Angular momentum drives proton-rich nucleosynthesis in hyperaccreting neutron stars in common envelopes

This study demonstrates that angular momentum-driven accretion disks in common envelope systems containing a neutron star and a massive companion facilitate the synthesis of significant rp-process products and alpha-rich elements like $^{44}$Ti and $^{56}$Ni, offering a new mechanism for galactic chemical evolution.

The Gist

Imagine two stars locked in a cosmic dance. One is a massive, aging giant, and the other is a tiny, incredibly dense "ghost" called a neutron star. As the giant star runs out of fuel, it swells up like a balloon, swallowing its tiny partner whole. This chaotic event is called a Common Envelope.

Usually, scientists thought that when the neutron star gets swallowed, it just gobbles up the giant star's gas and spits it back out in a messy, turbulent cloud. But this new paper suggests something much more organized and explosive is happening: The neutron star is spinning up a cosmic blender.

Here is the simple breakdown of what the scientists found, using some everyday analogies:

1. The Cosmic Blender (The Accretion Disk)

In previous studies, scientists assumed the gas falling onto the neutron star was like a slow, messy drip. But this paper says: No, it's a whirlwind.

Because the gas has a lot of "spin" (angular momentum), it doesn't fall straight down. Instead, it swirls around the neutron star, forming a flat, spinning disk—like water swirling down a drain, but made of super-hot gas.

• The Analogy: Think of a pizza dough tosser. If you just drop dough on a table, it's a mess. If you spin it, it flattens out into a perfect circle. The neutron star is the hand spinning the dough (the gas), creating a hot, flat disk.

2. The Cooking Pot (Nucleosynthesis)

Once the gas is in this spinning disk, it gets squeezed and heated to temperatures hotter than the center of the sun (billions of degrees). This is where the "cooking" happens. The paper explores how this extreme heat changes the ingredients (atoms) in the gas.

The scientists found that the "recipe" changes depending on where the neutron star is in the giant star's body:

• Scenario A: The Outer Shell (The Soup)
  When the neutron star is eating the giant star's outer layers (mostly hydrogen), the spinning disk acts like a proton-rich oven.

  • What happens: The heat is so intense that protons (hydrogen nuclei) slam into other atoms, building them up rapidly.
  • The Result: It creates rare, "proton-rich" elements that are usually hard to make. It's like a chef who can only use salt and pepper but manages to create a complex, spicy dish. This contributes to the "chemical soup" of the galaxy.

• Scenario B: The Mixed Zone (The Stew)
  As the neutron star dives deeper, it starts eating a mix of hydrogen and helium.

  • What happens: The cooking style shifts. Now, "alpha particles" (helium nuclei) become the main ingredient.
  • The Result: This creates a specific mix of elements like Titanium-44 and Nickel-56.
  • The Twist: The paper found a fascinating separation. The Titanium-44 is made in the outer edges of the spinning disk and ejected slowly. The Nickel-56 is made in the inner core of the disk and ejected very fast.
  • The Analogy: Imagine a centrifuge spinning salad. The heavy lettuce (Nickel) flies out to the edge fast, while the lighter dressing (Titanium) stays closer to the center. If this explosion happens, we would see the Nickel further out in space than the Titanium.

• Scenario C: The Core (The Furnace)
  If the neutron star dives all the way to the giant star's core (pure helium), the heat becomes so extreme that the atoms break apart and reform into the most stable, heavy elements possible (like Iron).

  • The Result: It's like a furnace so hot that it melts everything down and only the strongest, most durable bricks (Iron and Nickel) can survive.

3. Why This Matters

Why should we care about a neutron star eating a giant star?

1. It's a New Factory: This process creates elements that other explosions (like standard supernovae) might miss. It explains where some of the rare, proton-rich elements in the universe come from.
2. It's a Detective Clue: The paper suggests that if we look at the remnants of these explosions (like the famous Cassiopeia A supernova), we might be able to tell if they were caused by this "Common Envelope" event.
   • How? By checking the speed of the elements. If the Nickel is flying faster and is further away than the Titanium, it's a strong hint that a neutron star was spinning a disk inside a giant star before the explosion.

