Showing papers in "Nature Astronomy in 2021"

Phosphine gas in the cloud decks of Venus

Abstract: Measurements of trace gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbour, Venus, has cloud decks that are temperate but hyperacidic. Here we report the apparent presence of phosphine (PH3) gas in Venus’s atmosphere, where any phosphorus should be in oxidized forms. Single-line millimetre-waveband spectral detections (quality up to ~15σ) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 ppb abundance is inferred. The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life. Other PH3 spectral features should be sought, while in situ cloud and surface sampling could examine sources of this gas. The detection of ~20 ppb of phosphine in Venus clouds by observations in the millimetre-wavelength range from JCMT and ALMA is puzzling, because according to our knowledge of Venus, no phosphine should be there. As the most plausible formation paths do not work, the source could be unknown chemical processes—maybe even life?

HXMT identification of a non-thermal X-ray burst from SGR J1935+2154 and with FRB 200428

Abstract: Fast radio bursts (FRBs) are short pulses observed in the radio band from cosmological distances1. One class of models invokes soft gamma-ray repeaters (SGRs), or magnetars, as the sources of FRBs2. Some radio pulses have been observed from some magnetars3, but no FRB-like events have been detected in association with any magnetar burst, including one giant flare4. Recently, a pair of FRB-like bursts (termed FRB 200428) separated by 29 milliseconds were detected from the general direction of the Galactic magnetar SGR J1935+2154 (refs. 5,6). Here, we report the detection of a non-thermal X-ray burst in the 1–250 keV energy band with the Insight-HXMT satellite7, which we identify as having been emitted from SGR J1935+2154. The burst showed two hard peaks with a separation of 34 milliseconds, broadly consistent with that of the two bursts in FRB 200428. The delay time between the double radio peak and the X-ray peaks is about 8.62 s, fully consistent with the dispersion delay of FRB 200428. We thus identify the non-thermal X-ray burst to be associated with FRB 200428, whose high-energy counterpart is the two hard X-ray peaks. Our results suggest that the non-thermal X-ray burst and FRB 200428 share the same physical origin in an explosive event from SGR J1935+2154. Insight-HXMT detected a double-peaked X-ray burst from Galactic magnetar SGR J1935+2154, consistent with two fast radio bursts (FRBs) observed from the same object within seconds. This coincidence suggests a common physical origin, and gives insight into the mechanism behind the origin of FRBs.

Molecular water detected on the sunlit Moon by SOFIA

Abstract: Widespread hydration was detected on the lunar surface through observations of a characteristic absorption feature at 3 µm by three independent spacecraft1–3. Whether the hydration is molecular water (H2O) or other hydroxyl (OH) compounds is unknown and there are no established methods to distinguish the two using the 3 µm band4. However, a fundamental vibration of molecular water produces a spectral signature at 6 µm that is not shared by other hydroxyl compounds5. Here, we present observations of the Moon at 6 µm using the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). Observations reveal a 6 µm emission feature at high lunar latitudes due to the presence of molecular water on the lunar surface. On the basis of the strength of the 6 µm band, we estimate abundances of about 100 to 400 µg g−1 H2O. We find that the distribution of water over the small latitude range is a result of local geology and is probably not a global phenomenon. Lastly, we suggest that a majority of the water we detect must be stored within glasses or in voids between grains sheltered from the harsh lunar environment, allowing the water to remain on the lunar surface. The Stratospheric Observatory for Infrared Astronomy (SOFIA) looked at the Moon in the 6 µm wavelength region and found a signature of molecular water, distinguishing it from other forms of hydration. The authors estimate water abundances between 100 and 400 µg g−1 at high latitudes, trapped within impact glasses or possibly in between grains.

A peculiar hard X-ray counterpart of a Galactic fast radio burst

Abstract: Fast radio bursts (FRBs) are bright, millisecond-scale radio flashes of unknown physical origin1. Young, highly magnetized, isolated neutron stars—magnetars—have been suggested as the most promising candidates for FRB progenitors owing to their energetics and high X-ray flaring activity2,3. Here we report the detection with Konus-Wind of a hard X-ray event of 28 April 2020 temporally coincident with a bright, two-peak radio burst4,5 in the direction of Galactic magnetar SGR 1935+2154, with properties remarkably similar to those of FRBs. We show that the two peaks of the double-peaked X-ray burst coincide in time with the radio peaks and infer a common source and the association of these phenomena. An unusual hardness of the X-ray spectrum strongly distinguishes the 28 April event among multiple ‘ordinary’ flares from SGR 1935+2154. A recent non-detection5–7 of radio emission from about 100 typical soft bursts from SGR 1935+2154 favours the idea that bright, FRB-like magnetar signals are associated with rare, hard-spectrum X-ray bursts. The implied rate of these hard X-ray bursts (~0.04 yr−1 magnetar−1) appears consistent with the rate estimate4 of SGR 1935+2154-like radio bursts (0.007–0.04 yr−1 magnetar−1). In April 2020, the Konus-Wind instrument registered two X-ray bursts temporally coincident with two radio bursts from the Galactic magnetar SGR 1935+2154. The unusual spectral hardness of the X-ray bursts may be an indicator of fast-radio-burst-like radio emission from magnetars.

Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data

Abstract: The detection of liquid water by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) at the base of the south polar layered deposits in Ultimi Scopuli has reinvigorated the debate about the origin and stability of liquid water under present-day Martian conditions. To establish the extent of subglacial water in this region, we acquired new data, achieving extended radar coverage over the study area. Here, we present and discuss the results obtained by a new method of analysis of the complete MARSIS dataset, based on signal processing procedures usually applied to terrestrial polar ice sheets. Our results strengthen the claim of the detection of a liquid water body at Ultimi Scopuli and indicate the presence of other wet areas nearby. We suggest that the waters are hypersaline perchlorate brines, known to form at Martian polar regions and thought to survive for an extended period of time on a geological scale at below-eutectic temperatures. MARSIS provides enhanced coverage of the south polar region where there have been indications of a subglacial lake. These new data confirm the presence of a lake and suggest the existence of a complex hydrologic system including various smaller liquid bodies, probably composed of salty brines.

Interstellar detection of the highly polar five-membered ring cyanocyclopentadiene

Abstract: Much like six-membered rings, five-membered rings are ubiquitous in organic chemistry, frequently serving as the building blocks for larger molecules, including many of biochemical importance. From a combination of laboratory rotational spectroscopy and a sensitive spectral line survey in the radio band toward the starless cloud core TMC-1, we report the astronomical detection of 1-cyano-1,3-cyclopentadiene (1-cyano-CPD, c-C5H5CN), a highly polar, cyano derivative of cyclopentadiene. The derived abundance of 1-cyano-CPD is far greater than predicted from astrochemical models that well reproduce the abundance of many carbon chains. This finding implies that either an important production mechanism or a large reservoir of aromatic material may need to be considered. The apparent absence of its closely related isomer, 2-cyano-1,3-cyclopentadiene, may arise from that isomer’s lower stability or may be indicative of a more selective pathway for formation of the 1-cyano isomer, perhaps one starting from acyclic precursors. The absence of N-heterocycles such as pyrrole and pyridine is discussed in light of the astronomical finding of 1-cyano-CPD. A five-membered carbon ring molecule, cyanocyclopentadiene, has been detected in a molecular cloud at a higher abundance than expected. This result from the GOTHAM survey indicates a rich aromatic chemistry in molecular clouds that is not fully understood theoretically.

Ionized outflows from active galactic nuclei as the essential elements of feedback

Abstract: Outflows from active galactic nuclei (AGNs) are one of the fundamental mechanisms by which the central supermassive black hole interacts with its host galaxy. Detected in ≥50% of nearby AGNs, these outflows have been found to carry kinetic energy that is a large fraction of the AGN power, and thereby give ‘negative’ feedback to their host galaxies. To understand the physical processes that regulate them, it is important to have a robust estimate of their physical and dynamical parameters. In this Review Article, we summarize our current understanding of the physics of the ionized outflows detected via absorption in the ultraviolet and X-ray wavelength bands. We discuss the most relevant observations and our current knowledge and uncertainties in the measurements of the outflow parameters, as well as their origin and acceleration mechanisms. The commissioning and concept studies of large telescope missions with high-resolution spectrographs in ultraviolet/optical and X-rays along with rapid advancements in simulations offer great promise for discoveries in this field over the next decade. This Review Article summarizes our current understanding of ionized outflows in active galactic nuclei, observed in absorption in the ultraviolet and X-ray wavelengths, including the most relevant observations as well as their origin and acceleration mechanisms.

Detection of two bright radio bursts from magnetar SGR 1935 + 2154

Abstract: Fast radio bursts are millisecond-duration, bright radio signals (fluence 0.1–100 Jy ms) emitted from extragalactic sources of unknown physical origin. The recent CHIME/FRB and STARE2 detection of an extremely bright (fluence ~MJy ms) radio burst from the Galactic magnetar SGR 1935+2154 supports the hypothesis that (at least some) fast radio bursts are emitted by magnetars at cosmological distances. In follow-up observations totalling 522.7 h on source, we detect two bright radio bursts with fluences of 112 ± 22 Jy ms and 24 ± 5 Jy ms, respectively. Both bursts appear to be affected by interstellar scattering and we measure significant linear and circular polarization for the fainter burst. The bursts are separated in time by ~1.4 s, suggesting a non-Poissonian, clustered emission process—similar to those seen in some repeating fast radio bursts. Together with the burst reported by CHIME/FRB and STARE2, as well as a much fainter burst seen by FAST (fluence 60 mJy ms), our observations demonstrate that SGR 1935+2154 can produce bursts with apparent energies spanning roughly seven orders of magnitude, and that the burst rate is comparable across this range. This raises the question of whether these four bursts arise from similar physical processes, and whether the fast radio burst population distribution extends to very low energies (~1030 erg, isotropic equivalent). Two further radio bursts associated with magnetar SGR 1935+2154 have been detected with a Westerbork 25 m dish, bringing the total to four. These observations demonstrate that SGR 1935+2154, a putative Galactic analogue of a fast radio burst source, can emit bursts across seven orders of magnitude in energy.

