight stellar occultations by Chariklo. These new data not only confirm that Chariklo has two rings but also determine the orientation, width, and eccentricity of each ring, as well as the 3D shape of the asteroid.

Why do more massive stars host larger planets?

Vol. 652
10. Planets and planetary systems

Why do more massive stars host larger planets?

by M. Lozovsky, R. Helled, I. Pascucci, C. Dorn, J. Venturini, and R. Feldmann 2021, A&A, 652, A110 alt

The increasing database of exoplanets allows us to explore how planetary populations vary on stellar mass. For relatively small K and G stars (masses ~

ALMA observations of the variability of the quiet Sun at millimeter wavelengths

Vol. 652
9. The Sun and the Heliosphere

ALMA observations of the variability of the quiet Sun at millimeter wavelengths

by A. Nindos, S. Patsourakos, C. E. Alissandrakis, and T. S. Bastian 2021, A&A, 652, A92 alt

The chromosphere, which is the layer from which most of the solar radiation at millimeter wavelengths is emitted, is probably the least understood layer of the solar atmosphere. This is true even for the quiet chromosphere (that is, outside strong magnetic field concentrations). In this article the authors used high spatial and temporal resolution ALMA observations of a quiet Sun region at 1.26 and 3 mm and addressed the variability in the quiet chromosphere with a focus on the study of spatially resolved oscillations and transient brightenings, that is, small, weak events of energy release. Both phenomena may have a bearing on the heating of the chromosphere. Their study demonstrated that most fluctuations at 1.26 and 3 mm were related to p-mode oscillations with similar rms at both wavelengths. The oscillations were ubiquitous and provided more than 60% of the total fluctuations. The removal of oscillations from the data revealed several transient brightenings. All of them showed gradual rise and fall time profiles, and only a few had 1600 or 304 \AA\ counterparts. The combined power per unit area of the transients in the two ALMA bands was more than two orders of magnitude smaller than the chromospheric radiative losses. The minimum energy of the 1.26 mm transients was among the smallest ever reported, irrespective of the wavelength of observation, and their occurrence rate per unit area was higher than that of the 3 mm events.

Revisiting the archetypical wind accretor Vela X-1 in depth. Case study of a well-known X-ray binary and the limits of our knowledge

Vol. 647
7. Stellar structure and evolution

Revisiting the archetypical wind accretor Vela X-1 in depth. Case study of a well-known X-ray binary and the limits of our knowledge

by P. Kretschmar, I. El Mellah, S. Martínez-NúnΜƒez, et al. 2021, A&A, 652, A95

A star as a guide. In this paper, which is more similar to a review in some respects, Kretschmar and collaborators used the X-ray binary Vela X-1 to investigate any aspect of wind accretion limiting our knowledge. Vela X-1 is a bright X-ray source that was discovered by Uhuru, the first X-ray satellite, and the optical companion is a bright (V~6.9) and early O star. We do not miss data at any wavelength. The neutron star spins slowly (~280 s) and orbits the more massive star on a slightly eccentric orbit (~0.1) every ~9 days. This allows the neutron star to act as a probe of the O-star stellar wind and of the accretion process itself. This article triggered a discussion amongst the editors of A&A as well. We welcome this kind of review article on single objects, as a "spoke-object" of its astronomical class.

COALAS. I. ATCA CO(1–0) survey and luminosity function in the Spiderweb protocluster at z = 2.16

Vol. 652
4. Extragalactic astronomy

COALAS. I. ATCA CO(1–0) survey and luminosity function in the Spiderweb protocluster at z = 2.16

by S. Jin, H. Dannerbauer, B. Emonts, et al. 2021, A&A, 652, A11 alt

Galaxy clusters assemble and collapse around z=2 in the first half of the life of the Universe, but the process is still not completely understood. The COALAS project obtained molecular gas observations of the Spiderweb protocluster field at z = 2.16. A total of 475 hours of Australia Telescope Compact Array (ATCA) observations cover a large ~5 Mpc region and over 13 000 km/s in velocity. The authors detect CO(1-0) emission in 46 galaxies, which is the largest sample of molecular gas measurements in a protocluster to date.

