Distinct neutrino signatures and onset condition of quark deconfinement in accretion-induced collapse of white dwarfs

This study presents the first general relativistic simulations of accretion-induced collapse using hybrid equations of state, demonstrating that a first-order quark deconfinement phase transition triggers a second collapse and a distinctive neutrino burst, with a tightly constrained onset mass that makes AIC a uniquely sensitive probe for determining QCD phase transition thresholds and the existence of protohybrid stars.

The Gist

The Big Picture: A Star's "Second Wind"

Imagine a white dwarf star as a heavy, dense ball of dead stellar material. Usually, these stars sit quietly, but if they steal too much mass from a neighbor, they get too heavy to hold themselves up. They collapse inward, bounce back, and settle down. This is called an Accretion-Induced Collapse (AIC).

This paper simulates what happens inside that collapsing star if the extreme pressure turns the normal "nuclear soup" (made of protons and neutrons) into something stranger: quark matter.

Think of the star's core like a block of ice. Under normal pressure, it's hard ice (hadronic matter). But if you squeeze it hard enough, it melts into water (quark matter). The researchers wanted to see what happens if this "melting" occurs inside a collapsing star.

The Story of the Collapse

The simulation tells a story with two distinct chapters:

Chapter 1: The First Bounce
The star collapses until it hits a point where the nuclear force acts like a stiff spring, stopping the fall. The star bounces back, sending a shockwave out. This creates a "Protoneutron Star" (PNS)—a hot, dense baby neutron star. It releases a massive burst of neutrinos (ghostly particles that barely interact with anything), like a star sneezing.

Chapter 2: The Slow Squeeze and the Second Collapse
After the bounce, the star doesn't just sit there. It slowly cools down, losing heat like a cup of coffee. As it cools, it loses the thermal pressure that was helping it hold its shape, so it starts to shrink again.

Here is where the "quark melting" happens. As the star shrinks, the pressure in the center gets so high that the nuclear "ice" turns into "quark water."

• The Problem: Quark matter is "softer" (less resistant to squeezing) than nuclear matter.
• The Result: The star suddenly loses its structural support. It undergoes a second, faster collapse.

Chapter 3: The Hard Stop and the Second Burst
The collapse doesn't go on forever. The center eventually turns into a super-hard, stiff core of pure quark matter. This acts like a concrete wall, stopping the fall instantly.

• This sudden stop creates a second shockwave that shoots outward.
• This second shockwave triggers a second burst of neutrinos.

The Key Discovery: A Unique "Fingerprint"

The most important finding of this paper is how different this process is compared to other famous star explosions (like Core-Collapse Supernovae).

The "Heavy Envelope" vs. The "Naked Core"

• Normal Supernovae (CCSNe): These stars are like onions with many layers. When they collapse, they are still being fed by a massive, heavy outer shell (an envelope) that keeps pouring material onto the core. This extra weight masks the specific details of the "quark melting." It's like trying to hear a whisper in a noisy stadium; the crowd (the heavy envelope) drowns out the specific signal.
• AIC Stars: These stars are "naked." They have no heavy outer shell. Because there is no extra weight being dumped on them, the star's behavior is purely dictated by the physics of the core itself.

The Result:
Because the AIC star is "naked," the time it takes to reach the "quark melting" point and the strength of the second neutrino burst are extremely sensitive to the specific rules of how quarks behave.

• If the "melting point" (onset density) is slightly different, the timing of the second neutrino burst changes significantly.
• In normal supernovae, this timing is messy and hard to predict because of the heavy outer layers. In AIC, it is a clean, precise signal.

The "Detective" Analogy

Imagine you are a detective trying to figure out the exact composition of a mysterious substance.

• In a Supernova (CCSN): You are trying to analyze a sample, but someone keeps dumping sand on it. You can't tell exactly what the substance is because the sand is changing the measurements.
• In an AIC: You have a pure sample in a clean lab. If you see the substance react in a specific way, you know exactly what it is made of.

The paper argues that if we ever detect a neutrino signal from an AIC event in our galaxy, we could use that "clean signal" to finally solve a major mystery in physics: At exactly what pressure do protons and neutrons break apart into quarks?

Summary of Findings

1. Two Bursts: AIC events with quark phase transitions produce two distinct neutrino bursts separated by a few seconds. The second one is caused by the star collapsing a second time after turning into quark matter.
2. The "Sweet Spot": Even though the star is small, it gets hot enough over several seconds to trigger this quark transition, even in models where the transition usually requires very high pressure.
3. Precision Tool: Because AIC stars lack a heavy outer shell, the timing and energy of the neutrino bursts provide a much sharper, more accurate way to measure the properties of quark matter than we get from normal supernovae.
4. One Signal is Enough: The authors suggest that detecting just one of these events in our galaxy could give scientists enough data to rule out many theories about how matter behaves at its densest.

In short, the paper suggests that these specific types of star collapses are the universe's most precise "laboratories" for testing the laws of physics at the highest densities.

