Showing papers in "Nature Astronomy in 2019"

Tensions between the early and late Universe

Abstract: A Kavli Institute for Theoretical Physics workshop in July 2019 directed attention to the Hubble constant discrepancy. New results showed that it does not appear to depend on the use of any one method, team or source. Proposed solutions focused on the pre-recombination era.

Microlensing constraints on primordial black holes with Subaru/HSC Andromeda observations

Abstract: Primordial black holes (PBHs) have long been suggested as a viable candidate for the elusive dark matter. The abundance of such PBHs has been constrained using a number of astrophysical observations, except for a hitherto unexplored mass window of MPBH = [10−14, 10−9] solar masses. Here we carry out a dense-cadence, 7-hour-long observation of M31 with the Subaru Hyper Suprime-Cam (HSC) to search for microlensing of stars in M31 by PBHs lying in the halo regions of the Milky Way and M31. Given our simultaneous monitoring of tens of millions of stars in M31, if such light PBHs make up a significant fraction of dark matter, we expect to find many microlensing events. However, we identify only a single candidate event, which translates into stringent upper bounds on the abundance of PBHs in the mass range MPBH ≃ [10−11, 10−6] solar masses. The abundance of primordial black holes in the Galactic halo is constrained through their microlensing of stars in M31. Despite monitoring tens of millions of stars, only a single candidate event is found, providing stringent upper bounds on their abundance.

Two accreting protoplanets around the young star PDS 70

Abstract: Newly forming protoplanets are expected to create cavities and substructures in young, gas-rich protoplanetary disks1–3, but they are difficult to detect as they could be confused with disk features affected by advanced image analysis techniques4,5. Recently, a planet was discovered inside the gap of the transitional disk of the T Tauri star PDS 706,7. Here, we report on the detection of strong Hα emission from two distinct locations in the PDS 70 system, one corresponding to the previously discovered planet PDS 70 b, which confirms the earlier Hα detection8, and another located close to the outer edge of the gap, coinciding with a previously identified bright dust spot in the disk and with a small opening in a ring of molecular emission6,7,9. We identify this second Hα peak as a second protoplanet in the PDS 70 system. The Hα emission spectra of both protoplanets indicate ongoing accretion onto the protoplanets10,11, which appear to be near a 2:1 mean motion resonance. Our observations show that adaptive-optics-assisted, medium-resolution integral field spectroscopy with MUSE12 targeting accretion signatures will be a powerful way to trace ongoing planet formation in transitional disks at different stages of their evolution. Finding more young planetary systems in mean motion resonance would give credibility to the Grand Tack hypothesis in which Jupiter and Saturn migrated in a resonance orbit during the early formation period of our Solar System13. Two Hα emission peaks are detected within the disk of the T Tauri star PDS 70: one corresponds to protoplanet PDS 70 b, and the other is associated with a second accreting planet of few Jupiter masses at ~35 au. The two protoplanets are near 2:1 mean motion resonance, supporting migration scenarios of giant planets during planetary formation.

KAGRA: 2.5 Generation Interferometric Gravitational Wave Detector

Abstract: The recent detections of gravitational waves (GWs) reported by the LIGO and Virgo collaborations have made a significant impact on physics and astronomy. A global network of GW detectors will play a key role in uncovering the unknown nature of the sources in coordinated observations with astronomical telescopes and detectors. Here we introduce KAGRA, a new GW detector with two 3 km baseline arms arranged in an ‘L’ shape. KAGRA’s design is similar to the second generations of Advanced LIGO and Advanced Virgo, but it will be operating at cryogenic temperatures with sapphire mirrors. This low-temperature feature is advantageous for improving the sensitivity around 100 Hz and is considered to be an important feature for the third-generation GW detector concept (for example, the Einstein Telescope of Europe or the Cosmic Explorer of the United States). Hence, KAGRA is often called a 2.5-generation GW detector based on laser interferometry. KAGRA’s first observation run is scheduled in late 2019, aiming to join the third observation run of the advanced LIGO–Virgo network. When operating along with the existing GW detectors, KAGRA will be helpful in locating GW sources more accurately and determining the source parameters with higher precision, providing information for follow-up observations of GW trigger candidates.

Cosmological constraints from the Hubble diagram of quasars at high redshifts

Abstract: The concordance model (Λ cold dark matter (ΛCDM) model, where Λ is the cosmological constant) reproduces the main current cosmological observations1–4 assuming the validity of general relativity at all scales and epochs and the presence of CDM and of Λ, equivalent to dark energy with a constant density in space and time. However, the ΛCDM model is poorly tested in the redshift interval between the farthest observed type Ia supernovae5 and the cosmic microwave background. We present measurements of the expansion rate of the Universe based on a Hubble diagram of quasars. Quasars are the most luminous persistent sources in the Universe, observed up to redshifts of z ≈ 7.5 (refs. 6,7). We estimate their distances following a method developed by our group8–10, based on the X-ray and ultraviolet emission of the quasars. The distance modulus/redshift relation of quasars at z < 1.4 is in agreement with that of supernovae and with the concordance model. However, a deviation from the ΛCDM model emerges at higher redshift, with a statistical significance of ~4σ. If an evolution of the dark energy equation of state is allowed, the data suggest dark energy density increasing with time. The concordance cosmology model is poorly tested at high redshifts. Here the expansion rate of the Universe in the range 0.5 < z < 5.1 is measured based on a Hubble diagram of quasars, whose distances are estimated from their X-ray and ultraviolet emission.