Half of the CO emitters are gravitationally unbound to the cluster core, suggesting that the cluster core is collapsed while its outer regions are still assembling. One main result is the high proportion of CO-rich galaxies, which are at least ten times more numerous than in the field at the same redshift. This environmental effect reverses what is seen at z=0, where cluster galaxies are gas deficient with respect to the field.

Galactic spiral structure revealed by Gaia EDR3

Vol. 651
5. Galactic structure, stellar clusters and populations

Galactic spiral structure revealed by Gaia EDR3

by E. Poggio, R. Drimmel, T. Cantat-Gaudin, et al. 2021, A&A, 651, A104

Poggio and collaborators use Gaia Early Data Release 3 (EDR3) to map the density variations in the distribution of young stars, open clusters, and classical Cepheids in the Galactic disk. Using kernel density and wavelet techniques, they construct an over-density map exhibiting large-scale arches that extend in a clumpy but coherent way over the entire sampled volume, indicating the location of the spiral arm segments in the vicinity of the Sun. The new map is generally consistent with previous models of the Sagittarius-Carina spiral arm. However, it differs significantly for the arms in quadrant III (galactic longitudes 180Β°

Spectral index of synchrotron emission: insights from the diffuse and magnetised interstellar medium

Vol. 651
6. Interstellar and circumstellar matter

Spectral index of synchrotron emission: insights from the diffuse and magnetised interstellar medium

by M. Padovani, A. Bracco, V. Jelic, D. Galli, and E. Bellomi 2021, A&A, 651, A116 alt

The authors model the spectral index degree of polarization correlations of Galactic synchrotron radiation using different model cosmic ray spectra and magnetic field configurations from a fully 3D magnetohydrodynamic simulation. They find a substantial decrease in polarization from the theoretical maximum of about 70% arising from the tangled field structures along the line of sight and show the correlations with the mean transverse field strengths. Although not included here, the effect will clearly be exacerbated by Faraday rotation and ionization fluctuations along the diffuse line of sight. From the spectral index and polarization distributions, they provide an estimator for the transverse (hence total) field strength for a range of instruments and angular resolutions. The paper has broad applications since, with an input turbulence spectrum (e.g., Kolmogorov or Goldreich-Sridhar) structure (in this case limited to a 50 pc region), the same technique could be used for resolved extragalactic interstellar fields.

Sharpening quasar absorption lines with ESPRESSO

Vol. 651
4. Extragalactic astronomy

Sharpening quasar absorption lines with ESPRESSO

by P. Noterdaeme, S. Balashev, C. Ledoux, et al. 2021, A&A, 651, A78

Using EXPRESSO, with a resolution of around 2 km/s, the authors have revisited the high redshift quasi-stellar object HE 0001-2340, for which lower resolution observations were obtained with UVES. Those earlier data indicated anomalous Mg isotopic ratios for some of the intermediate redshift absorption systems. The new data yield essentially solar abundances. A striking result is the determination of the electron temperature for the high redshift component, about 1.6Γ—104K, resolved into multiple velocity components within a range of a single galactic disk. The paper is a spectacular demonstration of the frontier opened by high resolution, high fidelity analysis of even single lines of sight, which can be used to tease out details of the evolution of galactic interstellar gas.

A kinematic study of central compact objects and their host supernova remnants

Vol. 651
7. Stellar structure and evolution

A kinematic study of central compact objects and their host supernova remnants

by M. G. F. Mayer and W. Becker 2021, A&A, 651, A40

Supernova remnants (SNRs) are the leftovers of supernovae, the results of powerful explosions. In the beginning, their gas expands at ~20,000 km/s. This velocity decreases with time, depending on the density of the surrounding ambient medium, which is snowplowed away. For relatively young SNRs, such as the ones considered in this study (~350 to 27,000 yr), one could expect expansion velocities of ~5,000 km/s. Naively, one might imagine that SNRs do not change on timescales of a human lifetime. However, if observed at high angular resolution (as Chandra observations allow for) and over a sufficiently long temporal base (~10 yr), they can reveal their overall expansion, similar to that of an inflating balloon. A meticulous analysis of the X-ray data is needed to put this expansion on a firm statistical basis. This is done in the work by Meyer and Becker, which has revealed a hint of SNR expansion in two remnants. In addition, the authors were also able to measure or constrain the motion of the neutron star, the supernova leftover, excluding hyper-velocity objects..

