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The mergers of neutron stars expel a heavy-element enriched fireball that can be observed as a kilonova 1 – 4 . The kilonova’s geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. Current hydrodynamical merger models typically show aspherical ejecta 5 – 7 . Previously, Sr + was identified in the spectrum 8 of the only well-studied kilonova 9 – 11 AT2017gfo 12 , associated with the gravitational wave event GW170817. Here we combine the strong Sr + P Cygni absorption-emission spectral feature and the blackbody nature of kilonova spectrum to determine that the kilonova is highly spherical at early epochs. Line shape analysis combined with the known inclination angle of the source 13 also show the same sphericity independently. We conclude that energy injection by radioactive decay is insufficient to make the ejecta spherical. A magnetar wind or jet from the black-hole disk could inject enough energy to induce a more spherical distribution in the overall ejecta; however, an additional process seems necessary to make the element distribution uniform. Spectra taken after the kilonova associated with GW170817 show a high degree of spherical symmetry and a line shape is found that is consistent with a completely spherical expansion to within a few per cent.

Half of all of the elements in the Universe that are heavier than iron were created by rapid neutron capture. The theory underlying this astrophysical r-process was worked out six decades ago, and requires an enormous neutron flux to make the bulk of the elements 1 . Where this happens is still debated 2 . A key piece of evidence would be the discovery of freshly synthesized r-process elements in an astrophysical site. Existing models 3 – 5 and circumstantial evidence 6 point to neutron-star mergers as a probable r-process site; the optical/infrared transient known as a ‘kilonova’ that emerges in the days after a merger is a likely place to detect the spectral signatures of newly created neutron-capture elements 7 – 9 . The kilonova AT2017gfo—which was found following the discovery of the neutron-star merger GW170817 by gravitational-wave detectors 10 —was the first kilonova for which detailed spectra were recorded. When these spectra were first reported 11 , 12 , it was argued that they were broadly consistent with an outflow of radioactive heavy elements; however, there was no robust identification of any one element. Here we report the identification of the neutron-capture element strontium in a reanalysis of these spectra. The detection of a neutron-capture element associated with the collision of two extreme-density stars establishes the origin of r-process elements in neutron-star mergers, and shows that neutron stars are made of neutron-rich matter 13 . Reanalysis of the spectra associated with the merger of two neutron stars identifies strontium, spectroscopically establishing the origin of the heavy elements created by rapid neutron capture and proving that neutron stars comprise neutron-rich matter.

The formation of dust in the dense circumstellar medium of the bright supernova 2010jl is at first rapid and produces very large grains, which resist destruction, whereas later the dust production rate increases, meaning its source is ejecta; this links early and late dust mass evolution in supernovae with dense circumstellar media. A supernova source for large dust grains Dust grains are found practically throughout the Universe and are crucial to galactic evolution, planet formation and much else. Yet it is still not clear where all this dust comes from and how it survives in the harsh environments of star-forming galaxies. Recent work suggested that dust might form in supernova remnants, although subsequent observations of the bright supernova SN 2010jl proved inconclusive. Christa Gall et al . now report spectrographic observations of SN 2010jl consistent with the rapid (40–240 days) formation of dust in its dense circumstellar medium. The wavelength-dependent extinction of this dust points to the presence of very large grains, greater than a micrometre in diameter, that resist destruction. At later times (around 500–900 days), near-infrared thermal emissions suggest an accelerated growth in dust mass. The origin of dust in galaxies is still a mystery 1 , 2 , 3 , 4 . The majority of the refractory elements are produced in supernova explosions, but it is unclear how and where dust grains condense and grow, and how they avoid destruction in the harsh environments of star-forming galaxies. The recent detection of 0.1 to 0.5 solar masses of dust in nearby supernova remnants 5 , 6 , 7 suggests in situ dust formation, while other observations reveal very little dust in supernovae in the first few years after explosion 1 , 8 , 9 , 10 . Observations of the spectral evolution of the bright SN 2010jl have been interpreted as pre-existing dust 11 , dust formation 12 , 13 or no dust at all 14 . Here we report the rapid (40 to 240 days) formation of dust in its dense circumstellar medium. The wavelength-dependent extinction of this dust reveals the presence of very large (exceeding one micrometre) grains, which resist destruction 15 . At later times (500 to 900 days), the near-infrared thermal emission shows an accelerated growth in dust mass, marking the transition of the dust source from the circumstellar medium to the ejecta. This provides the link between the early and late dust mass evolution in supernovae with dense circumstellar media.