An X-ray burst from a magnetar enlightening the mechanism of fast radio bursts

Abstract: Fast radio bursts (FRBs) are millisecond radio pulses originating from powerful enigmatic sources at extragalactic distances. Neutron stars with large magnetic fields (magnetars) have been considered as the sources powering the FRBs, but the connection requires further substantiation. Here we report the detection by the AGILE satellite on 28 April 2020 of an X-ray burst in temporal coincidence with a bright FRB-like radio burst from the Galactic magnetar SGR 1935+2154. The burst observed in the hard X-ray band (18–60 keV) lasted about 0.5 s, it is spectrally cut off above 80 keV and implies an isotropically emitted energy of about 1040 erg. This event demonstrates that a magnetar can produce X-ray bursts in coincidence with FRB-like radio bursts. It also suggests that FRBs associated with magnetars can emit X-ray bursts. We discuss SGR 1935+2154 in the context of FRBs with low–intermediate radio energies in the range 1038–1040 erg. Magnetars with magnetic fields B ≈ 1015 G may power these FRBs, and new data on the search for X-ray emission from FRBs are presented. We constrain the bursting X-ray energy of the nearby FRB 180916 to be less than 1046 erg, smaller than that observed in giant flares from Galactic magnetars. In April 2020, the AGILE satellite registered an X-ray burst temporally coincident with a radio burst from the Galactic magnetar SGR 1935+2154. As seen in hard X-rays, the burst was cut off above 80 keV and had an isotropically emitted energy of about 1040 erg.

Evidence for GN-z11 as a luminous galaxy at redshift 10.957

Abstract: GN-z11 was photometrically selected as a luminous star-forming galaxy candidate at redshift z > 10 on the basis of Hubble Space Telescope imaging data1. Follow-up Hubble Space Telescope near-infrared grism observations detected a continuum break that was explained as the Lyα break corresponding to $$z = 11.09_{ - 0.12}^{ + 0.08}$$ (ref. 2). However, its accurate redshift remained unclear. Here we report a probable detection of three ultraviolet emission lines from GN-z11, which can be interpreted as the [C iii] λ1907, C iii] λ1909 doublet and O iii] λ1666 at z = 10.957 ± 0.001 (when the Universe was only ~420 Myr old, or ~3% of its current age). This is consistent with the redshift of the previous grism observations, supporting GN-z11 as the most distant galaxy known to date. Its ultraviolet lines probably originate from dense ionized gas that is rarely seen at low redshifts, and its strong [C iii] and C iii] emission is partly due to an active galactic nucleus or enhanced carbon abundance. GN-z11 is luminous and young, yet moderately massive, implying a rapid build-up of stellar mass in the past. Future facilities will be able to find the progenitors of such galaxies at higher redshift and probe the cosmic epoch at the beginning of reionization. The detection of three ultraviolet emission lines from GN-z11 can be interpreted as the [C iii] λ1907, C iii] λ1909 doublet and O iii] λ1666 at z = 10.957 ± 0.001, confirming GN-z11 as the most distant galaxy known to date and revealing the properties of its dense ionized gas.

A tidal disruption event coincident with a high-energy neutrino

Abstract: Cosmic neutrinos provide a unique window into the otherwise hidden mechanism of particle acceleration in astrophysical objects. The IceCube Collaboration recently reported the likely association of one high-energy neutrino with a flare from the relativistic jet of an active galaxy pointed towards the Earth. However a combined analysis of many similar active galaxies revealed no excess from the broader population, leaving the vast majority of the cosmic neutrino flux unexplained. Here we present the likely association of a radio-emitting tidal disruption event, AT2019dsg, with a second high-energy neutrino. AT2019dsg was identified as part of our systematic search for optical counterparts to high-energy neutrinos with the Zwicky Transient Facility. The probability of finding any coincident radio-emitting tidal disruption event by chance is 0.5%, while the probability of finding one as bright in bolometric energy flux as AT2019dsg is 0.2%. Our electromagnetic observations can be explained through a multizone model, with radio analysis revealing a central engine, embedded in a UV photosphere, that powers an extended synchrotron-emitting outflow. This provides an ideal site for petaelectronvolt neutrino production. Assuming that the association is genuine, our observations suggest that tidal disruption events with mildly relativistic outflows contribute to the cosmic neutrino flux.