Evidence for widespread hydrated minerals on asteroid (101955) Bennu.

Abstract: Early spectral data from the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission reveal evidence for abundant hydrated minerals on the surface of near-Earth asteroid (101955) Bennu in the form of a near-infrared absorption near 2.7 µm and thermal infrared spectral features that are most similar to those of aqueously altered CM-type carbonaceous chondrites. We observe these spectral features across the surface of Bennu, and there is no evidence of substantial rotational variability at the spatial scales of tens to hundreds of metres observed to date. In the visible and near-infrared (0.4 to 2.4 µm) Bennu’s spectrum appears featureless and with a blue (negative) slope, confirming previous ground-based observations. Bennu may represent a class of objects that could have brought volatiles and organic chemistry to Earth.

A Hubble constant measurement from superluminal motion of the jet in GW170817

Abstract: The Hubble constant (H0) measures the current expansion rate of the Universe, and plays a fundamental role in cosmology. Tremendous effort has been dedicated over the past decades to measure H0 (refs. 1–10). Gravitational wave (GW) sources accompanied by electromagnetic (EM) counterparts offer an independent standard siren measurement of H0 (refs. 11–13), as demonstrated following the discovery of the neutron star merger, GW170817 (refs. 14–16). This measurement does not assume a cosmological model and is independent of a cosmic distance ladder. The first joint analysis of the GW signal from GW170817 and its EM localization led to a measurement of $$H_0 = 74_{ - 8}^{ + 16}\,{\mathrm{km}}\,{\mathrm{s}}^{-1}\,{\mathrm{Mpc}}^{-1}$$ (median and symmetric 68% credible interval)13. In this analysis, the degeneracy in the GW signal between the source distance and the observing angle dominated the H0 measurement uncertainty. Recently, tight constraints on the observing angle using high angular resolution imaging of the radio counterpart of GW170817 have been obtained17. Here, we report an improved measurement $$H_0 = 70.3_{ - 5.0}^{ + 5.3}\,{\mathrm{km}}\,{\mathrm{s}}^{-1}\,{\mathrm{Mpc}}^{-1}$$ by using these new radio observations, combined with the previous GW and EM data. We estimate that 15 more GW170817-like events, having radio images and light curve data, as compared with 50–100 GW events without such data18,19, will potentially resolve the tension between the Planck and Cepheid–supernova measurements. Combining gravitational-wave and electromagnetic data with new radio observations of GW170817, an improved measurement of $$H_0 = 70.3_{-5.0}^{+5.3}\, {\mathrm{km}}\, {\mathrm{s}}^{-1}\,{\mathrm{Mpc}}^{-1}$$ is derived. Fifteen more GW170817-like events with radio data could resolve the Hubble constant tension.

Properties of Rubble-Pile Asteroid (101955) Bennu from OSIRIS-REx Imaging and Thermal Analysis

Abstract: Establishing the abundance and physical properties of regolith and boulders on asteroids is crucial for understanding the formation and degradation mechanisms at work on their surfaces. Using images and thermal data from NASA’s Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft, we show that asteroid (101955) Bennu’s surface is globally rough, dense with boulders, and low in albedo. The number of boulders is surprising given Bennu’s moderate thermal inertia, suggesting that simple models linking thermal inertia to particle size do not adequately capture the complexity relating these properties. At the same time, we find evidence for a wide range of particle sizes with distinct albedo characteristics. Our findings imply that ages of Bennu’s surface particles span from the disruption of the asteroid’s parent body (boulders) to recent in situ production (micrometre-scale particles).

Massive Stars as Major Factories of Galactic Cosmic Rays

Abstract: The identification of the main contributors to the locally observed fluxes of cosmic rays is a prime objective in the resolution of the long-standing enigma of the source of cosmic rays. We report on a compelling similarity of the energy and radial distributions of multi-TeV cosmic rays extracted from observations of very-high-energy γ-rays towards the Galactic Centre and two prominent clusters of young massive stars, Cygnus OB2 and Westerlund 1. We interpret this resemblance as evidence that cosmic rays responsible for the diffuse very-high-energy γ-ray emission from the Galactic Centre are accelerated by the ultracompact stellar clusters located in the heart of the Galactic Centre. The derived 1/r decrement of the cosmic ray density with the distance from a star cluster is a distinct signature of continuous cosmic ray injection into the interstellar medium over a few million years. The lack of brightening of the γ-ray images towards the stellar clusters excludes the leptonic origin of γ-ray radiation. The hard, ∝E−2.3-type, power-law energy spectra of parent protons continues up to ~1 PeV. The efficiency of conversion of the kinetic energy of stellar winds to cosmic rays can be as high as 10%, implying that young massive stars may operate as proton PeVatrons with a dominant contribution to the flux of the highest-energy Galactic cosmic rays. Ultracompact stellar clusters in the Galactic Centre are likely to be major contributors to the Galactic cosmic ray flux in the multi-TeV energy range. Observations of the diffuse gamma-ray emission from the Galactic Centre and two young massive star clusters correlate with the cosmic-ray distribution.