At cosmic dawn, the first stars and galaxies are believed to form from and be deeply embedded in clouds of dense, pristine gas. Here we present a study of the James Webb Space Telescope/NIRSpec data of the most distant, spectroscopically confirmed galaxy observed to date, JADES-GS-z14-0 (GS-z14 for short), at z = 14.179, combined with recently reported far-infrared measurements of the [O iii]-88 μm and [C ii]-158 μm line transitions and underlying dust-continuum emission. Based on the observed prominent damped Lyα (DLA) absorption profile, we determine a substantial neutral atomic hydrogen (H i) column density, log(NHI/cm−2)=22.27−0.09+0.08 , consistent with previous estimates though seemingly at odds with the dynamical and gas mass of the galaxy. Using various independent but complementary approaches, considering the implied neutral gas mass from the DLA measurement, the star formation rate surface density, and the metal abundance, we demonstrate that the total gas mass of GS-z14 is of the order Mgas = 109.5–109.8 M⊙. This implies a substantial gas mass fraction, fgas ≈ 0.7–0.9 and that the bulk of the interstellar medium (ISM) is in the form of H i, with mass ratios MHI/MH2≈3 . We show that the derived gas mass is fully consistent with the nondetection of [C ii]-158 μm, assuming an appropriate scaling to the neutral gas. The low dust-to-gas ratio, AV/NHI = (1.3 ± 0.6) × 10−23 mag cm2, derived in the line of sight through the DLA further indicates that the absorbing gas is more pristine than the central, star-forming regions probed by the [O iii]-88 μm emission. These results highlight the implications for far-infrared line-detection searchers attainable with the Atacama Large Millimeter/submillimeter Array and demonstrate that the bright, relatively massive galaxy GS-z14 at z = 14.179 is deeply embedded in a substantial, pristine H i gas reservoir dominating its baryonic matter content.

The role of internal and environmental factors in the star formation activity of galaxies is still a matter of debate, in particular at higher redshifts. Leveraging the most recent release of the COSMOS catalog, COSMOS2020, as well as density measurements from our previous study, we disentangle the impact of environment and stellar mass on the star formation rate (SFR) and specific SFR (sSFR) of a sample of ∼210,000 galaxies within a redshift range of 0.4 < z < 4, and present our findings in three cosmic epochs: (1) out to z ∼ 1, the average SFR and sSFR decline in extremely dense environments and at the high-mass end of the distribution, which is mostly due to the presence of the massive quiescent population; (2) at 1 < z < 2, the environmental dependence diminishes, while mass is still the dominant factor in star formation activity; and (3) beyond z ∼ 2, our sample is dominated by star-forming galaxies and we observe a reversal of the trends seen in the local Universe—the average SFR increases with increasing environmental density. Our analysis shows that both environmental and mass quenching efficiencies increase with stellar mass at all redshifts, with mass being the dominant quenching factor in massive galaxies compared to environmental effects. At 2 < z < 4, negative values of environmental quenching efficiency suggest that the fraction of star-forming galaxies in dense environments exceeds that in less-dense regions, likely due to the greater availability of cold gas, higher merger rates, and tidal effects that trigger star formation activity.