The Bottom Line

This paper changes the story of what happens when stars collide. Instead of a messy, chaotic crash, it's a highly organized, high-speed spinning disk that acts as a cosmic kitchen. Depending on the ingredients and the heat, it cooks up different flavors of elements, leaving behind a unique "fingerprint" (the speed and type of elements) that astronomers can use to solve the mystery of how these violent cosmic events actually work.

Technical Summary

Here is a detailed technical summary of the paper "Angular momentum drives proton-rich nucleosynthesis in hyperaccreting neutron stars in common envelopes" by Hall-Smith et al. (2026).

1. Problem Statement

Common Envelope (CE) evolution occurs when a binary system undergoes rapid mass transfer, leading to a shared envelope surrounding both the core of the expanding star and its companion. If the companion is a compact object (e.g., a neutron star), it spirals inward, accreting material at hyper-accretion rates.

Previous nucleosynthesis studies of these systems (e.g., Keegans et al. 2019; Anninos et al. 2026) primarily focused on non-rotating scenarios or systems with negligible angular momentum. In these models, accretion drives turbulent convection with extremely short timescales (milliseconds), limiting the duration material spends at extreme temperatures. Consequently, these models predicted limited nucleosynthesis, often restricted to iron-peak elements or trace amounts of heavy elements, and failed to account for the formation of accretion disks.

The Gap: This study addresses the critical role of angular momentum in accreting material. It investigates how the formation of an accretion disk alters the thermodynamic trajectory (temperature and density evolution) of the material, potentially allowing for longer residence times at high temperatures and enabling different nucleosynthetic pathways, specifically the rapid proton-capture process (rp-process).

2. Methodology

System Configuration

• Progenitor System: A binary consisting of a 1.5 $M_\odot$ neutron star and a 15 $M_\odot$ massive companion (Zero Age Main Sequence metallicity of 2%).
• Evolutionary Stage: The companion is at the end of core hydrogen burning, expanding to engulf the neutron star. The study models the "plunge-in" phase where the neutron star spirals from the hydrogen-rich outer envelope toward the helium-rich core.
• Accretion Rates: Investigated rates ranging from $1 \times 10^{-5}$to$512 \times 10^{-5} M_\odot \text{s}^{-1}$.

Trajectory Modeling

The authors developed a three-phase trajectory model for material accreting onto the neutron star, incorporating a specific angular momentum of $j_{ang} = 10^{17} \text{ cm}^2 \text{s}^{-1}$:

1. Infall Phase: Free-fall accretion until centrifugal force balances gravity, forming a disk at radius $r_{disk}$.
2. Disk Evolution: Modeled using an $\alpha$-disk prescription (Shakura & Sunyaev 1973). This phase is significantly longer (thousands of seconds) than convective turnover times, allowing for sustained nuclear burning.
3. Ejection Phase: Material is ejected via a disk wind, following an exponential expansion followed by a power-law homologous expansion.

Nucleosynthesis Calculations

• Code: Used the NuGrid nucleosynthesis tool PPN1 for post-processing.
• Reaction Network: A comprehensive network of 5,234 isotopes (from neutrons/protons up to $^{216}\text{Po}$).
  • Crucial Update: Unlike previous studies using limited seed nuclei, this model includes all stable solar seed nuclei up to $^{209}\text{Bi}$. This allows for the accurate modeling of heavy element synthesis.
• Reaction Rates: Utilized the updated JINA-CEE Reaclib (June 2021), supplemented with KADoNiS neutron capture rates and specific beta-decay rates.

3. Key Contributions

1. Inclusion of Angular Momentum: Demonstrated that angular momentum forces the formation of a long-lived accretion disk, extending the timescale of nuclear burning from milliseconds to thousands of seconds.
2. Expanded Seed Nuclei: Showed that limiting initial compositions to light isotopes (up to $^{62}\text{Ni}$) severely underestimates nucleosynthesis yields. Including heavy seeds up to Bismuth is essential for modeling the full chart of nuclides.
3. Proton-Rich (rp-process) Mechanism: Identified a new mechanism for galactic chemical evolution where CE events eject significant amounts of proton-rich isotopes, challenging the view that such environments are solely r-process (neutron-rich) or iron-peak dominated.
4. Spatial Separation of Isotopes: Mapped the production of specific isotopes ($^{44}\text{Ti}$ vs. $^{56}\text{Ni}$) to specific radial zones within the accretion disk, linking them to distinct ejection velocities.