Exogenic basalt on asteroid (101955) Bennu

Abstract: When rubble-pile asteroid 2008 TC3 impacted Earth on 7 October 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material1,2. However, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption3,4. Here, we report the presence of metre-scale exogenic boulders on the surface of near-Earth asteroid (101955) Bennu—the 0.5-km-diameter, rubble-pile target of the OSIRIS-REx mission5 that has been spectroscopically linked to the CM carbonaceous chondrite meteorites6. Hyperspectral data indicate that the exogenic boulders have the same distinctive pyroxene composition as the howardite–eucrite–diogenite (HED) meteorites that come from (4) Vesta, a 525-km-diameter asteroid that has undergone differentiation and extensive igneous processing7–9. Delivery scenarios include the infall of Vesta fragments directly onto Bennu or indirectly onto Bennu’s parent body, where the latter’s disruption created Bennu from a mixture of endogenous and exogenic debris. Our findings demonstrate that rubble-pile asteroids can preserve evidence of inter-asteroid mixing that took place at macroscopic scales well after planetesimal formation ended. Accordingly, the presence of HED-like material on the surface of Bennu provides previously unrecognized constraints on the collisional and dynamical evolution of the inner main belt. Six bright boulders of exotic material on near-Earth asteroid (101955) Bennu stand out from the average asteroidal surface. This unexpected record of impactors offers clues to the formation history of Bennu.

Internal mixing of rotating stars inferred from dipole gravity modes

Abstract: During most of their life, stars fuse hydrogen into helium in their cores. The mixing of chemical elements in the radiative envelope of stars with a convective core is able to replenish the core with extra fuel. If effective, such deep mixing allows stars to live longer and change their evolutionary path. Yet localized observations to constrain internal mixing are absent so far. Gravity modes probe the deep stellar interior near the convective core and allow us to calibrate internal mixing processes. Here we provide core-to-surface mixing profiles inferred from observed dipole gravity modes in 26 rotating stars with masses between 3 and 10 solar masses. We find a wide range of internal mixing levels across the sample. Stellar models with stratified mixing profiles in the envelope reveal the best asteroseismic performance. Our results provide observational guidance for three-dimensional hydrodynamical simulations of transport processes in the deep interiors of stars. Kepler space telescope observations of 26 intermediate-mass rotating stars (slowly pulsating B-type stars) are analysed to isolate the gravity modes that probe the stars’ deep interiors. Internal mixing levels are unexpectedly varied and best reproduced with models incorporating radially stratified mixing profiles.

Hierarchical mergers of stellar-mass black holes and their gravitational-wave signatures

Abstract: While most of the compact-binary mergers detected by LIGO and Virgo are expected to consist of first-generation black holes formed from the collapse of stars, others might instead be of second (or higher) generation, containing the remnants of previous black-hole mergers. We review theoretical findings, astrophysical modelling and current gravitational-wave evidence of hierarchical stellar-mass black-hole mergers. Such a subpopulation of hierarchically assembled black holes presents distinctive gravitational-wave signatures, namely higher masses, possibly within the pair-instability mass gap, and dimensionless spins clustered at the characteristic value of ~0.7. To produce hierarchical mergers, astrophysical environments need to overcome the relativistic recoils imparted to black-hole merger remnants, a condition that prefers hosts with escape speeds of ≳100 km s−1. Promising locations for efficient production of hierarchical mergers include nuclear star clusters and accretion disks surrounding active galactic nuclei, though environments that are less efficient at retaining merger products such as globular clusters may still contribute significantly to the detectable population of repeated mergers. While GW190521 is the single most promising hierarchical-merger candidate to date, constraints from large population analyses are becoming increasingly more powerful. Theoretical findings and current gravitational-wave evidence of hierarchical stellar-mass black-hole mergers are reviewed. Promising locations for the production of such systems include nuclear star clusters, accretion disks in active galactic nuclei, and potentially globular clusters.

Lunar regolith and substructure at Chang'E-4 landing site in South Pole-Aitken basin

Abstract: The South Pole–Aitken (SPA) basin is the oldest and largest impact structure on the Moon, and it gives particular insight on the lunar interior composition1–3. However, the surface of the SPA basin has been substantially modified by consequent impacts and basalt flooding. The exploration of the surficial material and the substructure of the SPA basin is one of the main scientific goals of the Chinese spacecraft Chang’E-4 that landed in the Von Karman crater inside the SPA basin4,5. Here we report the lunar penetrating radar profiles along the track of the lunar rover Yutu-2, which show a three-unit substructure at the landing site. The top unit consists of the ~12-m-thick lunar regolith and ~120 m multilayered ejecta that were delivered from several adjacent craters. The middle unit is the mare basalts filling the Von Karman crater. The lowest unit is another ejecta layer with a thickness of ≥200 m, likely from the Leibnitz crater. These discoveries fully support the local stratigraphy and geological explanation presented previously6. Our results reveal that the surface materials at the Chang’E-4 landing site are unambiguously dominated by the ejecta from the Finsen crater with a minor contribution from other neighbouring craters. The regolith measured by Yutu-2 is representative of the initial lunar deep interior materials, rather than the later erupted basalts. Three different layers can be distinguished in the first 500 metres of depth beneath the South Pole–Aitken basin on the Moon: a first layer made up by regolith and ejecta material from different craters, followed by a middle unit of mare basalts and finally a >200-m-thick layer of ejecta from the Leibnitz crater.

