Nature Astronomy最新文献

Centaurs are transitional objects between primitive trans-Neptunian objects and Jupiter-family comets. Their compositions and activities provide fundamental clues regarding the processes affecting the evolution of and interplay between these small bodies. Here we report observations of centaur 29P/Schwassmann–Wachmann 1 (29P) with the James Webb Space Telescope (JWST). We identified localized jets with heterogeneous compositions driving the outgassing activity. We employed the NIRSpec mapping spectrometer to study the fluorescence emissions of CO and obtain a definitive detection of CO₂ for this target. The exquisite sensitivity of the instrument also enabled carbon and oxygen isotopic signatures to be probed. Molecular maps reveal complex outgassing distributions, such as jets and anisotropic morphology, which indicate that 29P’s nucleus is dominated by active regions with heterogeneous compositions. These distributions could reflect that it has a bilobate structure with compositionally distinct components or that strong differential erosion takes place on the nucleus. As there are no missions currently planning to visit a centaur, these observations demonstrate JWST’s unique capabilities in characterizing these objects.

Ascertaining the morphology and composition of the icy mantles covering dust grains in dense, cold regions of the interstellar medium is essential to developing accurate astrochemical models, determining conditions for ice formation, constraining chemical interactions in and on icy grains and understanding how ices withstand space radiation. The widely observed infrared spectroscopic signature of H₂O ice at ~3 μm discriminates crystalline from amorphous structures in interstellar ices. Weaker bands seen only in laboratory ice spectra at ~2.7 μm, termed ‘dangling OH’ (dOH), are attributed to water molecules not fully bound to neighbouring water molecules and are often considered as tracing the degree of ice compaction. We exploit the high sensitivity of JWST NIRCam to detect two dOH features at 2.703 and 2.753 μm along multiple lines of sight probing the dense cloud Chamaeleon I, attributing these signatures to unbound dOH in cold water ice and dOH in interaction with other molecular species. These detections open a path to using the dOH features as tracers of the formation, composition, morphology and evolution of icy grains during the star and planet formation process.

Cosmological accretion shocks created during the formation of galaxy clusters are a ubiquitous phenomenon all around the universe. These shocks and their features are intimately related with the gravitational energy at stake during galaxy cluster formation. Studying a sample of simulated galaxy clusters and their associated accretion shocks, we show that objects in our sample sit in a plane within the three-dimensional space of cluster total mass, shock radius and Mach number (a measure of shock intensity). Using this relation, and considering that forthcoming new observations will be able to measure shock radii and intensities, we put forward the idea that the dark matter content of galaxy clusters could be indirectly measured with an error up to around 30% at the 1σ confidence level. This procedure would be a new and independent method to measure the dark matter mass in cosmic structures and a novel constraint to the accepted Lambda cold dark matter paradigm.

Protoplanetary disks, the birthplace of planets, are expected to be gravitationally unstable in their early phase of evolution. IM Lup, a well-known T-Tauri star, is surrounded by a protoplanetary disk with spiral arms. The disk was probably caused by gravitational instability. The IM Lup disk has been observed using various methods, but developing a unified explanatory model is challenging. Here we present a physical model of the IM Lup disk that offers a comprehensive explanation for diverse observations spanning from near-infrared to millimetre wavelengths. Our findings underscore the importance of dust fragility in retaining the observed millimetre emission and reveal the preference for moderately porous dust to explain the observed millimetre polarization. We also find that the inner disk region is probably heated by gas accretion, which provides a natural explanation for bright millimetre emission within 20 au. The actively heated inner region in the model casts a 100 au-scale shadow that aligns seamlessly with the observation of near-infrared scattered light. The accretion heating also supports the fragile-dust scenario in which accretion efficiently heats the disk midplane. Due to the fragility of the dust, it is unlikely that a potential embedded planet at 100 au formed through pebble accretion in the smooth disk, which suggests that local dust enhancement boosted pebble accretion or that there are alternative pathways, such as outward migration or gravitational fragmentation.