Uncovering the birth of the Milky Way through accurate stellar ages with Gaia

Abstract: Knowledge of the ages of the stars formed over a galaxy’s lifetime is fundamental to an understanding of its formation and evolution. However, stellar ages are difficult to obtain since they cannot be measured from observations, but require comparison with stellar models1. Alternatively, age distributions can be derived by applying the robust technique of colour–magnitude diagram fitting2, which until now has been used primarily to study nearby galaxies. Accurate distances to individual Milky Way stars now provided by the Gaia spacecraft mission3 have allowed us to derive ages from a thick-disk colour–magnitude diagram and from the two-sequenced colour–magnitude diagram of the kinematically hot local halo4, whose blue sequence has been linked to a major accretion event, Gaia-Enceladus5,6. Because accurate stellar ages were lacking, the time of the merger and its role in our Galaxy’s early evolution remained unclear. Here we show that the stars in both halo sequences share identical age distributions, and are older than most of the thick-disk stars. The sharp halo age distribution cutoff at ten billion years ago can be identified with the time of accretion of Gaia-Enceladus to the Milky Way. Together with state-of-the-art cosmological simulations of galaxy formation7, these robust ages allow us to order the early sequence of events that shaped our Galaxy. We identify the red-sequence stars as the first stars formed within the Milky Way progenitor, and their kinematics indicate that these stars constitute the long-sought in situ halo of the Milky Way. Stars in the Milky Way halo are older than those in its thick disk, with their ten-billion-year age distribution cutoff marking the accretion of Gaia-Enceladus to the Milky Way. The red-sequence halo stars are those formed first in the Milky Way progenitor, constituting its long-sought in situ halo.

Water vapour in the atmosphere of the habitable-zone eight-Earth-mass planet K2-18 b

Abstract: In the past decade, observations from space and the ground have found water to be the most abundant molecular species, after hydrogen, in the atmospheres of hot, gaseous extrasolar planets1–5. Being the main molecular carrier of oxygen, water is a tracer of the origin and the evolution mechanisms of planets. For temperate, terrestrial planets, the presence of water is of great importance as an indicator of habitable conditions. Being small and relatively cold, these planets and their atmospheres are the most challenging to observe, and therefore no atmospheric spectral signatures have so far been detected6. Super-Earths—planets lighter than ten Earth masses—around later-type stars may provide our first opportunity to study spectroscopically the characteristics of such planets, as they are best suited for transit observations. Here, we report the detection of a spectroscopic signature of water in the atmosphere of K2-18 b—a planet of eight Earth masses in the habitable zone of an M dwarf7—with high statistical confidence (Atmospheric Detectability Index5 = 5.0, ~3.6σ (refs. 8,9)). In addition, the derived mean molecular weight suggests an atmosphere still containing some hydrogen. The observations were recorded with the Hubble Space Telescope/Wide Field Camera 3 and analysed with our dedicated, publicly available, algorithms5,9. Although the suitability of M dwarfs to host habitable worlds is still under discussion10–13, K2-18 b offers an unprecedented opportunity to gain insight into the composition and climate of habitable-zone planets. K2-18 b is a planet with a mass around eight times that of the Earth that lies within the standard habitable zone of its star. Hubble spectra show the presence of an atmosphere around K2-18 b containing significant amounts of water vapour (up to a few tens of per cent, depending on the spectral model), but also a non-negligible amount of H2–He.

The dynamic geophysical environment of (101955) Bennu based on OSIRIS-REx measurements

Abstract: The top-shaped morphology characteristic of asteroid (101955) Bennu, often found among fast-spinning asteroids and binary asteroid primaries, may have contributed substantially to binary asteroid formation. Yet a detailed geophysical analysis of this morphology for a fast-spinning asteroid has not been possible prior to the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission. Combining the measured Bennu mass and shape obtained during the Preliminary Survey phase of the OSIRIS-REx mission, we find a notable transition in Bennu’s surface slopes within its rotational Roche lobe, defined as the region where material is energetically trapped to the surface. As the intersection of the rotational Roche lobe with Bennu’s surface has been most recently migrating towards its equator (given Bennu’s increasing spin rate), we infer that Bennu’s surface slopes have been changing across its surface within the last million years. We also find evidence for substantial density heterogeneity within this body, suggesting that its interior is a mixture of voids and boulders. The presence of such heterogeneity and Bennu’s top shape are consistent with spin-induced failure at some point in its past, although the manner of its failure cannot yet be determined. Future measurements by the OSIRIS-REx spacecraft will provide insight into and may resolve questions regarding the formation and evolution of Bennu’s top-shape morphology and its link to the formation of binary asteroids.