4. Key Results

The study analyzed nucleosynthesis across three distinct regions of the companion star as the neutron star spirals inward:

A. Hydrogen-Rich Outer Envelope ($1 \times 10^{-5} M_\odot \text{s}^{-1}$)

• Conditions: Peak temperatures reach $\sim 1.8$ GK.
• Process: High proton abundance drives the rp-process.
• Yields: Material reaches the Sn-Sb-Te cycle (waiting points at $^{100}\text{Cd}$, etc.). Significant mass fractions of neutron-deficient isotopes are synthesized.
• Significance: This provides a viable mechanism for the production of p-nuclides and proton-rich isotopes in the galaxy, distinct from standard supernovae.

B. Mixed Hydrogen-Helium Region ($32 \times 10^{-5} M_\odot \text{s}^{-1}$)

• Conditions: Peak temperatures range from 2 to 6.21 GK.
• Process: A transition occurs where $\alpha$-capture dominates at lower temperatures, followed by proton captures at higher temperatures.
• Yields:
  • $\alpha$-capture products: High yields of $^{28}\text{Si}$, $^{32}\text{S}$, $^{36}\text{Ar}$, $^{40}\text{Ca}$, and $^{44}\text{Ti}$.
  • Iron Peak: Production of $^{56}\text{Ni}$ occurs at the highest temperatures (inner disk).
  • Spatial Inversion: There is a strong inverse relationship between $^{44}\text{Ti}$ and $^{56}\text{Ni}$. $^{44}\text{Ti}$ is produced in the outer disk (lower peak T, slower ejection), while $^{56}\text{Ni}$ is produced in the inner disk (higher peak T, faster ejection).
  • Limitation: The rp-process does not reach the Sn-Sb-Te cycle in this region due to reduced proton availability compared to the outer envelope.

C. Helium Core Region ($16 \times 10^{-5}$to$512 \times 10^{-5} M_\odot \text{s}^{-1}$)

• Conditions: Extreme densities and temperatures (up to 8.72 GK), reaching Nuclear Statistical Equilibrium (NSE).
• Yields:
  • **Low/Mid Accretion ($16 \times 10^{-5}$):** Dominated by$\alpha$-captures, but high neutron flux from$(\alpha, n)$ reactions produces trace amounts of neutron-rich isotopes.
  • **High Accretion ($512 \times 10^{-5}$):** Material settles into NSE, producing almost exclusively **iron-group elements** (peaking at$^{56}\text{Ni}$). The rp-process is suppressed as the system favors the most tightly bound nuclei.

5. Significance and Implications

• Galactic Chemical Evolution: This study establishes that Common Envelope events involving neutron stars are a significant, previously underappreciated source of proton-rich (rp-process) isotopes and p-nuclides in the universe.
• Observational Diagnostics: The spatial separation of $^{44}\text{Ti}$ and $^{56}\text{Ni}$ offers a unique observational signature.
  • Since $^{56}\text{Ni}$ is ejected faster (inner disk) and $^{44}\text{Ti}$ slower (outer disk), the resulting transient remnant should show iron (decay product of $^{56}\text{Ni}$) located further out than the $^{44}\text{Ti}$ emission.
  • This velocity distribution could distinguish CE-driven supernova-like outbursts from other explosion mechanisms (e.g., core-collapse supernovae). The authors suggest this pattern may be relevant to remnants like Cassiopeia A or SN 1987A.
• Modeling Necessity: The work underscores that accurate modeling of CE nucleosynthesis must include angular momentum (disk formation) and a full range of seed nuclei. Neglecting these factors leads to incomplete and potentially incorrect predictions of chemical yields.

In conclusion, the paper demonstrates that angular momentum transforms the accretion environment from a short-lived convective burst into a long-lived disk, fundamentally altering the nucleosynthetic output to include significant proton-rich yields and creating a distinct spatial-velocity signature for heavy isotopes.