Optical follow-up of the neutron star–black hole mergers S200105ae and S200115j

Abstract: LIGO and Virgo’s third observing run revealed the first neutron star–black hole (NSBH) merger candidates in gravitational waves. These events are predicted to synthesize r-process elements1,2 creating optical/near-infrared ‘kilonova’ emission. The joint gravitational wave and electromagnetic detection of an NSBH merger could be used to constrain the equation of state of dense nuclear matter3, and independently measure the local expansion rate of the Universe4. Here, we present the optical follow-up and analysis of two of the only three high-significance NSBH merger candidates detected to date, S200105ae and S200115j, with the Zwicky Transient Facility5. The Zwicky Transient Facility observed ~48% of S200105ae and ~22% of S200115j’s localization probabilities, with observations sensitive to kilonovae brighter than −17.5 mag fading at 0.5 mag d−1 in the g- and r-bands; extensive searches and systematic follow-up of candidates did not yield a viable counterpart. We present state-of-the-art kilonova models tailored to NSBH systems that place constraints on the ejecta properties of these NSBH mergers. We show that with observed depths of apparent magnitude ~22 mag, attainable in metre-class, wide-field-of-view survey instruments, strong constraints on ejecta mass are possible, with the potential to rule out low mass ratios, high black hole spins and large neutron star radii. The optical follow-up and analysis of two neutron star–black hole merger candidates with the Zwicky Transient Facility did not yield viable counterparts. However, state-of-the-art kilonova models constrain the ejecta properties of these mergers.

Availability of subsurface water-ice resources in the northern mid-latitudes of Mars

Abstract: Multiple nations and private entities are pushing to make landing humans on Mars a reality The majority of proposed mission architectures envision ‘living off the land’ by leveraging Martian water-ice deposits for fuel production and other purposes Fortunately for mission designers, water ice exists on Mars in plentiful volumes The challenge is isolating accessible ice deposits within regions that optimize other preferred landing-site conditions Here we present the first results of the Mars Subsurface Water Ice Mapping (SWIM) project, which has the aim of searching for buried ice resources across the mid-latitudes Through the integration of orbital datasets in concert with new data-processing techniques, the SWIM project assesses the likelihood of ice by quantifying the consistency of multiple, independent data sources with the presence of ice Concentrating our efforts across the majority of the northern hemisphere, our composite ice-consistency maps indicate that the broad plains of Arcadia and the extensive glacial networks across Deuteronilus Mensae match the greatest number of remote-sensing criteria for accessible ice-rich, subsurface material situated equatorwards of the contemporary ice-stability zone The Mars Subsurface Water Ice Mapping (SWIM) project aims at determining the regions where near-surface ice is most likely to be present, according to the combination of all the available datasets Focusing on the northern mid-latitudes, they identify in particular Deuteronilus Mensae and Arcadia Planitia as promising sites

A non-energetic mechanism for glycine formation in the interstellar medium

Abstract: The detection of the amino acid glycine and its amine precursor methylamine on the comet 67P/Churyumov-Gerasimenko by the Rosetta mission provides strong evidence for a cosmic origin of amino acids on Earth. How and when such molecules form along the process of star formation remains debated. Here we report the laboratory detection of glycine formed in the solid phase through atom and radical–radical addition surface reactions under dark interstellar cloud conditions. Our experiments, supported by astrochemical models, suggest that glycine forms without the need for ‘energetic’ irradiation (such as ultraviolet photons and cosmic rays) in interstellar water-rich ices, where it remains preserved, during a much earlier star-formation stage than previously assumed. We also confirm that solid methylamine is an important side-reaction product. A prestellar formation of glycine on ice grains provides the basis for a complex and ubiquitous prebiotic chemistry in space enriching the chemical content of planet-forming material. Amino acid glycine is shown here to form in the laboratory at temperatures of <15 K without the need for energetic processing (such as ultraviolet irradiation or particle bombardment). The implication is that amino acids could potentially form at the very earliest stages of star formation and persist until planetary systems are established.

HAWC observations of the acceleration of very-high-energy cosmic rays in the Cygnus Cocoon

Abstract: Cosmic rays with energies up to a few PeV are known to be accelerated within the Milky Way1,2. Traditionally, it has been presumed that supernova remnants were the main source of these very-high-energy cosmic rays3,4, but theoretically it is difficult to accelerate protons to PeV energies5,6 and observationally there simply is no evidence of the remnants being sources of hadrons with energies above a few tens of TeV7,8. One possible source of protons with those energies is the Galactic Centre region9. Here, we report observations of 1–100 TeV γ rays coming from the ‘Cygnus Cocoon’10, which is a superbubble that surrounds a region of massive star formation. These γ rays are likely produced by 10–1,000 TeV freshly accelerated cosmic rays that originate from the enclosed star-forming region Cyg OB2. Until now it was not known that such regions could accelerate particles to these energies. The measured flux likely originates from hadronic interactions. The spectral shape and the emission profile of the Cocoon changes from GeV to TeV energies, which reveals the transport of cosmic particles and historical activity in the superbubble. Following HAWC observations of the Cygnus Cocoon, massive star-forming regions can now be considered to be sources of very-high-energy (TeV to PeV) Galactic cosmic rays.