Jupiter’s β€œcold” formation in the protosolar disk shadow. An explanation for the planet’s uniformly enriched atmosphere

Vol. 651
1. Letter to the Editor

Jupiter’s β€œcold” formation in the protosolar disk shadow. An explanation for the planet’s uniformly enriched atmosphere

by K. Ohno and T. Ueda 2021, A&A, 651, L2 alt

Spacecraft observations have revealed that Jupiter's atmosphere is uniformly enriched in heavy elements, with elemental abundances of O, C, N, S, P, Ar, Kr, and Xe all a factor of two to four larger than protosolar abundances. While a general enrichment of heavy elements can be expected due to planetesimal and pebble dissolution or to core erosion posterior to its formation, it is intriguing that even highly volatile elements, such as N and Ar, are as enriched as other elements. Previously proposed explanations have invoked a formation of Jupiter in very cold environments ( 30 au), or the entrapment of hyper-volatiles in water ice ("clathration"), but have faced a number of difficulties. Ohno and Ueda propose a novel idea, namely that the dust pileup that occurs at the H2O snow line casts a shadow on the disk, cooling a 2-10 au wide zone exterior to the H2O snow line and enabling the condensation of N2 and noble gases there. They compute the temperature structure of a shadowed protosolar disk and attendant volatile radial distribution and are able to reproduce a uniform enrichment pattern at Jupiter if the small-dust surface density drops by a factor of at least 30 across the H2O snow line. Unlike in many previous models, a formation of the proto-Jupiter near the current orbit is therefore a viable hypothesis. Interestingly, because the disk shadow barely extends to Saturn's orbit, the new scenario does not predict a uniform elemental enrichment at Saturn and can therefore be tested by in situ measurements from a Saturn entry probe.

The 10 parsec sample in the Gaia era

Vol. 650
14. Catalogs and data

The 10 parsec sample in the Gaia era

by C. ReylΓ©, K. Jardine, P. FouquΓ©, J. A. Caballero, R. L. Smart, and A. Sozzetti 2021, A&A, 650, A201 alt

A new sample of the nearest objects has been released, containing complete relevant data for all known stars within 10 pc from the Sun. It highlights the richness and variety of the solar neighborhood with objects of all multiplicity combinations, sizes, and ages. The Gaia mission has revolutionized the exploration of our galaxy, including much improvement regarding knowledge on the nearest stars. The new database contains updated astrometry and photometry from the Gaia Early Data Release 3, complemented with literature magnitudes, spectral types, and radial velocities, along with a precious list of references to aid future studies. This updated census of 541 objects within 10 pc, which are mostly stars but also a surprising number of brown dwarfs and exoplanets, reveals a high multiplicity frequency (28% of them are in binary or multiple systems). The vast majority of stars are M-dwarfs (61%), which are the most common type of stars in the Milky Way. Our neighbors are the most important ones in terms of future exploration, as they are the easiest systems to be scrutinized and will, one day, be the first targets for human space travel.

The similarity of the interstellar comet 2I/Borisov to Solar System comets from high-resolution optical spectroscopy

Vol. 650
1. Letter to the Editor

The similarity of the interstellar comet 2I/Borisov to Solar System comets from high-resolution optical spectroscopy

by C. Opitom, E. Jehin, D. HutsemΓ©kers, Y. Shinnaka, J. Manfroid, et al. 2021, A&A, 650, L19 alt

Discovered in August 2019 on an orbit with an eccentricity in excess of 3, and showing signs of activity already at discovery, comet 2I/ Borisov is the first comet of unambiguous interstellar origin. Previous spectroscopic studies at visible-to-radio wavelengths have led to the detection of several atomic, radical, and molecular features due to CN, C2, NH, [OI], NH2, OH, HCN, CO, and most recently atomic Ni, overall indicative of a composition similar to that of CO-rich and carbon-chain depleted Solar System comets. Using UVES at the VLT for a total of ~21 hours, Opitom et al. reobserved all of the abovementioned atomic and radical species in the coma of 2I/Borisov, and they report for the first time the detection of CH, [FeI], and additional forbidden lines due to [OI], while C3 and several ionic species remain undetected. These new observations enable the first determination of the Fe/Ni relative abundance, the ortho-to-para ratio of NH2, and the green-to-red (G/R) line ratio of atomic oxygen in 2I/Borisov. In these respects, and except for a high G/R ratio, which is presumably related to its CO abundance, 2I/Borisov does not appear to stand out from solar system comets, pointing to a similarity of environments between its unknown birthplace and those of CO-rich comets in our own Solar System.

