Letters in 2019

The long-term evolution and stratigraphy of the CO₂ ice residual southern polar cap of Mars can be explained by a model that includes the active coupling of near-surface CO₂ with the atmosphere through the permeable H₂O ice layers.

Most stars in the Galactic nuclear disk formed at least 8 Gyr ago, with a starburst event about 1 Gyr ago that formed roughly 5% of its mass. This long quiescence has implications on when the Galactic bar was formed and its gas transport efficiency.

A detailed study of young stellar populations from high-quality stacked spectra of 28,663 massive early-type galaxies reveals ubiquitous residual star formation, measuring average mass fractions of 0.5% in young stars in the last 2 Gyr of their evolution.

Enceladus’s tiger stripes at the south pole formed in cascade and spaced equally after the first fracture—probably Baghdad Sulcus—was created by the release of accumulated tensile stress, caused in turn by secular cooling.

Spectropolarimetric data combined with high-resolution numerical simulations reveal a resonance cavity above a highly magnetic sunspot. This technique conclusively determines the three-dimensional structure of solar active regions from relatively commonplace two-dimensional data.

Protonated buckminsterfullerene (C₆₀H⁺) may be the most abundant interstellar analogue of C₆₀, and its gas-phase laboratory infrared spectrum is presented here. C₆₀H⁺ has more infrared active modes than C₆₀, giving it a richer spectrum that may be detectable with future infrared space missions.

Nineteen dwarf galaxies from the ALFALFA catalogue support previous observations of dwarf galaxies that suggested a deficiency in dark matter, challenging the formation theory of low-mass galaxies within the standard cold dark matter model.

Multi-frequency observations by the upgraded LIGO/Virgo and the proposed LISA space gravitational wave detector will be used to detect intermediate-mass black hole binaries in their inspiral, merger and ring-down phases out to cosmological distances.

The global geological map of Saturn’s moon Titan, created using radar observations from Cassini, shows a clear latitudinal dependence, with young dune fields dominant at the equator, plains at mid-latitudes and lakes and old dissected terrains at the poles. Titan’s geomorphology is controlled mostly by climate and topography.

An angular extension at gamma-ray energies of 52 arcseconds is detected for the Crab nebula, revealing the emission region of the highest-energy gamma rays; simulations of the electromagnetic emission provide a non-trivial test of our understanding of particle acceleration in the Crab nebula.

Diffuse X-ray emission is detected around an ultraluminous X-ray source, interpreted as a wind-powered expanding nebula. Its energetics suggests that a super-Eddington regime can be longer than the spin-up time of the central neutron star.

SPHERE at the VLT observed Hygiea, the fourth largest body in the main belt and the parent body of a big asteroid family, at unprecedented spatial resolution. Its unexpected spherical shape without any impact crater is explained by numerical simulations with a big impact that fluidized the body, reassembling it in a rotational equilibrium regime.

Thousands of compact and massive star clusters have formed at a steady rate over the past 1 Gyr around the central giant elliptical galaxy of the Perseus cluster, showing that progenitor globular clusters can form over cosmic history from cooled intracluster gas.

Idealized synchrotron emission, incorporating time-dependent electron cooling, can fit ~95% of all time-resolved spectra of single-peaked gamma-ray bursts. The presented analysis probes the microphysical processes operating within these ultra-relativistic outflows.

2I/Borisov is the first interstellar comet we know to have visited the Solar System, and the second interstellar visitor after ‘Oumuamua. Initial observations with the William Herschel Telescope and Gemini North Telescope reveal cometary features indistinguishable from Solar System comets, apart from its hyperbolic orbit.

ALMA observations have revealed a pair of symmetric spiral arms in the protostellar disk around HH 111 VLA 1. This discovery would seem to confirm hydrodynamical models that suggest that symmetric spiral structures arise in disks as a consequence of active accretion.

A highly magnified, strongly lensed star-forming galaxy is detected in X-rays. It is a low-mass, low-metallicity starburst that is a likely analogue to the first generation of galaxies, which may have played a role in reionizing the Universe.

Water ice on the surfaces of asteroids can survive only up to 10⁶ years. Laboratory experiments show that space weathering by energetic particles and micrometeorites can regenerate enough water by oxidizing organics and dehydrating minerals to explain the spectral features of hydration observed on asteroid surfaces.

GRB 070809 shows an unusual optical radiation component at t ≈ 0.47 d, at odds with a forward shock afterglow. If interpreted as a kilonova, the possibility arises of using short gamma-ray bursts to identify neutron star mergers beyond LIGO’s reach.

A theoretical thermonuclear runaway that converts helium to carbon in the interior of an evolved star—the helium core flash—has not yet been confirmed observationally. Asteroseismic models of hot subdwarf stars show that the helium core flash should be detectable with the Transiting Exoplanet Survey Satellite.