Detection of the Milky Way reflex motion due to the Large Magellanic Cloud infall

Abstract: The Large Magellanic Cloud is the most massive satellite galaxy of the Milky Way, with an estimated mass exceeding a tenth of the mass of the Milky Way1–5. Just past its closest approach of about 50 kpc, and flying past the Milky Way at an astonishing speed of 327 km s−1 (ref. 6), the Large Magellanic Cloud can affect our Galaxy in a number of ways, including dislodging the Milky Way disk from the Galactic centre of mass7–9. Here, we report evidence that the Milky Way disk is moving with respect to stellar tracers in the outer halo in a direction that points at an earlier location on the Large Magellanic Cloud trajectory. The resulting reflex motion is detected in the kinematics of outer halo stars and Milky Way satellite galaxies with accurate distances, proper motions and line-of-sight velocities. Our results indicate that dynamical models of our Galaxy cannot neglect gravitational perturbations induced by the Large Magellanic Cloud infall, nor can observations of the stellar halo be treated in a reference frame that does not correct for disk reflex motion. Future spectroscopic surveys of the stellar halo combined with Gaia astrometry will allow for sophisticated modelling of the Large Magellanic Cloud trajectory across the Milky Way, constraining the dark matter distribution in both galaxies with unprecedented detail. The Milky Way disk is found to be moving with respect to the outer halo of the Galaxy as a result of the gravitational pull of the Large Magellanic Cloud as it falls into the Milky Way. Dynamical models of our Galaxy need to take this effect into account.

Highly polarized microstructure from the repeating FRB 20180916B

Abstract: Fast radio bursts (FRBs) are bright, coherent, short-duration radio transients of as-yet unknown extragalactic origin. FRBs exhibit a variety of spectral, temporal and polarimetric properties that can unveil clues into their emission physics and propagation effects in the local medium. Here, we present the high-time-resolution (down to 1 μs) polarimetric properties of four 1.7 GHz bursts from the repeating FRB 20180916B, which were detected in voltage data during observations with the European Very Long Baseline Interferometry Network. We observe a range of emission timescales that spans three orders of magnitude, with the shortest component width reaching 3–4 μs (below which we are limited by scattering). We demonstrate that all four bursts are highly linearly polarized (≳80%), show no evidence of significant circular polarization (≲15%), and exhibit a constant polarization position angle (PPA) during and between bursts. On short timescales (≲100 μs), however, there appear to be subtle PPA variations (of a few degrees) across the burst profiles. These observational results are most naturally explained in an FRB model in which the emission is magnetospheric in origin, in contrast to models in which the emission originates at larger distances in a relativistic shock. High-time-resolution observations of the repeating fast radio burst source FRB 20180916B reveal changes to the polarization properties of the emission on timescales of a few microseconds, indicating an origin in the source magnetosphere.

Collisional history of Ryugu’s parent body from bright surface boulders

Abstract: The asteroid (162173) Ryugu and other rubble-pile asteroids are likely re-accumulated fragments of much larger parent bodies that were disrupted by impacts. However, the collisional and orbital pathways from the original parent bodies to subkilometre rubble-pile asteroids are not yet well understood1–3. Here we use Hayabusa2 observations to show that some of the bright boulders on the dark, carbonaceous (C-type) asteroid Ryugu4 are remnants of an impactor with a different composition as well as an anomalous portion of its parent body. The bright boulders on Ryugu can be classified into two spectral groups: most are featureless and similar to Ryugu’s average spectrum4,5, while others show distinct compositional signatures consistent with ordinary chondrites—a class of meteorites that originate from anhydrous silicate-rich asteroids6. The observed anhydrous silicate-like material is likely the result of collisional mixing between Ryugu’s parent body and one or multiple anhydrous silicate-rich asteroid(s) before and during Ryugu’s formation. In addition, the bright boulders with featureless spectra and less ultraviolet upturn are consistent with thermal metamorphism of carbonaceous meteorites7,8. They might sample different thermal-metamorphosed regions, which the returned sample will allow us to verify. Hence, the bright boulders on Ryugu provide new insights into the collisional evolution and accumulation of subkilometre rubble-pile asteroids. The Hayabusa2 team has discovered two types of bright boulder on the dark, carbonaceous asteroid Ryugu. One type has a spectrum consistent with material from an anhydrous silicate-rich asteroid, likely introduced by one or more collisions in Ryugu’s past.