Kinematic detection of a planet carving a gap in a protoplanetary disk

Abstract: We still do not understand how planets form or why extrasolar planetary systems are so different from our own Solar System. However, the past few years have dramatically changed our view of the disks of gas and dust around young stars. Observations with the Atacama Large Millimeter/submillimeter Array and extreme adaptive-optics systems have revealed that most—if not all—disks contain substructure, including rings and gaps1–3, spirals4–6, azimuthal dust concentrations7 and shadows cast by misaligned inner disks5,8. These features have been interpreted as signatures of newborn protoplanets, but the exact origin is unknown. Here we report the kinematic detection of a few-Jupiter-mass planet located in a gas and dust gap at 130 au in the disk surrounding the young star HD 97048. An embedded planet can explain both the disturbed Keplerian flow of the gas, detected in CO lines, and the gap detected in the dust disk at the same radius. While gaps appear to be a common feature in protoplanetary disks2,3, we present a direct correspondence between a planet and a dust gap, indicating that at least some gaps are the result of planet–disk interactions. Pinte et al. report the kinematic detection of a few-Jupiter-mass planet orbiting at 130 au from the young star HD 97048. The radial position of the planet coincides with a gap in both the gas and dust components of the protoplanetary disk, showing that at least some gaps can be linked to the presence of planets.

The origin of radio emission from radio-quiet active galactic nuclei

Abstract: The central nuclei of galaxies, where supermassive black holes (SMBHs) are thought to reside, can experience phases of activity when they become active galactic nuclei (AGNs). An AGN can eject winds and jets and produce radiation across the entire electromagnetic spectrum. The fraction of the bolometric emission in the radio spans a factor of approximately 105 across the different classes of AGNs. The weakest radio sources, radio-quiet (RQ) AGNs, are typically 1,000 times fainter than the radio-loud (RL) AGNs, and represent the majority of the AGN population. In RQ AGNs, the absence of luminous jets allows us to probe radio emission from a wide range of possible mechanisms: star formation, AGN-driven wind, free-free emission from photoionized gas, low-power jets and the innermost accretion disk coronal activity. All these mechanisms can now be probed with unprecedented precision and spatial resolution, owing to the current and forthcoming generation of highly sensitive radio arrays. This article reviews radio emission mechanisms in radio-quiet active galactic nuclei (AGNs), from star formation and AGN winds, to free-free emission from photoionized gas and AGN disk coronal activity. These mechanisms can be probed by sensitive radio observatories.

The many flavours of photometric redshifts

Abstract: Since more than 70 years ago, the colours of galaxies derived from flux measurements at different wavelengths have been used to estimate their cosmological distances. Such distance measurements, called photometric redshifts, are necessary for many scientific projects, ranging from investigations of the formation and evolution of galaxies and active galactic nuclei to precision cosmology. The primary benefit of photometric redshifts is that distance estimates can be obtained relatively cheaply for all sources detected in photometric images. The drawback is that these cheap estimates have low precision compared with resource-expensive spectroscopic ones. The methodology for estimating redshifts has been through several revolutions in recent decades, triggered by increasingly stringent requirements on the photometric redshift accuracy. Here, we review the various techniques for obtaining photometric redshifts, from template-fitting to machine learning and hybrid schemes. We also describe state-of-the-art results on current extragalactic samples and explain how survey strategy choices affect redshift accuracy. We close with a description of the photometric redshift efforts planned for upcoming wide-field surveys, which will collect data on billions of galaxies, aiming to investigate, among other matters, the stellar mass assembly and the nature of dark energy. A review of the various techniques to obtain photometric redshifts, from template-fitting to machine learning and hybrid schemes, and a description of the latest results on extragalactic samples and how survey strategy choices impact redshift accuracy.

A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds

Abstract: With no analogues in the Solar System, the discovery of thousands of exoplanets with masses and radii intermediate between Earth and Neptune was one of the big surprises of exoplanet science. These super-Earths and sub-Neptunes probably represent the most common outcome of planet formation. Mass and radius measurements indicate a diversity in bulk composition much wider than for gas giants; however, direct spectroscopic detections of molecular absorption and constraints on the gas mixing ratios have largely remained limited to planets more massive than Neptune. Here we analyse a combined Hubble/Spitzer Space Telescope dataset of 12 transits and 20 eclipses of the sub-Neptune exoplanet GJ 3470 b, whose mass of 12.6 M⊕ places it near the halfway point between previously studied Neptune-like exoplanets (22–23 M⊕) and exoplanets known to have rocky densities (7 M⊕). Obtained over many years, our dataset provides a robust detection of water absorption (>5σ) and a thermal emission detection from the lowest irradiated planet to date. We reveal a low-metallicity, hydrogen-dominated atmosphere similar to that of a gas giant, but strongly depleted in methane gas. The low metallicity (O/H = 0.2–18.0) sets important constraints on the potential planet formation processes at low masses as well as the subsequent accretion of solids. The low methane abundance indicates that methane is destroyed much more efficiently than previously predicted, suggesting that the CH_4/CO transition curve has to be revisited for close-in planets. Finally, we also find a sharp drop in the cloud opacity at 2–3 µm, characteristic of Mie scattering, which enables narrow constraints on the cloud particle size and makes GJ 3470 b a key target for mid-infrared characterization with the James Webb Space Telescope.