The EXTraS project: Exploring the X-ray Transient and variable Sky

Vol. 650
15. Numerical methods and codes

The EXTraS project: Exploring the X-ray Transient and variable Sky

by A. De Luca, R. Salvaterra, A. Belfiore, S. Carpano, et al. 2021, A&A, 650, A167 alt

Everything flows. Time is a fundamental perception in our life. In this paper, De Luca and collaborators investigate the timing properties of ~400,000 X-ray sources found in the XMM-Newton database for over 10 years of observations. They characterize periodic and aperiodic (either short- and long-term) variability, spanning more than eight orders of magnitude in time and six orders of magnitude in flux. They also searched for new transient events that are too faint to be detected as sources in the overall images. This investigation provides a thorough characterization of variability in its entire aspect and a legacy for future studies. The Exploring the X-ray Transient and variable Sky (EXTraS) project was funded by the European Union and all the results and methods are publicly available at the project website.

Atmosphere of Betelgeuse before and during the Great Dimming event revealed by tomography

Vol. 650
1. Letters to the Editor

Atmosphere of Betelgeuse before and during the Great Dimming event revealed by tomography

by K. Kravchenko, A. Jorissen, S. Van Eck, et al. 2021, A&A, 650, L17

Between October 2019 and February 2020, Betelgeuse exhibited a rapid and unprecedented decrease in brightness that has been widely dubbed as "the Great Dimming". This has been the subject of numerous studies, and several, sometimes inter-related hypotheses have been put forward to explain this behavior. These have ranged from the formation of a dust cloud, to a temperature change in the atmosphere, to a cool spot covering about half of the stellar surface. Some have even suggested that the dimming could signal imminent core collapse, which could make Betelgeuse the first observable supernova in our Galaxy for over 400 years. However, a key piece of the puzzle relates the brightness changes to the atmospheric dynamics. In this study, Kravchenko et al. use a high signal-to-noise time series of spectra spanning about 5 years, which bracket the Great Dimming event. This allowed them to probe the conditions as a function of depth in the stellar atmosphere. They uncovered the presence of multiple shocks that occurred in 2018 and 2019 that were amplified by the underlying 400-day pulsation cycle, resulting in an outflow and a corresponding increase in opacity that led to the dimming.

Description of turbulent dynamics in the interstellar medium: Multifractal-microcanonical analysis I. Application to Herschel observations of the Musca filament

Vol. 649
6. Interstellar and circumstellar matter

Description of turbulent dynamics in the interstellar medium: Multifractal-microcanonical analysis I. Application to Herschel observations of the Musca filament

by H. Yahia, N. Schneider, S. Bontemps, et al. 2021, A&A, 649, A33 alt

The flow of energy in turbulence between the source and dissipation scales is a self-similar irreversible cascade process. For engineers this loss of energy from the system is a phenomenological nuisance since star formation is the core mystery: the impediment to collapse presented by the large-scale motions must be eliminated before the gas can become gravitationally dominated, but how does this dissipation occur? The signature of this process is non-Gaussianity, called β€œintermittency," which is the revelation of spatial and velocity correlations. This can be understood through the analysis of the singular points in the flow where the velocity gradients are steepest, hence regions of strong coupling. The authors provide a superb introduction to the multifractals and present new methods to reveal and analyze these structures in far-infrared molecular cloud images with Herschel. Here the focus is on the Musca filament as a demonstration, comparing their results with magnetohydrodynamic (MHD) simulations and demonstrating the presence of a cascade over an order of magnitude in length with a driving at parsec scales with intermittency operating in a spectacular array of small-scale filaments and knots.