Concepts and status of Chinese space gravitational wave detection projects

Abstract: Gravitational wave (GW) detection in space probes the GW spectrum that is inaccessible from the Earth. In addition to the LISA project led by the European Space Agency, and the DECIGO detector proposed by the Japan Aerospace Exploration Agency, two Chinese space-based GW observatories—TianQin and Taiji—are planned to be launched in the 2030s. TianQin has a unique concept in its design with a geocentric orbit. Taiji’s design is similar to LISA, but is more ambitious with a longer arm distance. Both facilities are complementary to LISA, considering that TianQin is sensitive to higher frequencies and Taiji probes similar frequencies but with a higher sensitivity. In this Perspective we explain the concepts of both facilities and introduce the development milestones of the TianQin and Taiji projects in testing key technologies to pave the way for future space-based GW detections. Considering that LISA, TianQin and Taiji have similar scientific goals, are all scheduled to be launched around the 2030s and will operate concurrently, we discuss possible collaborations among them to improve GW source localization and characterization. The concepts and development milestones of the two Chinese space-based gravitational wave observatories, TianQin and Taiji, are introduced. Considering their similar goals and operation window with LISA, possible collaborations among them are discussed.

An investigation of spectral line stacking techniques and application to the detection of HC11N

Abstract: As the inventory of interstellar molecules continues to grow, the gulf between small species, whose individual rotational lines can be observed with radio telescopes, and large ones, such as polycyclic aromatic hydrocarbons best studied in bulk via infrared and optical observations, is slowly being bridged. Understanding the connection between these two molecular reservoirs is critical to understanding the interstellar carbon cycle, but will require pushing the boundaries of how far we can probe molecular complexity while still retaining observational specificity. Towards this end, we present a method for detecting and characterizing new molecular species in single-dish observations towards sources with sparse line spectra. We have applied this method to data from the ongoing GOTHAM (GBT Observations of TMC-1: Hunting Aromatic Molecules) Green Bank Telescope large programme, discovering six new interstellar species. Here we highlight the detection of HC11N, the largest cyanopolyyne in the interstellar medium. The authors present a technique to detect (weak) molecular emission lines towards sources with sparse line spectra. This method supports the current GOTHAM survey of TMC-1, and is applied to the detection of the cyanopolyyne species HC11N.

The sonic scale of interstellar turbulence

Abstract: Understanding the physics of turbulence is crucial for many applications, including weather, industry and astrophysics. In the interstellar medium1,2, supersonic turbulence plays a crucial role in controlling the gas density and velocity structure, and ultimately the birth of stars3–8. Here we present a simulation of interstellar turbulence with a grid resolution of 10,0483 cells that allows us to determine the position and width of the sonic scale (ls)—the transition from supersonic to subsonic turbulence. The simulation simultaneously resolves the supersonic and subsonic cascade, with the velocity as a function of scale, v(l) ∝ lp, where we measure psup = 0.49 ± 0.01 and psub = 0.39 ± 0.02, respectively. We find that ls agrees with the relation $${\ell }_{{\rm{s}}}={\phi }_{{\rm{s}}}{L\,}{{\mathcal{M}}}^{-1/{p}_{\sup }}$$ , where $${\mathcal{M}}$$ is the three-dimensional Mach number, L is either the driving scale of the turbulence or the diameter of a molecular cloud, and ϕs is a dimensionless factor of order unity. If L is the driving scale, we measure $${\phi }_{{\rm{s}}}=0.4{2}_{-0.09}^{+0.12}$$ , primarily because of the separation between the driving scale and the start of the supersonic cascade. For a supersonic cascade extending beyond the cloud scale, we get $${\phi }_{{\rm{s}}}=0.9{1}_{-0.20}^{+0.25}$$ . In both cases, ϕs ≲ 1, because we find that the supersonic cascade transitions smoothly to the subsonic cascade over a factor of 3 in scale, instead of a sharp transition. Our measurements provide quantitative input for turbulence-regulated models of filament structure and star formation in molecular clouds. A high-resolution simulation of interstellar turbulence determines the position and width of the transition from supersonic to subsonic turbulence, providing quantitative input for models of filament structure and star formation in molecular clouds.

Ubiquitous aromatic carbon chemistry at the earliest stages of star formation

Abstract: Benzonitrile (c-C6H5CN, where ‘c’ indicates a cyclic structure), a polar proxy for benzene (c-C6H6), has the potential to serve as a highly convenient radio probe for aromatic chemistry, provided that this ring can be found in other astronomical sources beyond the molecule-rich prestellar cloud TMC-1. Here we present radio astronomical evidence of benzonitrile in four other prestellar, and possibly protostellar, sources: Serpens 1A, Serpens 1B, Serpens 2 and MC27/L1521F. These detections establish that benzonitrile is not unique to TMC-1; rather, aromatic chemistry appears to be widespread throughout the earliest stages of star formation, probably persisting at least until the initial formation of a protostar. The abundance of benzonitrile far exceeds predictions from models that well reproduce the abundances of carbon chains such as HC7N, a cyanpolyyne with the same heavy atoms, indicating that the chemistry responsible for planar carbon structures (as opposed to linear ones) in primordial sources is favourable but not well understood. The abundance of benzonitrile relative to carbon chain molecules displays sizable variations between sources within the Taurus and Serpens clouds, implying the importance of physical conditions and initial elemental reservoirs of the clouds themselves. Benzonitrile, a proxy for the aromatic ring molecule benzene, has now been detected at multiple locations in the Taurus and Serpens molecular clouds, suggesting a widespread aromatic chemistry in the interstellar medium. Chemical models underestimate the abundance of aromatic molecules, highlighting the need for further study.