Molybdenum isotopic evidence for the late accretion of outer Solar System material to Earth

Abstract: Earth grew through collisions with Moon-sized to Mars-sized planetary embryos from the inner Solar System, but it also accreted material from greater heliocentric distances1,2, including carbonaceous chondrite-like bodies, the likely source of Earth’s water and highly volatile species3,4. Understanding when and how this material was added to Earth is critical for constraining the dynamics of terrestrial planet formation and the fundamental processes by which Earth became habitable. However, earlier studies inferred very different timescales for the delivery of carbonaceous chondrite-like bodies, depending on assumptions about the nature of Earth’s building materials5–11. Here we show that the Mo isotopic composition of Earth’s primitive mantle falls between those of the non-carbonaceous and carbonaceous reservoirs12–15, and that this observation allows us to quantify the accretion of carbonaceous chondrite-like material to Earth independently of assumptions about its building blocks. As most of the Mo in the primitive mantle was delivered by late-stage impactors10, our data demonstrate that Earth accreted carbonaceous bodies late in its growth history, probably through the Moon-forming impact. This late delivery of carbonaceous material probably resulted from an orbital instability of the gas giant planets, and it demonstrates that Earth’s habitability is strongly tied to the very late stages of its growth. Measurements of Mo in meteorites constrain the time when the Earth accreted carbonaceous material from the outer Solar System (a likely source of Earth’s water and volatiles) to late in the Earth’s growth history—probably in the same event that formed the Moon.

Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu

Abstract: C-type asteroids are among the most pristine objects in the Solar System, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith-covered surface with low thermal conductivity and particle sizes in the centimetre range. This includes observations of C-type asteroid (162173) Ryugu1–3. However, on arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover of sand- to pebble-sized particles was found to be absent4,5 (R.J. et al., manuscript in preparation). Rather, the surface is largely covered by cobbles and boulders, seemingly incompatible with the remote-sensing infrared observations. Here we report on in situ thermal infrared observations of a boulder on the C-type asteroid Ryugu. We found that the boulder’s thermal inertia was much lower than anticipated based on laboratory measurements of meteorites, and that a surface covered by such low-conductivity boulders would be consistent with remote-sensing observations. Our results furthermore indicate high boulder porosities as well as a low tensile strength in the few hundred kilopascal range. The predicted low tensile strength confirms the suspected observational bias6 in our meteorite collections, as such asteroidal material would be too frail to survive atmospheric entry7. The MASCOT lander observed a boulder on the surface of asteroid Ryugu up close. The boulder’s low thermal inertia is closer to fine regolith or comets rather than stony boulders, indicating high porosity and low tensile strength. Orbit measurements confirm that Ryugu’s surface is covered with similar boulders.

A comprehensive three-dimensional radiative magnetohydrodynamic simulation of a solar flare

Abstract: Solar and stellar flares are the most intense emitters of X-rays and extreme ultraviolet radiation in planetary systems1,2. On the Sun, strong flares are usually found in newly emerging sunspot regions3. The emergence of these magnetic sunspot groups leads to the accumulation of magnetic energy in the corona. When the magnetic field undergoes abrupt relaxation, the energy released powers coronal mass ejections as well as heating plasma to temperatures beyond tens of millions of kelvins. While recent work has shed light on how magnetic energy and twist accumulate in the corona4 and on how three-dimensional magnetic reconnection allows for rapid energy release5,6, a self-consistent model capturing how such magnetic changes translate into observable diagnostics has remained elusive. Here, we present a comprehensive radiative magnetohydrodynamics simulation of a solar flare capturing the process from emergence to eruption. The simulation has sufficient realism for the synthesis of remote sensing measurements to compare with observations at visible, ultraviolet and X-ray wavelengths. This unifying model allows us to explain a number of well-known features of solar flares7, including the time profile of the X-ray flux during flares, origin and temporal evolution of chromospheric evaporation and condensation, and sweeping of flare ribbons in the lower atmosphere. Furthermore, the model reproduces the apparent non-thermal shape of coronal X-ray spectra, which is the result of the superposition of multi-component super-hot plasmas8 up to and beyond 100 million K. A state-of-the-art magnetohydrodynamic simulation of a solar flare from emergence to eruption is able to reproduce observations at visible, UV and X-ray wavelengths, and suggests that non-thermal particles at high energy may play a less prominent role than usually assumed in flare models.

