Heavy Elements in a Swirling Storm: How Neutron Stars Inside Common Envelopes Forge the Universe’s Rarest Atoms

In this study, Anninos et al. explore an unusual cosmic environment where a neutron star spirals into the swollen outer layers of a red giant star, a stage astronomers call a Common Envelope (CE) phase. As the neutron star dives deeper, it pulls in enormous amounts of gas at “hyper-Eddington” accretion rates, far higher than normal stellar feeding. The authors model what happens in the extreme, super-hot atmosphere that forms around the neutron star, paying close attention to how intense heating, rapid cooling, and violent motion might create heavy elements, including those made by the r-process, a rapid sequence of neutron-capture reactions responsible for many of the Universe’s heaviest nuclei.

Building the Model: How the Simulations Work

The paper begins by describing how the CE environment becomes a nuclear laboratory. As material from the red giant falls inward, it slams into the neutron star’s surface and forms an outward-moving accretion shock. Beneath the shock, temperatures climb high enough to break atomic nuclei apart, while a thin neutrino cooling layer near the surface radiates energy away. Because the flow is unstable, rising and falling gas parcels stir the region into vigorous convection, repeatedly cycling through hot (disassembly) and cool (reassembly) phases. These cycles allow nuclei to undergo photodisintegration, recombination, and neutron- or proton-capture sequences.

Tools of the Trade: Hydrodynamics and Nuclear Networks

To model this environment, the authors use the Cosmos++ code, which handles relativistic fluid flow, shock heating, and nuclear reactions while resolving structures less than a kilometer thick near the neutron star surface. Their simulations span a wide range of accretion rates, from 0.3 to 3×10⁴ solar masses per year, and follow “tracer particles” that record each parcel’s temperature, density, and composition as it moves. These tracers are later processed with a 7150-isotope network, enabling the team to compute detailed element yields that go far beyond simplified nuclear models.

From One Dimension to Two: The Role of Convection

Comparing one-dimensional and two-dimensional simulations, the authors show that multi-dimensional convection is essential. In 2D models, gas loops repeatedly between the shock and the neutron star surface, producing the rapid heating–cooling cycles required for heavy-element formation. Some tracers gain enough energy to become unbound, eventually escaping into space carrying newly forged elements. Others fall back toward the surface and end their journey as free nucleons. This chaotic motion differentiates the CE environment from more uniform r-process sites like neutron-star mergers.

A Diverse Set of Nuclear Outcomes

A major finding is that different tracer histories lead to very different nuclear products. Some trajectories remain mostly proton-rich, producing light p-nuclei near the iron group. Others follow paths that allow a full r-process, forming nuclei as heavy as lead and bismuth. In one especially interesting case, track 146, the gas experiences non-monotonic reheating during its final expansion. This creates a previously unrecognized multi-step process that combines r-process seeds, (γ,n) photodisintegration, and proton-capture reactions, yielding rare neutron-deficient p-isotopes. This mechanism may contribute to the origin of some puzzling solar-system p-nuclei.

What the Escaping Material Looks Like

When the authors combine all escaping tracers, they find that the resulting ejecta are dominated by p-nuclei, which show high overproduction factors relative to solar values. Although only about a quarter of tracer particles escape the neutron star’s gravity, those that do carry significant quantities of nuclei with atomic mass A > 100. The simulations suggest that CE accretion, usually overlooked compared to neutron star mergers, could be an important site for both r-process and p-process nucleosynthesis, thanks to the high entropies and rapid expansion seen in their models.

Conclusion: A New Candidate Site for Rare Elements

Overall, Anninos and collaborators demonstrate that the turbulent, high-entropy environment around a hyperaccreting neutron star inside a common envelope can forge a surprisingly rich mix of heavy and rare nuclei. Their work shows how repeated convective cycling, neutrino-driven cooling, and non-monotonic reheating can combine to produce nuclear species unlike those made in more familiar astrophysical settings. This broadens the landscape of possible cosmic forges for the Universe’s heaviest elements and highlights CE systems as a promising, if complex, nucleosynthesis site.

Source: Anninos