The number density of impact craters on a planetary surface is used to determine its age, which requires a model for the production rate of craters of different sizes. On Mars, however, estimates of the production rate of small craters (<60 m) from orbital imagery and from extrapolation of lunar impact data do not match. Here we provide a new independent estimate of the impact rate by analysing the seismic events recorded by the seismometer onboard NASA’s InSight lander. Some previously confirmed seismically detected impacts are part of a larger class of marsquakes (very high frequency, VF). Although a non-impact origin cannot be definitively excluded for each VF event, we show that the VF class as a whole is plausibly caused by meteorite impacts. We use an empirical scaling relationship to convert between seismic moment and crater diameter. Applying area and time corrections to derive a global impact rate, we find that 280–360 craters >8 m diameter are formed globally per year, consistent with previously published chronology model rates and above the rates derived from freshly imaged craters. Our work shows that seismology is an effective tool for determining meteoroid impact rates and complements other methods such as orbital imaging.

The shape of the dark matter (DM) halo is key to understanding the hierarchical formation of the Galaxy. Despite extensive efforts in recent decades, however, its shape remains a matter of debate, with suggestions ranging from strongly oblate to prolate. Here, we present a new constraint on its present shape by directly measuring the evolution of the Galactic disk warp with time, as traced by accurate distance estimates and precise age determinations for about 2,600 classical Cepheids. We show that the Galactic warp is mildly precessing in a retrograde direction at a rate of ω = −2.1 ± 0.5 (statistical) ± 0.6 (systematic) km s⁻¹ kpc⁻¹ for the outer disk over the Galactocentric radius [7.5, 25] kpc, decreasing with radius. This constrains the shape of the DM halo to be slightly oblate with a flattening (minor axis to major axis ratio) in the range 0.84 ≤ q_Φ ≤ 0.96. Given the young nature of the disk warp traced by Cepheids (less than 200 Myr), our approach directly measures the shape of the present-day DM halo. This measurement, combined with other measurements from older tracers, could provide vital constraints on the evolution of the DM halo and the assembly history of the Galaxy.

The radial structure of a galaxy is a fundamental property that reflects its growth and assembly history. Although it is straightforward to measure that of external galaxies, it is challenging for the Milky Way because of our inside perspective. Traditionally, the radial structure of the Milky Way has been assumed to be characterized by a single-exponential disk and a central bulge component. Here we report (1) a measurement of the age-resolved Galactic surface brightness profile in a wide radial range from R = 0 to 17 kpc and (2) the corresponding size of the Milky Way in terms of a half-light radius. We find a broken surface brightness profile with a nearly flat distribution between 3.5 and 7.5 kpc, in contrast to a canonical single-exponential disk. This broken profile results in a half-light radius of 5.75 ± 0.38 kpc, significantly larger than that inferred from a single-exponential disk profile but consistent with that of local disk galaxies of similar mass. We also confirm that the size growth history of the Milky Way is broadly consistent with high-redshift galaxies but with systematically smaller size. Our results suggest that the Milky Way has a more complex radial structure and larger size than previously expected.

Supergranules, which are solar flow features with a lateral scale of 30,000–40,000 km and a lifetime of ~24 h, form a prominent component of the Sun’s convective spectrum. However, their internal flows, which can be probed only by helioseismology, are not well understood. We analyse dopplergrams recorded by the Solar Dynamics Observatory satellite to identify and characterize ~23,000 supergranules. We find that the vertical flows peak at a depth of ~10,000 km, and remain invariant over the full range of lateral supergranular scales, contrary to numerical predictions. We also infer that, within the local seismic resolution (≳5,000 km), downflows are ~40% weaker than upflows, indicating an apparent mass-flux imbalance. This may imply that the descending flows also comprise plumes, which maintain the mass balance but are simply too small to be detected by seismic waves. These results challenge the widely used mixing-length description of solar convection.