Observing black holes spin

Abstract: The spin of a black hole retains the memory of how the black hole grew, and can be a potent source of energy for powering relativistic jets. To understand the diagnostic power and astrophysical significance of black hole spin, however, we must first devise observational methods for measuring spin. Here, I describe the current state of black hole spin measurements, highlighting the progress made by X-ray astronomers, as well as the current excitement of gravitational wave- and radio astronomy-based techniques. Today’s spin measurements are already constraining models for the growth of supermassive black holes and giving new insights into the dynamics of stellar core collapse, as well as hinting at the physics of relativistic jet production. Future X-ray, radio and gravitational wave observatories will transform black hole spin into a precision tool for astrophysics and test fundamental theories of gravity. Current black hole spin measurements, in X-rays, radio and gravitational waves, are already constraining models for the growth of black holes, the dynamics of stellar core-collapse and the physics of relativistic jet production.

Cosmic ray measurements from Voyager 2 as it crossed into interstellar space

Abstract: The interaction of the interstellar and solar winds is complex, as revealed by differences in intensities and anisotropies of low-energy ions (>0.5 MeV per nucleon) originating inside the heliosphere and those of higher-energy Galactic cosmic rays (>70 MeV per nucleon) originating outside, in the Milky Way. On 5 November 2018, Voyager 2 observed a sharp decrease in the intensity of low-energy ions and a simultaneous increase in the intensity of cosmic rays, indicating that Voyager 2 had crossed the heliopause at 119 au and entered interstellar space about six years after Voyager 1. Unlike Voyager 1, which found that two interstellar flux tubes had invaded the heliosheath and served as precursors to the heliopause, Voyager 2 found no similar precursors. However, just beyond the heliopause Voyager 2 discovered a boundary layer, in which low-energy particles streamed outward along the magnetic field and cosmic ray intensities were only 90% of those further out. As it crossed the heliopause, Voyager 2 observed a sharp decrease in measurements of the low-energy ions that originate within the heliosphere, and an increase in the cosmic rays from the Milky Way, without any of the precursor flux tubes that Voyager 1 experienced. Outside the heliopause, a boundary layer exists.

Trans-Neptunian binaries as evidence for planetesimal formation by the streaming instability

Abstract: A critical step toward the emergence of planets in a protoplanetary disk is the accretion of planetesimals, bodies 1–1,000 km in size, from smaller disk constituents. This process is poorly understood, partly because we lack good observational constraints on the complex physical processes that contribute to planetesimal formation1. In the outer Solar System, the best place to look for clues is the Kuiper belt, where icy planetesimals survive to this day. Here we report evidence that Kuiper belt planetesimals formed by the streaming instability, a process in which aerodynamically concentrated clumps of pebbles gravitationally collapse into 100-kilometre-class bodies2. Gravitational collapse has previously been suggested to explain the ubiquity of equal-sized binaries in the Kuiper belt3–5. We analyse new hydrodynamical simulations of the streaming instability to determine the model expectations for the spatial orientation of binary orbits. The predicted broad inclination distribution with approximately 80% of prograde binary orbits matches the observations of trans-Neptunian binaries6. The formation models that imply predominantly retrograde binary orbits (for example, ref. 7) can be ruled out. Given its applicability over a wide range of protoplanetary disk conditions8, it is expected that the streaming instability also seeded planetesimal formation elsewhere in the Solar System, and beyond. The predominantly prograde orientation and broad inclination distribution of trans-Neptunian binary objects is reproduced by a three-dimensional hydrodynamical simulation of planetesimal formation driven by the streaming instability, showing evidence of the activation of the streaming instability in the solar protoplanetary disk.

Modelling the coincident observation of a high-energy neutrino and a bright blazar flare

Abstract: In September 2017, the IceCube Neutrino Observatory recorded a very-high-energy neutrino in directional coincidence with a blazar in an unusually bright gamma-ray state, TXS0506 + 056 (refs1,2). Blazars are prominent photon sources in the Universe because they harbour a relativistic jet whose radiation is strongly collimated and amplified. High-energy atomic nuclei known as cosmic rays can produce neutrinos; thus, the recent detection may help in identifying the sources of the diffuse neutrino flux3 and the energetic cosmic rays. Here we report a self-consistent analysis of the physical relation between the observed neutrino and the blazar, in particular the time evolution and spectral behaviour of neutrino and photon emission. We demonstrate that a moderate enhancement in the number of cosmic rays during the flare can yield a very strong increase in the neutrino flux, which is limited by co-produced hard X-rays and teraelectronvolt gamma rays. We also test typical radiation models4,5 for compatibility and identify several model classes6,7 as incompatible with the observations. We investigate to what degree the findings can be generalized to the entire population of blazars, determine the relation between their output in photons, neutrinos and cosmic rays, and suggest how to optimize the strategy of future observations. With a lepto-hadronic jet model and recent multi-messenger data, it is shown that a moderate enhancement in cosmic rays during a blazar flare can yield an increased neutrino flux, which is limited by co-produced hard X-rays and TeV gamma rays.