Six transiting planets and a chain of Laplace resonances in TOI-178

Vol. 649
10. Planets and planetary systems

Six transiting planets and a chain of Laplace resonances in TOI-178

by A. Leleu, Y. Alibert, N. C. Hara, et al. 2021, A&A, 649, A26 alt

Planets in mean motion resonance chains are expected to be a common outcome of planetary formation. However, few systems have been observed to be in Laplace chains (three body resonance), and, so far, planets in only one system (K2-138) have been measured both in transit and in radial velocity. Recently, the TOI system has received attention as initial measurements with TESS indicated three planet candidates near the 2:3:3 resonance, that is, a potential co-orbital system with two planets oscillating around the same period. Additional data from CHEOPS, ESPRESSO, NGTS, and SPECULOOS, presented by Leleu et al., reveal a much richer system: TOI-178 in fact harbors at least six planets in the super-Earth to mini-Neptune class (radii from 1.15 to 2.87 R_earth, sampling both sides of the Fulton radius valley), and the five outermost planets form a 2:4:6:9:12 chain of Laplace resonances. The system also appears to be remarkably co-planar, with the projected inclinations of the various planets differing by only 0.1Β°. Another remarkable feature, which differs notably from the TRAPPIST-1 system, is the variation of densities from planet to planet. They range from 0.177 to 1.02 times the Earth density and in a way that is not correlated to the distance from the star, something that would be expected if the differences were due to the primordial gas envelopes being partially removed via evaporation. With these unique features, and given that the star brightness (H = 8.8) will allow further characterization, TOI-178 is likely to become an emblematic system for understanding planetary formation and evolution.

The LOFAR LBA Sky Survey. I. Survey description and preliminary data release

Vol. 648
14. Catalogs and data

The LOFAR LBA Sky Survey. I. Survey description and preliminary data release

by F. de Gasperin, W. L. Williams, P. Best, et al. 2021, A&A, 648, A104

The sky at frequencies below 100 MHz has to date been systematically mapped to only 100 mJy noise levels and arcminute resolution. The LOFAR LBA Sky Survey (LoLSS) is using the LOFAR array to map the Northern Hemisphere in the 42 to 66 MHz frequency band to millijansky noise levels and 15" resolution. Gasperin et al. describe the first data release of the survey, which uses a preliminary processing pipeline to make public images and catalogs for a 750 square degree contiguous sky patch. At a 5 mJy noise level and 45" angular resolution, this early release represents both a major step forward and a taste of what future releases will deliver.

Spatially resolved spectroscopy across stellar surfaces. Paper IV: F, G, and K-stars: Synthetic 3D spectra at hyper-high resolution and paper V: Observational prospects: toward Earth-like exoplanet detection

Vol. 648
8. Stellar astmospheres

Spatially resolved spectroscopy across stellar surfaces. Paper IV: F, G, and K-stars: Synthetic 3D spectra at hyper-high resolution and paper V: Observational prospects: toward Earth-like exoplanet detection

by D. Dravins, H.-G. Ludwig, and B. Freytag 2021, A&A, 648, A16 and 2021, A&A, 648, A17

Transits of exoplanets across their host stars have brought us a great wealth of exoplanetary information, ranging from the chemical compositions of planetary atmospheres to the statistical properties of exoplanetary systems. These transits also represent powerful probes of the atmosphere of the host star, probably less intuitively, since the difference between occulted and unocculted observations isolates the light from a tiny fraction of the star along the path of the transit. This provides a linear resolution on the surface of those stars, which is set by the diameter of the planet and is otherwise unmatched except on our Sun. In a pair of papers, Dravins, Ludwig, and Freytag use 3D model atmospheres to synthesize the signal from such high spectral resolution differential spectroscopy and find that its observation at a very high signal-to-noise ratio is able to probe fine details of the atmospheric structure of the star.

Multiwavelength analysis of the X-ray spur and southeast of the Large Magellanic Cloud

Vol. 648
6. Interstellar and circumstellar matter

Multiwavelength analysis of the X-ray spur and southeast of the Large Magellanic Cloud

by J. R. Knies, M. Sasaki, Y. Fukui, et al. 2021, A&A, 648, A90

The authors study the extended region known as the Large Magellanic Cloud (LMC) X-ray spur that extends southeast from the 30 Dor star-forming region using XMM soft X-ray imaging, H I 21 cm emission, and a range of atomic and molecular tracers. By tessellating the X-ray- and channel-separated H I imaging, they produce a novel correlation map between the X-ray emission and the neutral gas, which they separate into components associated with the LMC disk, from the Small Magellanic Cloud (SMC), and an intermediate component that is connected to 30 Dor. A striking result of the separation of the three H I components is their correlations, and anti-correlations, with the X-rays. These maps show that while the stellar feedback dominates the interstellar medium around R136 and the core of 30 Dor, the large kiloparsec-scale structures are likely the product of a collision of gas originating in the SMC with the disk of the LMC that began about 5 Myr ago and is still ongoing. An array of spectroscopic probes is used to tease out the fine structure and reconstruct the history of the interaction, including Planck dust maps, CO, optical recombination, and collisionally excited lines that trace ionization and shocks.

The 800pc long tidal tails of the Hyades star cluster. Possible discovery of candidate epicyclic overdensities from an open star cluster

Vol. 647
5. Galactic structure, stellar cluster and populations

The 800pc long tidal tails of the Hyades star cluster. Possible discovery of candidate epicyclic overdensities from an open star cluster

by T. Jerabkova, H. M. J. Boffin, G. Beccari, G. de Marchi, J. H. J. de Bruijne, and T. Prusti 2021, A&A, 647, A137 alt

Open star clusters live and suffer in the potential of the mighty Milky Way. Some get quickly dissolved, while others are simply torn apart by our Galaxy, being stretched over a more extended period of time. New Gaia observations reveal the extended tidal tails of the Hyades, a nearby prototypical star cluster. Large-scale tidal tails for this cluster can now be recovered, extending almost one kiloparsec. Exquisite details, such as the epicyclic overdensities, can be appreciated, helping us ascertain details about the cluster's birth and evolution. Detailed N-body simulations can interpret these observations and map the potential of the Galaxy.

The MUSE Extremely Deep Field: The cosmic web in emission at high redshift

Vol. 647
3. Cosmology

The MUSE Extremely Deep Field: The cosmic web in emission at high redshift

by R. Bacon, D. Mary, T. Garel, J. Blaizot, M. Maseda, et al. 2021, A&A, 647, A107 alt

Galaxies form within large cosmic filaments of gas and dark matter, which are delineated by massive bright galaxies. Smaller galaxies are also believed to gather with the massive galaxies in these filaments, but they are too faint to be observed. With the MUSE Extremely Deep Field, a 140-hour deep MUSE observation in the Hubble Ultra-Deep Field, Bacon et al. have discovered diffuse extended Ly-alpha emission from redshift 3.1 to 4.5, tracing cosmic filaments on scales of several megaparsecs. These structures match the overdensities of individual Ly-alpha emitting galaxies, which are located at the centers of the very extended Ly-alpha diffuse emission. However, 70% of the total Ly-alpha luminosity from these filaments originates outside any individual Ly-alpha emitting galaxy. The origin of this diffuse Ly-alpha emission has remained an open question. The authors postulate that fluorescent Ly-alpha emission powered by the cosmic UV background can only account for a small fraction of it; they conclude that the bulk of this diffuse emission must come from the unresolved Ly-alpha emission of a large population of ultra-low luminosity Ly-alpha emitting galaxies, provided that they are tightly clustered together. If these Ly-alpha emitters are powered by star formation, then their luminosity function needs to extend down to very low star formation rates, below 10^-4 M_sol/yr. These observations provide the first detection of the cosmic web in Ly-alpha emission in typical filamentary environments as well as the first observational clue indicating the existence of a large population of ultra-low luminosity Ly-alpha emitters at high redshift.

First direct measurement of auroral and equatorial jets in the stratosphere of Jupiter

Vol. 647
1. Letters

First direct measurement of auroral and equatorial jets in the stratosphere of Jupiter

by T. CavaliΓ©, B. Benmahi, V. Hue, et al. 2021, A&A, 647, L8

The circulation in Jupiter's upper troposphere at cloud level has been measured for decades using the cloud-tracking technique. It consists of alternating prograde and retrograde zonal jets (i.e., east-west) with velocities up to ~100m/s that peak at low latitudes. More than 1000 km higher, in the ionosphere, supersonic jets have been discovered at polar latitudes in the auroral region. In this paper, CavaliΓ© and collaborators use the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to discover jets 150-200 km above the clouds in the stratosphere. Benefiting from the combination of high spectral and spatial resolving powers as well as high sensitivity and near instantaneous imaging with ALMA, they directly probe the wind speeds using Doppler shifts induced by the winds on spectral lines formed in the stratosphere. They discover strong nonzonal jets located under the main auroral ovals with velocities of 300-400 m/s. Auroral jets may thus extend from the ionosphere down to the stratosphere, forming huge vortices in which chemical compounds may be dynamically confined and bombarded by energetic magnetospheric electrons, resulting in a richer chemical inventory. In addition, zonal jets with velocities peaking at 150 m/s were found around Jupiter’s equator, the first direct measurement of the winds generated by the quasi-quadrennial oscillation in Jupiter’s equatorial stratosphere.

The past and future obliquity of Saturn as Titan migrates

Vol. 647
10. Planets and planetary systems

The past and future obliquity of Saturn as Titan migrates

by M. Saillenfest, G. Lari, G. BouΓ©, and A. Courtot 2021, A&A, 647, A92

The rotation axis of Saturn is tilted with respect to its orbital plane. The authors of the present study show that the past migration of Titan can explain this axial tilt. Titan has been drifting away from Saturn since its formation at a rate which the Cassini mission measured. The present study extends this previous work by simulating millions of trajectories to quantify this phenomenon and link the axial tilt of Saturn with the migration rate of Titan. These many simulations cover the uncertainty range on the migration rate of Titan, on the initial obliquity of Saturn, and on its moment of inertia. They show that the initial obliquity of Saturn was much smaller than its current value, and that the migrating Titan helped trap the precession of the rotation axis of Saturn into a secular resonance with Neptune. This resonance is in turn responsible for the tilt of Saturn. This study also looks into the future evolution of the spin axis of Saturn, suggesting that its obliquity is still increasing and could reach a much higher value of 55 to 65Β° within 5 Gyrs.

3D dynamics of the Orion cloud complex. Discovery of coherent radial gas motions at the 100-pc scale

Vol. 647
5. Galactic structure, stellar clusters and populations

3D dynamics of the Orion cloud complex. Discovery of coherent radial gas motions at the 100-pc scale

by J. Grossschedl, J. Alves, S. Meingast, G. Herbst-Kiss 2021, A&A, 647, A91 alt

The Orion complex is one of the nearest and largest collections of molecular clouds actively forming stars in the Milky Way. New observations using the proper motions of young stellar objects, as measured by Gaia and combined with gas radial velocities from CO archival data, helped to map the gas and stars in this region. They measured the 3D dynamics of a large area (~250 square degrees) across the Orion cloud complex, which includes Orion A, Orion B, and a couple of outer cometary clouds. The authors report the main discovery of coherent radial gas motions at large scales (~100 pc). Also, the energetics make it necessary to have the input of several past supernovae in the region. This is important for the formation history and the subsequent evolution of the young stellar objects in large molecular clouds, including a test for the classical feedback-driven scenario.

APOGEE discovery of a nitrogen-enhanced star being tidally disrupted from NGC 6723 and captured by the Milky Way bulge

Vol. 647
5. Galactic structure, stellar clusters and populations

APOGEE discovery of a nitrogen-enhanced star being tidally disrupted from NGC 6723 and captured by the Milky Way bulge

by J. G. Fernandez-Trincado, T. C. Beers, D. Minniti, L. Carigi, et al. 2021, A&A, 647, A64 alt

Metal-poor nitrogen-enriched stars are suited for chemical tagging and play a crucial role in reconstructing the formation and evolutionary history of the Milky Way. The authors studied the globular cluster NGC 6723 in the central region of the Milky Way with high resolution spectroscopy as part of the APOGEE-2 survey. They serendipitously discovered a star, 2M18594405-3651518, located outside the cluster (near the tidal radius) but moving on a similar orbit. Abundance analysis shows that nitrogen is enriched, and the abundances of the star agree with that in globular clusters. This means that the star has been recently ejected from the cluster, providing the first piece of evidence that some of the nitrogen-enhanced stars in the bulge likely originate from the tidal disruption of globular clusters.

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