Successful Common Envelope Ejection and Binary Neutron Star Formation in 3D Hydrodynamics (2011.06630v2)

Abstract: A binary neutron star merger has been observed in a multi-messenger detection of gravitational wave (GW) and electromagnetic (EM) radiation. Binary neutron stars that merge within a Hubble time, as well as many other compact binaries, are expected to form via common envelope evolution. Yet five decades of research on common envelope evolution have not yet resulted in a satisfactory understanding of the multi-spatial multi-timescale evolution for the systems that lead to compact binaries. In this paper, we report on the first successful simulations of common envelope ejection leading to binary neutron star formation in 3D hydrodynamics. We simulate the dynamical inspiral phase of the interaction between a 12$M_\odot$ red supergiant and a 1.4$M_\odot$ neutron star for different initial separations and initial conditions. For all of our simulations, we find complete envelope ejection and final orbital separations of $a_{\rm f} \approx 1.3$-$5.1 R_\odot$ depending on the simulation and criterion, leading to binary neutron stars that can merge within a Hubble time. We find $\alpha_{\rm CE}$-equivalent efficiencies of $\approx 0.1$-$2.7$ depending on the simulation and criterion, but this may be specific for these extended progenitors. We fully resolve the core of the star to $\lesssim 0.005 R_\odot$ and our 3D hydrodynamics simulations are informed by an adjusted 1D analytic energy formalism and a 2D kinematics study in order to overcome the prohibitive computational cost of simulating these systems. The framework we develop in this paper can be used to simulate a wide variety of interactions between stars, from stellar mergers to common envelope episodes leading to GW sources.

• The paper demonstrates 3D hydrodynamic simulations that successfully eject the common envelope to form binary neutron stars.
• It models the inspiral of a 12 M☉ red supergiant and a 1.4 M☉ neutron star, focusing on core resolution and varied initial separations.
• Results show final orbital separations from 1.3 to 5.1 R☉ and efficiency values from 0.1 to 2.7, informing merger rate predictions.

Analysis of Successful Common Envelope Ejection for Binary Neutron Star Formation

The paper "Successful Common Envelope Ejection and Binary Neutron Star Formation in 3D Hydrodynamics," authored by Law-Smith et al., addresses a significant challenge in astrophysics: the understanding of common envelope (CE) phases during binary star evolution, specifically leading to the formation of binary neutron stars (BNS). For decades, the common envelope phase has been a complex stage to model due to its multi-dimensional and multi-physical nature. This research presents the first successful simulation of CE ejection in a context that predicts the formation of BNS systems, utilizing state-of-the-art 3D hydrodynamic simulations.

Methodological Approach

This paper focuses on the dynamical inspiral phase of CE evolution, simulating the interaction between a 12 $M_\odot$ red supergiant and a 1.4 $M_\odot$ neutron star. The simulations explore different initial separations and conditions to evaluate the outcomes of CE ejection. A significant departure from previous studies is the excision of the supergiant’s outer layers containing less than 0.1% of the binding energy, trimming the star to 10 $R_\odot$. This allows the researchers to concentrate computational resources on the dense core, simulating interactions with a fully resolved core to $\lesssim 0.005 R_\odot$.

Additionally, the 3D simulations are informed by an adjusted 1D analytic energy framework and a 2D kinematics paper. This multi-step approach balances computational feasibility with accuracy, overcoming the challenges of simulating full-scale stellar environments.

Numerical Results and Outcomes

The findings are robust, with complete envelope ejection achieved across all simulated models. Final orbital separations following successful CE ejection range from $a_{\text{f}}^* \approx 1.3$ to $2.8 R_\odot$ when utilizing the condition that the entire envelope outside the helium core is unbound. When considering just the material outside the secondary's current orbit, separations span from $a_{\text{f}}^{**} \approx 2.5$ to $5.1 R_\odot$.

The paper adopts the $\alpha$-formalism to assess efficiency, with values spanning from 0.1 to 2.7, suggesting complex dependencies on initial conditions and specific progenitor characteristics. These results provide crucial insights into the conditions conducive to BNS formation within a Hubble time.

Implications and Future Directions

The implications of this research are profound for both theoretical astrophysics and observational astronomy. Understanding the dynamics of CE phases impacts predictions on BNS merger rates, a crucial aspect following the detection of gravitational waves from such events. Furthermore, the research introduces a validated framework applicable to other stellar interactions, such as BH/NS systems, presenting a versatile tool for further exploration into stellar evolution and mergers.

Future avenues include extending the framework to incorporate accretion effects on the neutron star, which can significantly influence both GW and EM observational signatures. Long-term evolution studies involving CE remnants could enhance predictions of phenomena such as kilonovae or exotic supernova types.

Overall, this paper represents significant progress in the computational modeling of one of astrophysics' intricate mechanisms, providing a framework for future exploratory extensions of binary interactions in stellar evolution.