Identification of the long stellar stream of the prototypical massive globular cluster ω Centauri

Abstract: Omega Centauri (ω Cen) is the Milky Way’s most massive globular cluster, and has long been suspected of being the remnant core of an accreted dwarf galaxy. If this scenario is correct, ω Cen should be tidally limited and tidal debris should be spread along its orbit. Here we use N-body simulations to show that the recently discovered ‘Fimbulthul’ structure is the long-sought-for tidal stream of ω Cen, extending up to 28° from the cluster. Follow-up high-resolution spectroscopy of five stream stars shows that they are closely grouped in velocity, and have metallicities consistent with having originated in that cluster. Informed by our N-body simulations, we devise a selection filter that we apply to Gaia mission data to also uncover the stream in the highly contaminated and crowded field within 10° of ω Cen. Further modelling of the stream may help to constrain the dynamical history of the dwarf galaxy progenitor of this disrupting system and guide future searches for its remnant stars in the Milky Way. Stellar streams are the outstretched remnants of globular clusters torn apart by tidal forces. A data-driven search method for identifying streams finds stream material from ω Centauri, the most massive globular cluster within the Milky Way.

A water budget dichotomy of rocky protoplanets from 26Al-heating

Abstract: In contrast to the water-poor planets of the inner Solar System, stochasticity during planetary formation1,2 and order-of-magnitude deviations in exoplanet volatile contents3 suggest that rocky worlds engulfed in thick volatile ice layers4,5 are the dominant family of terrestrial analogues6,7 among the extrasolar planet population. However, the distribution of compositionally Earth-like planets remains insufficiently constrained3, and it is not clear whether the Solar System is a statistical outlier or can be explained by more general planetary formation processes. Here we use numerical models of planet formation, evolution and interior structure to show that a planet’s bulk water fraction and radius are anti-correlated with initial 26Al levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals8 before their accretion onto larger protoplanets and yields a system-wide correlation9,10 of planetary bulk water abundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets11. Qualitatively, our models suggest two main scenarios for the formation of planetary systems: high-26Al systems, like our Solar System, form small, water-depleted planets, whereas those devoid of 26Al predominantly form ocean worlds. For planets of similar mass, the mean planetary transit radii of the ocean planet population can be up to about 10% larger than for planets from the 26Al-rich formation scenario. The initial abundance of 26Al in a planetary system determines the surface environment of its solid planets. High levels of 26Al will dehydrate planetesimals and produce water-poor worlds similar to the terrestrial planets in our Solar System; sub-solar levels of 26Al will preferentially generate ocean planets.

Low-frequency gravity waves in blue supergiants revealed by high-precision space photometry

Abstract: Almost all massive stars explode as supernovae and form a black hole or neutron star. The remnant mass and the impact of the chemical yield on subsequent star formation and galactic evolution strongly depend on the internal physics of the progenitor star, which is currently not well understood. The theoretical uncertainties of stellar interiors accumulate with stellar age, which is particularly pertinent for the blue supergiant phase. Stellar oscillations represent a unique method of probing stellar interiors, yet inference for blue supergiants is hampered by a dearth of observed pulsation modes. Here we report the detection of diverse variability in blue supergiants using the K2 and TESS space missions. The discovery of pulsation modes or an entire spectrum of low-frequency gravity waves in these stars allow us to map the evolution of hot massive stars towards the ends of their lives. Future asteroseismic modelling will provide constraints on ages, core masses, interior mixing, rotation and angular momentum transport. The discovery of variability in blue supergiants is a step towards a data-driven empirical calibration of theoretical evolution models for the most massive stars in the Universe. Leveraging the precision of K2 and TESS, Bowman et al. have detected variability in galactic and Magellanic blue supergiants that is due to low-frequency gravity waves in their interiors.

Rotational disruption of dust grains by radiative torques in strong radiation fields

Abstract: Massive stars, supernovae, and kilonovae are among the most luminous radiation sources in the Universe. Observations usually show near- to mid-infrared (NIR–MIR, λ ≈ 1–5 μm) emission excess from H ii regions around young massive star clusters. Early-phase observations in optical-to-NIR wavelengths of type Ia supernovae also reveal unusual properties of dust extinction and dust polarization. The most common explanation for such NIR−MIR excess and unusual dust properties is the predominance of small grains (size a ≲ 0.05 μm) relative to large grains (a ≳ 0.1 μm) in the local environment of these strong radiation sources. However, why small grains might be predominant in these environments is unclear. Here we report a mechanism of dust destruction based on centrifugal stress within extremely fast-rotating grains spun-up by radiative torques, which we term radiative torque disruption (RATD). We find that RATD can disrupt large grains located within a distance of about a parsec from a massive star of luminosity L ≈ 104L⊙, where L⊙ is the solar luminosity, or from a supernova. This disruption effect increases the abundance of small grains relative to large grains and successfully reproduces the observed NIR−MIR excess and anomalous dust extinction/polarization. We apply the RATD mechanism for kilonovae and find that dust within about 0.1 parsec would be dominated by small grains. Small grains produced by RATD can also explain the steep far-ultraviolet rise in extinction curves towards starburst and high-redshift galaxies, and the decrease of the escape fraction of Lyman α photons from H ii regions surrounding young massive star clusters. A predominance of small grains (tens of nanometres in size) over larger grains and the corresponding near- to mid-infrared excess radiation from H ii regions around massive stars and supernovae has been difficult to explain. Hoang et al. propose a radiative torque disruption method for large dust grains that fits with the observational constraints.

