Letters in 2021

LOFAR observations of a galaxy group reveal multiple generations of cosmic-ray bubbles. The bubble buoyancy power offsets the radiative cooling of the intragroup medium, while magnetic fields prevent mixing between the bubbles and the external medium.

While pulsar timing observations are currently unable to distinguish a binary black hole astrophysical foreground from a cosmological background, integrated bounds on the ultra-low-frequency gravitational wave spectrum from other cosmological probes may help to break this degeneracy.

The authors present 19 detections of coherent low-frequency radio emission from M dwarfs using the Low Frequency Array. The sample includes both chromospherically active and quiescent stars, but radio luminosities are independent of coronal and chromospheric activity indicators.

Basalt samples from the Moon gathered during the Apollo 17 mission hold information on the lunar magnetic field as it was 3.7 billion years ago. Its mean intensity was ~50 μT and its inclination 34 ± 10°. Such results suggest that the lunar dynamo was active at the time and was axially aligned and dipolar.

[C ii] 158 μm emission associated with a strong low-ionization absorber at z = 5.978 indicates a dark matter halo mass of around 4 × 10¹¹ solar masses, one to two orders of magnitude more massive than typical values predicted from cosmological simulations.

A high-resolution calculation reproducing solar-like differential rotation shows that the strong magnetic field generated by a small-scale dynamo has a substantial impact on thermal convection and is a key step in understanding the 11 yr solar cycle.

The discovery of a multiply imaged, probably of type Ia, supernova in a galaxy at redshift 1.95 enables a time-delay measurement with an uncertainty of <1%. The prediction that a new image will appear in the year 2037 ± 2 allows the use of this system as a cosmological probe.

Cosmological hydrodynamical simulations of the ΛCDM Universe show that isolated quenched ultra-diffuse galaxies are formed as backsplash galaxies that were once satellites of another galactic group or cluster halo but are today a few megaparsecs away from them.

An ab initio closed-form analytical solution for the geometric albedo and the integral phase function, which is valid for any law of reflection provided that it depends only on the scattering angle, is proposed. Such solutions can be applied to determine the scattering properties of planetary atmospheres or surfaces.

The nearby Ophiuchus star-forming region should be able to give us insight into the incorporation of short-lived radionuclides into the early Solar System. These radionuclides, such as ²⁶Al, originated from multiple sources and gently warmed the protosolar disk over an extended period. Another heating event, therefore—perhaps an FU Orionis-type outburst—presumably led to the resetting of the radiogenic clock.

Like a seismograph, Saturn’s rings are sensitive to oscillations coming from the planet’s interior. State-of-the-art modelling shows that Cassini’s measurements of ring waves point to a convectively stable diffuse core within Saturn, which extends for 60% of its radius and contains 17 Earth masses of ice and rock.

The spectral properties of a short gamma-ray burst indicate that, contrary to expectations, it arose from the collapse of a massive star rather than from a compact binary merger. This discovery also confirms that most collapsars do not produce ultra-relativistic jets.

A gamma-ray burst (GRB) is reported to show a sharp 1-second spike, characteristic of short GRBs, but with other observational properties resembling those of long GRBs. This burst may belong to a class of core-collapse-origin GRBs with genuinely short durations.

The millimetre image of the Centaurus A nucleus by the Event Horizon Telescope reveals a highly collimated, asymmetrically edge-brightened jet. The source’s event horizon shadow should be visible at terahertz frequencies, consistent with the universal scale invariance of black holes.

Chandra X-ray spectroscopy reveals a 10⁸ K plasma outflow in the low-luminosity active galactic nucleus M81. Magnetohydrodynamical simulations show this energetic wind to originate from a hot accretion flow and impact its host galaxy.

A full 2D radiation–hydrodynamic model of a protoplanetary disk shows that rocky planets can be formed early, and not tens of million years after the dispersal of the gas disk as usually assumed, by means of gas-driven migration of planetesimals around 1 au. The model reproduces well the structure of the inner Solar System.

Three planets orbit the Sun-like star ν² Lupi. CHEOPS data show that all of them are transiting and show remarkable diversity. In particular, dry and gas-poor inner planet b has experienced extensive atmospheric loss, while planets c and d are water rich and have a small gaseous envelope of primordial origin.

Electron-capture supernovae are thought to come from progenitors with a narrow range of masses, and thus they are rare. Here the authors present six indicators of an electron-capture supernova origin, and find that supernova 2018zd fulfils all six criteria.

The Hayabusa2 spacecraft found dark boulders with very high porosity (>70%, as high as cometary nuclei) at the bottom of small craters on Ryugu. Such boulders are probably the most pristine parts of the planetesimals that formed Ryugu’s parent body and might have been captured by Hayabusa2 sampling.

Leveraging asteroseismology, stellar abundances and kinematics to derive precise ages for a sample of 95 stars, Montalbán et al. determine that the Milky Way was already host to a substantial population of stars when it was just 3.8 billion years old, at the time of the Gaia-Enceladus accretion event.