Technical Summary

Technical Summary: Distinct neutrino signatures and onset condition of quark deconfinement in accretion-induced collapse of white dwarfs

Problem Statement
The internal structure of dense matter and the nature of the Quantum Chromodynamics (QCD) phase transition (PT) from hadronic to deconfined quark matter remain open questions in nuclear astrophysics. While first-order QCD phase transitions have been studied in the context of core-collapse supernovae (CCSNe), the specific dynamics of such transitions in Accretion-Induced Collapse (AIC) systems have not been explored with realistic, long-term simulations. AIC progenitors (white dwarfs) differ significantly from CCSN progenitors (massive stars) by lacking a massive envelope and possessing lower total masses. It is unclear whether the thermodynamic conditions in AIC systems—specifically the interplay between neutrino cooling, contraction, and high temperatures—can trigger a QCD PT, and how the absence of a massive envelope alters the resulting hydrodynamics and neutrino observables compared to CCSNe.

Methodology
The authors present the first general relativistic, neutrino-radiation hydrodynamics simulations of AIC extending to several seconds after core bounce.

• Progenitor Models: The study utilizes double degenerate binary merger channels, generating spherically symmetric, non-rotating white dwarf (WD) progenitors with gravitational masses of $\sim 1.53 M_\odot$ and central densities of $10^9$ g cm$^{-3}$.
• Equation of State (EOS): The simulations employ realistic hadron-quark hybrid EOSs based on the relativistic density functional (RDF) formalism. Four specific EOS models (RDF-1.9, RDF-1.2, RDF-1.5, RDF-1.1) were selected to cover a range of onset densities ($\rho_{\text{onset}}$) for the hadron-quark mixed phase, varying from $4.6 \times 10^{14}$ to $8.8 \times 10^{14}$ g cm$^{-3}$.
• Numerical Setup: The simulations use the Gmunu code, solving Einstein's field equations under the conformal flatness condition. Neutrino transport is handled via an energy-integrated two-moment scheme with an implicit-explicit Runge-Kutta time integrator.
• Microphysics: The weak interaction library Weakhub is used. To bridge the gap between the initial cold WD state and the hot collapse phase, an effective deleptonization scheme mimics the proton fraction evolution during the collapse, switching to full two-moment transport upon core bounce.
• Comparative Analysis: The study includes a systematic comparison with CCSN simulations using the same hybrid EOSs but with a massive progenitor (12 $M_\odot$) to isolate the effects of the envelope.

Key Results

1. Phase Transition Dynamics: In all AIC models, the protoneutron star (PNS) undergoes a slow contraction driven by Kelvin-Helmholtz cooling after the initial core bounce. This contraction eventually reaches the conditions for a first-order QCD PT. The thermally suppressed onset density allows low-mass PNSs to enter the mixed phase even for EOSs with high onset densities.
2. Second Dynamical Collapse and PHS Formation: The softening of the EOS upon entering the mixed phase triggers a second dynamical collapse. This collapse is halted by the formation of a stiffer deconfined quark core, resulting in the birth of a quasistable Proto-Hybrid Star (PHS). The PHS consists of a deconfined quark core, a mixed-phase mantle, and a residual hadronic envelope.
3. Distinctive Neutrino Signatures:
   • Second Neutrino Burst: The second collapse generates a second outward-propagating shock, producing a distinct second neutrino burst. This burst is dominated by electron antineutrinos ($\bar{\nu}_e$) due to positron capture in a low-proton-fraction cavity formed near the core.
   • Energy and Luminosity: The second burst exhibits higher average energies across all neutrino species compared to the first burst. The total luminosity of the second burst varies significantly with the EOS, reaching up to $\sim 10^{52}$ erg s$^{-1}$.
4. Onset Mass Universality: Unlike CCSNe, where the presence of a massive envelope leads to continuous mass accretion and variable onset masses, AIC models exhibit a tightly constrained onset mass (variation $< 0.07\%$). The absence of a massive envelope means the PNS evolution is not supported by sustained accretion, making the timing of the PT highly sensitive to the EOS properties.
5. Empirical Relations: The authors establish empirical relations between the PT onset density ($\rho_{\text{mixed,c}}$) and neutrino observables. In AIC, the time delay between the two bursts ($t_Q$), the total neutrino luminosity, and the average $\bar{\nu}_e$ energy show a strong, distinct dependence on $\rho_{\text{mixed,c}}$. Specifically, $t_Q$ increases significantly with $\rho_{\text{mixed,c}}$, and $\langle \epsilon_{\bar{\nu}_e} \rangle$ decreases rapidly, a trend more pronounced than in CCSNe.

Significance and Claims
The paper claims that AIC systems offer a unique and potentially superior laboratory for constraining QCD phase transition thresholds compared to CCSNe.

• Enhanced Sensitivity: Due to the lack of a massive envelope and the narrow progenitor mass range ($\sim 1.44 - 2.5 M_\odot$), AIC neutrino signals are less contaminated by accretion dynamics. This results in a "cleaner" environment where observables like the time delay between bursts and burst energies are directly linked to the hybrid EOS properties.
• Observational Potential: The authors suggest that a single Galactic AIC neutrino detection could place strong constraints on the onset density of the first-order QCD PT and the characteristics of hybrid equations of state. The distinct second neutrino burst serves as a key observational probe for the existence of proto-hybrid stars.
• Multimessenger Implications: The study posits that these events may also produce detectable gravitational waves, gamma-ray bursts, and r-process nucleosynthesis, motivating future multidimensional simulations that include rotation and magnetic fields.

The authors maintain modesty regarding future work, noting that current results are based on spherically symmetric, non-rotating models. They acknowledge that rotation, magnetic fields, and advanced microphysics (e.g., inelastic scattering) will be necessary for realistic multimessenger predictions and that a comprehensive assessment of detectability in current and next-generation detectors remains a task for future research.