Core-collapse supernovae are explosions of massive stars at the end of their evolution. They are responsible for metal production and for halting star formation, having a significant impact on galaxy evolution. The details of these processes depend on the nature of supernova progenitors, but it is unclear if Type Ic supernovae (without hydrogen or helium lines in their spectra) originate from core-collapses of very massive stars (>30 M ⊙ ) or from less massive stars in binary systems. Here we show that Type II (with hydrogen lines) and Ic supernovae are located in environments with similar molecular gas densities, therefore their progenitors have comparable lifetimes and initial masses. This supports a binary interaction for most Type Ic supernova progenitors, which explains the lack of hydrogen and helium lines. This finding can be implemented in sub-grid prescriptions in numerical cosmological simulations to improve the feedback and chemical mixing. Type Ic supernovae (SNe) originate either from the core collapse of very massive stars or from less massive stars in binary systems. Here, the authors show that progenitors of Type II and Ic SNe have comparable lifetimes and initial masses, which supports binary interaction for most Type Ic SNe progenitors.

Planetary materials show systematic variations in their nucleosynthetic isotope compositions that resonate with orbital distance. The origin of this pattern remains debated, limiting how these isotopic signatures can be used to trace the precursors of terrestrial planets. Here we test the hypothesis that interstellar ices carried supernova-produced nuclides by searching for a supernova nucleosynthetic fingerprint in aqueous alteration minerals from carbonaceous and non-carbonaceous chondrite meteorites. We focus on zirconium, a refractory element that includes the neutron-rich isotope 96 Zr formed in core-collapse supernovae. Leaching experiments reveal extreme 96 Zr enrichments in alteration minerals, showing that they incorporated supernova material hosted in interstellar ices. We show that the Solar System’s zirconium isotope variability reflects mixing between these ices and an ice-free rocky component. Finally, the presence of supernova nuclides in a volatile carrier supports models where the Solar System’s nucleosynthetic variability was imparted by thermal processing of material in the protoplanetary disk and during planetary accretion. This study shows that supernova material was delivered to the early Solar System by interstellar ices, explaining isotope variations and revealing that even inner planets like Earth and Mars accreted water-rich material from the outer Solar System.

Far-infrared measurements of galaxies in the early Universe would reveal their detailed properties, but have been lacking for the more typical galaxies where most stars form; here an archetypal, early Universe star-forming galaxy is detected at far-infrared wavelengths, allowing its dust mass, total star-formation rate and dust-to-gas ratio to be calculated. A star-forming early galaxy observed Infrared and millimetre-wavelength observations of galaxies in the early Universe would reveal their detailed properties, but until now such data have not been available for the dusty metal-rich galaxies typical of those in which most stars form. Here Darach Watson et al . present infrared spectra and millimetre-wavelength continuum observations of an archetypal star-forming galaxy, the high-redshift galaxy A1689-zD1, which allow the calculation of its dust mass, total star-formation rate and dust-to-gas ratio. They calculate its redshift to be z = 7.5±0.2, with a total star-formation rate of about 12 solar masses per year. The galaxy is highly evolved with a large stellar mass, and its dust-to-gas ratio is close to that of the Milky Way. Candidates for the modest galaxies that formed most of the stars in the early Universe, at redshifts z  > 7, have been found in large numbers with extremely deep restframe-ultraviolet imaging 1 . But it has proved difficult for existing spectrographs to characterize them using their ultraviolet light 2 , 3 , 4 . The detailed properties of these galaxies could be measured from dust and cool gas emission at far-infrared wavelengths if the galaxies have become sufficiently enriched in dust and metals. So far, however, the most distant galaxy discovered via its ultraviolet emission and subsequently detected in dust emission is only at z = 3.2 (ref. 5 ), and recent results have cast doubt on whether dust and molecules can be found in typical galaxies at z  ≥ 7 6 , 7 , 8 . Here we report thermal dust emission from an archetypal early Universe star-forming galaxy, A1689-zD1. We detect its stellar continuum in spectroscopy and determine its redshift to be z = 7.5 ± 0.2 from a spectroscopic detection of the Lyman-α break. A1689-zD1 is representative of the star-forming population during the epoch of reionization 9 , with a total star-formation rate of about 12 solar masses per year. The galaxy is highly evolved: it has a large stellar mass and is heavily enriched in dust, with a dust-to-gas ratio close to that of the Milky Way. Dusty, evolved galaxies are thus present among the fainter star-forming population at z  > 7.