3D insights into SN 1987A: ALMA observations compared to hydrodynamical explosion simulations

We obtain three-dimensional distributions of CO and SiO molecules from high spatial resolution (0.03–0.06") ALMA observations of SN 1987A at two different epochs. The evolution between these two epochs is consistent with homologous expansion. From these 3D maps, we reconstruct the 3D mass distributions of the ejecta in CO and SiO molecules, which we compare with those obtained by state-of-the-art, long-time hydrodynamical supernova explosion models computed with the Prometheus-HotB code for 10 different progenitors, including both red and blue supergiants. The models which best match the mass distributions correspond to explosions of binary-merger blue supergiant progenitors; at least two such models approximately reproduce the observed CO morphology. In contrast, the SiO velocity distribution and morphology are not as well reproduced in these models, indicating insufficient mixing of Si into the outer layers already at the progenitor stage. The theoretical models suggest a strong correlation between the centre of mass of the densest carbon- and oxygen-rich ejecta and the direction of the neutron-star kick. If such a correlation also applies to the CO emission in the ejecta of SN 1987A, the kick of the compact remnant is expected to point towards the observer, at an angle of approximately $45^\circ$ to the north.

💡 Research Summary

This paper presents a detailed three‑dimensional (3‑D) reconstruction of the molecular ejecta of SN 1987A using high‑resolution Atacama Large Millimeter/sub‑millimeter Array (ALMA) observations of CO and SiO at two distinct epochs (approximately 10 000 and 11 800 days after explosion). The authors combine data from several ALMA programmes, achieving angular resolutions of 0.03–0.06 arcsec, and process the cubes with standard CASA pipelines, subtracting continuum emission and correcting velocities to the LMC systemic frame (+287 km s⁻¹). By assuming homologous (free) expansion, they convert line‑of‑sight velocities into physical distances along the line of sight (z‑axis) and, together with the known distance to the Large Magellanic Cloud (51.4 kpc), map each voxel into a 3‑D Cartesian grid.

The resulting 3‑D maps reveal distinct morphologies for the two molecules. CO emission is relatively smooth, forming a flattened, disc‑like structure with a central low‑emission cavity, and shows a modest blueshift. SiO, in contrast, is clumpy, concentrated in high‑velocity knots that extend farther from the centre, and displays a stronger asymmetry with a pronounced blueshifted wing. Integrated line profiles confirm these differences: CO peaks near ±2000 km s⁻¹, while SiO exhibits significant flux at velocities exceeding 3000 km s⁻¹, especially on the blue side.

To interpret these observations, the authors compare them with a suite of ten long‑term hydrodynamical explosion simulations performed with the Prometheus‑HotB code. The models span a range of progenitor types, including red supergiants (RSG), blue supergiants (BSG), and binary‑merger BSG configurations, each evolved from core‑collapse through the early shock phase (seconds) to the late nebular phase (thousands of years). The simulations incorporate neutrino‑driven mechanisms, SASI, convection, and Rayleigh‑Taylor mixing, and they output the spatial distribution of key isotopes (C, O, Si) as a function of velocity.

Quantitative comparison focuses on the mass‑velocity distribution of CO (tracing C‑ and O‑rich material) and SiO (tracing Si‑rich material). Two binary‑merger BSG models reproduce the observed CO morphology and velocity spread best: they generate a dense, centrally concentrated C/O core whose centre of mass is offset from the geometric centre, matching the observed disc‑like CO shell and the central intensity dip. These models also reproduce the overall asymmetry and the modest blueshift seen in CO.

However, none of the models adequately reproduce the SiO observations. In the simulations Si remains largely confined to inner layers, resulting in a deficit of high‑velocity SiO (>3000 km s⁻¹) and a smoother distribution than observed. The authors attribute this mismatch to insufficient pre‑explosion Si mixing in the progenitor structures and/or limited resolution of the mixing instabilities (e.g., Rayleigh‑Taylor fingers) in the simulations. They suggest that more vigorous early‑time mixing or alternative progenitor chemistry would be required to match the SiO data.

A notable theoretical result emerges from the simulations: the centre of mass of the densest C/O‑rich ejecta correlates strongly with the direction of the nascent neutron‑star (NS) kick. If the brightest CO clump indeed traces this dense C/O core, the NS kick should be directed toward the observer, inclined roughly 45° north of the line of sight. This prediction aligns with indirect evidence from X‑ray and optical asymmetries and offers a novel way to infer the NS motion in SN 1987A, where the compact object has not yet been directly detected.

In summary, the paper demonstrates that ALMA’s sub‑arcsecond imaging of molecular lines provides a powerful probe of the three‑dimensional structure of young supernova ejecta. The comparison with state‑of‑the‑art 3‑D explosion models shows that binary‑merger blue‑supergiant progenitors can explain the CO distribution, but current models still struggle with SiO, highlighting the need for improved treatment of early mixing and progenitor composition. The identified correlation between dense C/O ejecta and NS kick direction offers an exciting avenue for future observational tests, especially with forthcoming JWST, ELT, and next‑generation radio facilities.