Visualization of the challenges and limitations of the long-term sunspot number record

Abstract: The solar cycle periodically reshapes the magnetic structure and radiative output of the Sun and determines its impact on the heliosphere roughly every 11 years. Besides this main periodicity, it shows century-long variations (including periods of abnormally low solar activity called grand minima). The Maunder Minimum (1645–1715) has generated significant interest as the archetype of a grand minimum in magnetic activity for the Sun and other stars, suggesting a potential link between the Sun and changes in terrestrial climate. Recent reanalyses of sunspot observations have yielded a conflicted view on the evolution of solar activity during the past 400 years (a steady increase versus a constant level). This has ignited a concerted community-wide effort to understand the depth of the Maunder Minimum and the subsequent secular evolution of solar activity. The goal of this Perspective is to review recent work that uses historical data to estimate long-term solar variability, and to provide context to users of these estimates that may not be aware of their limitations. We propose a clear visual guide than can be used to easily assess observational coverage for different periods, as well as the level of disagreement between currently proposed sunspot group number series. The sunspot number time series is an essential tool to determine the secular variations of solar activity, but particular care must be taken to handle and present incomplete temporal coverage. The authors present the current state of research and propose a new way to visualize long-term solar activity data.

The prevalence of repeating fast radio bursts

Abstract: Fast radio bursts are extragalactic, sub-millisecond radio impulses of unknown origin1,2. Their dispersion measures, which quantify the observed frequency-dependent dispersive delays in terms of free-electron column densities, greatly exceed predictions from models3 of the Milky Way interstellar medium. The excess dispersions are probably accrued as fast radio bursts propagate through their host galaxies, gaseous galactic halos and the intergalactic medium4,5. Despite extensive follow-up observations of the published sample of 72 burst sources6, only two have been observed to repeat7,8, and it is unknown whether the remainder are truly one-off events. Here I show that the volumetric occurrence rate of the fast radio bursts that have not been observed to repeat thus far probably exceeds the rates of candidate cataclysmic progenitor events, and also probably exceeds the birth rates of candidate compact-object sources. This analysis is based on the high detection rate of bursts with low dispersion measures by the Canadian Hydrogen Intensity Mapping Experiment (CHIME)9. Within the existing suite of astrophysical scenarios for fast radio burst progenitors, I conclude that most observed cases must originate from sources that emit several bursts over their lifetimes. Of the 72 known fast radio burst (FRB) sources only two have been observed to emit repeat bursts. By calculating the volumetric occurrence rate of non-repeating FRBs, Vikram Ravi shows that there are not enough candidate cataclysmic progenitor events for most FRBs to be one-off phenomena, and therefore most FRBs must repeat.

Obliquity-driven sculpting of exoplanetary systems

Abstract: NASA’s Kepler mission revealed that ~30% of Solar-type stars harbour planets with sizes between that of Earth and Neptune on nearly circular and coplanar orbits with periods less than 100 days1–4. Such short-period compact systems are rarely found with planet pairs in mean-motion resonances (MMRs)—configurations in which the planetary orbital periods exhibit a simple integer ratio—but there is a significant overabundance of planet pairs lying just wide of the first-order resonances5. Previous work suggests that tides raised on the planets by the host star may be responsible for forcing systems into these configurations by draining orbital energy to heat6–8. Such tides, however, are insufficient unless there exists a substantial and as-yet-unidentified source of extra dissipation9,10. Here we show that this cryptic heat source may be linked to ‘obliquity tides’ generated when a large axial tilt (obliquity) is maintained by secular resonance-driven spin–orbit coupling. We present evidence that typical compact, nearly coplanar systems frequently experience this mechanism, and we highlight additional features in the planetary orbital period and radius distributions that may be its signatures. Extrasolar planets that maintain large obliquities will exhibit infrared light curve features that are detectable with forthcoming space missions. The observed period ratio distribution can be explained if typical tidal quality factors for super-Earths and sub-Neptunes are similar to those of Uranus and Neptune. Compact exoplanetary systems frequently experience spin–orbit coupling driven by secular resonances, which can shape their architecture, allowing the planet to maintain a large obliquity and inducing the piling up of planets just wide of the first-order resonance.