Water activity in Venus's uninhabitable clouds and other planetary atmospheres

Abstract: The recent suggestion of phosphine in Venus’s atmosphere has regenerated interest in the idea of life in clouds. However, such analyses usually neglect the role of water activity, which is a measure of the relative availability of water, in habitability. Here we compute the water activity within the clouds of Venus and other Solar System planets from observations of temperature and water-vapour abundance. We find water-activity values of sulfuric acid droplets, which constitute the bulk of Venus’s clouds, of ≤0.004, two orders of magnitude below the 0.585 limit for known extremophiles. Considering other planets, ice formation on Mars imposes a water activity of ≤0.537, slightly below the habitable range, whereas conditions are biologically permissive (>0.585) at Jupiter’s clouds (although other factors such as their composition may play a role in limiting their habitability). By way of comparison, Earth’s troposphere conditions are, in general, biologically permissive, whereas the atmosphere becomes too dry for active life above the middle stratosphere. The approach used in the current study can also be applied to extrasolar planets. Calculations of water activity reveal that this parameter can be a substantial barrier to habitability for clouds of Solar System planets. In particular, water activity within droplets of Venus’s clouds is more than 100-fold below the threshold for biotic activity of known extremophiles.

Martian water loss to space enhanced by regional dust storms

Abstract: Mars has lost most of its initial water to space as atomic hydrogen and oxygen. Spacecraft measurements have determined that present-day hydrogen escape undergoes large variations with season that are inconsistent with long-standing explanations. The cause is incompletely understood, with likely contributions from seasonal changes in atmospheric circulation, dust activity and solar extreme ultraviolet input. Although some modelling and indirect observational evidence suggest that dust activity can explain the seasonal trend, no previous study has been able to unambiguously distinguish seasonal from dust-driven forcing. Here we present synoptic measurements of dust, temperature, ice, water and hydrogen on Mars during a regional dust event, demonstrating that individual dust events can boost planetary H loss by a factor of five to ten. This regional storm occurred in the declining phase of the known seasonal trend, establishing that dust forcing can override this trend to drive enhanced escape. Because similar regional storms occur in most Mars years, these storms may be responsible for a large fraction of Martian water loss and represent an important driver of Mars atmospheric evolution.

Micro cold traps on the Moon

Abstract: Water ice is thought to be trapped in large permanently shadowed regions in the Moon’s polar regions, due to their extremely low temperatures. Here, we show that many unmapped cold traps exist on small spatial scales, substantially augmenting the areas where ice may accumulate. Using theoretical models and data from the Lunar Reconnaissance Orbiter, we estimate the contribution of shadows on scales from 1 km to 1 cm, the smallest distance over which we find cold-trapping to be effective for water ice. Approximately 10–20% of the permanent cold-trap area for water is found to be contained in these micro cold traps, which are the most numerous cold traps on the Moon. Consideration of all spatial scales therefore substantially increases the number of cold traps over previous estimates, for a total area of ~40,000 km2, about 60% of which is in the south. A majority of cold traps for water ice is found at latitudes > 80° because permanent shadows equatorward of 80° are typically too warm to support ice accumulation. Our results suggest that water trapped at the lunar poles may be more widely distributed and accessible as a resource for future missions than previously thought. A reconstruction of the distribution of cold traps on the Moon at spatial scales varying from 1 km to 1 cm shows that the smallest ones are also the most numerous, 10–20% of the total. The total surface area of the Moon that can efficiently trap water is revised substantially upward, to 40,000 km2.

Discovery and confirmation of the shortest gamma ray burst from a collapsar

Abstract: Gamma-ray bursts (GRBs) are among the brightest and most energetic events in the universe. The duration and hardness distribution of GRBs has two clusters, now understood to reflect (at least) two different progenitors. Short-hard GRBs (SGRBs; T90 2 s) have been attributed to the collapse of peculiar massive stars (collapsars). The discovery of SN 1998bw/GRB 980425 marked the first association of a LGRB with a collapsar and AT 2017gfo/GRB 170817A/GW170817 marked the first association of a SGRB with a binary neutron star merger, producing also gravitational wave (GW). Here, we present the discovery of ZTF20abwysqy (AT2020scz), a fast-fading optical transient in the Fermi Satellite and the InterPlanetary Network (IPN) localization regions of GRB 200826A; X-ray and radio emission further confirm that this is the afterglow. Follow-up imaging (at rest-frame 16.5 days) reveals excess emission above the afterglow that cannot be explained as an underlying kilonova (KN), but is consistent with being the supernova (SN). Despite the GRB duration being short (rest-frame T90 of 0.65 s), our panchromatic follow-up data confirms a collapsar origin. GRB 200826A is the shortest LGRB found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets.