Letters in 2021

Previously unresolved radio features of nearby Centaurus A reveal transition regions for both the feeding of this active galaxy and the feedback mechanism for recycling energy back into the surrounding medium.

A population of free-floating planets in Upper Scorpius that is larger than that predicted by core-collapse models suggests that the ejection of planets due to dynamical interactions early in the lifetime of a forming planetary system is more common than prevoiusly thought.

High-spectral-resolution data from the CARMENES spectrometer could resolve the neutral oxygen triple line at 777.4 nm in the atmosphere of ultrahot Jupiter KELT-9b. The results show the presence of non-local thermodynamic equilibrium processes and reveal macro- and microturbulence induced by fast winds with speeds ranging between 3 and 13 km s⁻¹.

An evolutionary model of the solar protoplanetary disk that includes the decrease of its viscosity with time and the accretion of gas from the interstellar medium shows that planetesimals formed simultaneously in two locations: at the water snowline (~5 au) and at the silicate sublimation line (~1 au), explaining the observed isotopic dichotomy of iron meteorites.

The MicrOmega imaging spectrometer performed a first characterization of the sample returned from asteroid Ryugu by Hayabusa2. Compositional homogeneity dominates down to millimetre scales, with signatures of hydrated phases and organics. At the submillimetre scale, NH-rich compounds and alteration products such as carbonates are detected.

The Hayabusa2 spacecraft returned 5.4 g of material from the asteroid Ryugu. A first analysis of the samples found an estimated density of 1,282 ± 231 kg m⁻³, considerably lower than even the most porous meteorites. Together with preliminary spectral analysis, these results indicate that Ryugu is similar to CI chondrites, but darker, more porous and more brittle.

The authors report the linear correlation of X-ray and Hα surface brightnesses in the material stripped from a galaxy, providing evidence for the mixing of galactic interstellar and hot intra-cluster medium as the origin of the multi-phase stripped tails observed previously.

A high-frequency quasiperiodic oscillation in the soft X-rays from unusual transient AT2018cow points towards the presence of a compact object in the remnant: either a neutron star with spin period of 4 ms or a low-mass black hole.

An energetic eruptive filament on EK Draconis most probably launched a coronal mass ejection with a mass ten times larger than the largest solar coronal mass ejection. Studying such ejections provides insight into stellar angular momentum loss and the habitability of orbiting planets.

The Zhurong rover has explored its landing site in Utopia Planitia, Mars, and is travelling south towards the highland–lowland boundary, focusing initially on the composition and physical properties of the rocks along the way.

The authors report time-series interferometric observations of a microlensing event from the ground. The lens images rotate during the series, giving the direction of motion of the lens and a very accurate Einstein ring radius. The lens is a 1.1-solar mass object at a distance of 5–6 kpc.

The 20-million-year-old, solar-type star V1298 Tau hosts a multiplanet system. The two outermost planets, gas giants with masses of 0.64 and 1.16 Jupiter masses, respectively, defy current formation models as their mass–radius relationship should be reached much later in the stages of planetary evolution.

The resonant chain of the TRAPPIST-1 planets is dynamically fragile, as small perturbations during its lifetime would have disrupted it. N-body simulations show that the system could not have interacted with more than 0.05 Earth masses of material after its formation. Thus, any water in the planets must come from the planets’ original accretion.

The Chang’e-4 rover observed a small crater formed less than one million years ago, finding glassy materials with spectral characteristics similar to those of carbonaceous chondrites that are identified as remnants of the original impactor that have not yet been affected by weathering processes.

The detection of the HF molecule in a lensed galaxy at z = 4.4 suggests a rapid chemical enrichment. Wolf–Rayet stars are the most likely providers of the fluorine.

General relativistic magnetohydrodynamic simulations with general relativistic radiative transfer reproduce remarkably well the broadband spectrum and morphology of the innermost jet of M87, allowing some first rough constraints on the spin of M87*.

As the Perseverance rover landed on the Martian surface, the sensors on NASA’s InSight Mars lander picked up no seismic or acoustic waves. This non-detection provides information on the crust and atmosphere of Mars.

A population study of near-infrared spectra of 19 hot giant planets shows a correlation between the strength of the 1.4 μm water band and temperature, which is broadly regulated by irradiation. However, the observed scatter around the mean is indicative of the effect of individual planetary formation pathways on the composition.

From its optical light curve, the white dwarf in the binary system TW Pictoris appears to be switching between two different intensities of accretion on timescales of hours. This behaviour is reminiscent of that seen in transitional millisecond pulsars, where the switching occurs several times a minute.

Globular cluster NGC 2005 in the Large Magellanic Cloud (LMC) bears the elemental hallmarks of being an accreted object: a surviving fragment of a galaxy that fell into the LMC long enough ago to have erased any dynamical signature of accretion.