Formation of neutron stars via accretion-induced collapse and core-merger-induced collapse inside planetary nebulae from white dwarf binaries

Technical Summary: Formation of Neutron Stars via AIC and CMIC inside Planetary Nebulae

Problem Statement
While the formation of neutron stars (NSs) via accretion-induced collapse (AIC) and merger-induced collapse (MIC) is a known theoretical pathway, the specific scenario of these events occurring inside planetary nebulae (PNe) has not been seriously considered in previous literature. Existing studies have addressed NS formation after PNe or core-collapse supernovae within PNe under specific third-body conditions, but the direct formation of an NS within a PN via binary evolution channels involving white dwarfs (WDs) and giant stars remains unexplored. This paper addresses the gap in understanding whether oxygen-neon-magnesium (ONeMg) WD binaries can evolve to form NSs while still embedded in the nebular structures of their progenitor systems.

Methodology
The study employs two distinct computational approaches to model two proposed evolutionary channels:

1. Stellar Evolution Simulations (MESA):

   • Code: The Modules for Experiments in Stellar Astrophysics (MESA) code (version 15140) is used to simulate 1D binary evolution.
   • System: A binary system consisting of a 1.2 $M_\odot$ non-magnetic ONeMg WD and a 1.5 $M_\odot$ Red Giant (RG) star with an initial orbital period of 120 days.
   • Physics: The simulation incorporates Roche lobe overflow (RLOF) mass transfer using the Kolb & Ritter (1990) prescription, stellar winds (Reimers for RGB, Blocker for AGB), and angular momentum evolution.
   • Accretion Physics: Mass retention efficiency is calculated based on stable hydrogen and helium burning prescriptions (Hillman et al. 2015; Kato & Hachisu 2004). The WD mass growth is tracked until it approaches the Chandrasekhar limit.

2. Binary Population Synthesis (BPS):

   • Code: An updated version of the BSE (Binary Star Evolution) code is used to simulate $10^7$ primordial binaries.
   • Initial Conditions: Primary masses follow the Kroupa et al. (1993) initial mass function; mass ratios follow a flat distribution; initial separations follow a power-law distribution; metallicity is fixed at $Z=0.02$.
   • Physics: The model includes mass transfer, stellar winds, tidal interactions, common envelope (CE) evolution (using the $\alpha$-formalism with $\alpha_{CE}=1.0$ and $\lambda=0.5$), and supernova kicks.
   • Focus: The simulation specifically tracks ONeMg WD binaries where the WD merges with the core of a giant companion during the CE phase, provided the total core mass exceeds the Chandrasekhar limit ($1.44 M_\odot$).

Key Contributions and Proposed Channels
The paper proposes two novel evolutionary pathways for NS formation within PNe:

1. Symbiotic Nebula Channel (AIC):

   • Mechanism: In a WD-RG (or WD-AGB) binary, the giant companion loses mass via stellar winds and/or stable RLOF. This mass forms a nebula surrounding the binary. The central accreting ONeMg WD grows in mass via stable accretion until it reaches the Chandrasekhar limit and undergoes AIC.
   • Nebula Ionization: The resulting nebula (termed a "symbiotic nebula") is ionized by the hot core of the giant star or the accreting WD itself.
   • Outcome: The formation of an NS that likely retains its WD companion, creating an NS-WD binary system.

2. Core-Merger-Induced Collapse (CMIC) Channel:

   • Mechanism: During the unstable mass transfer phase of an ONeMg WD binary, the system enters a Common Envelope (CE) phase. The WD spirals inward and merges with the hot core of the giant companion.
   • Collapse Condition: If the combined mass of the WD and the giant's core exceeds the Chandrasekhar limit, the merger triggers a collapse (CMIC) rather than a Type Ia supernova.
   • Outcome: The formation of a newborn NS (potentially a pulsar) embedded within the expanding CE material, which may manifest as a planetary nebula or a pulsar wind nebula.

Results

• MESA Simulation Results:

  • The simulation demonstrates that an ONeMg WD can accrete sufficient mass from an RG companion to reach the Chandrasekhar limit.
  • During the evolution, the RG loses approximately $0.4 M_\odot$, with roughly half accreted by the WD and the remainder ejected to form a surrounding nebula.
  • The core mass of the giant companion remains sufficiently massive ($>0.3 M_\odot$) to reach effective temperatures capable of ionizing the nebula, even after envelope stripping.
  • The study notes that previous similar simulations (e.g., Ablimit 2023) did not discuss the resulting PNe; this work explicitly links the mass loss to nebula formation.
  • In cases involving highly magnetized WDs, the ejected mass could be significantly higher, potentially forming bipolar nebulae via jet launching.

• BPS Simulation Results:

  • The Galactic formation rate for NSs formed via CMIC inside PNe is estimated at $\sim 3.6 \times 10^{-4} \text{ yr}^{-1}$ (assuming a star formation rate of $2 M_\odot \text{ yr}^{-1}$).
  • The distributions of CE mass and total core mass for systems leading to CMIC show a wide range suitable for forming both nebulae and NSs.
  • The hot WD and giant cores during the spiral-in process can reach temperatures ($\sim 30,000$ K) sufficient to ionize the CE, while the newborn NS may further power the nebula via a pulsar wind.

Significance and Claims
The paper claims to be the first to seriously consider and model the formation of neutron stars inside planetary nebulae via binary evolution.

• Novelty: It introduces the concept of "symbiotic nebulae" formed by WD-RG mass transfer leading to AIC, and expands the definition of CMIC to include mergers within the CE of WD binaries resulting in NSs within PNe.
• Population: The author suggests that the Milky Way may host dozens of NS-WD binary systems formed via the AIC channel described.
• Observational Prospects: The paper posits that next-generation ground- and space-based telescopes may soon provide compelling evidence for these scenarios, particularly through observations of pulsar wind nebulae located inside expanding planetary nebulae.
• Theoretical Impact: These findings offer new insights into rare phenomena such as WD collapses and explosions occurring within the specific environment of planetary nebulae, refining our understanding of binary stellar evolution.