Hydrodynamic moving-mesh simulations of the common envelope phase in binary stellar systems (1512.04529v1)

Abstract: The common envelope (CE) phase is an important stage in binary stellar evolution. It is needed to explain many close binary stellar systems, such as cataclysmic variables, Type Ia supernova progenitors, or X-ray binaries. To form the resulting close binary, the initial orbit has to shrink, thereby transferring energy to the primary giant's envelope that is hence ejected. The details of this interaction, however, are still not understood. Here, we present new hydrodynamic simulations of the dynamical spiral-in forming a CE system. We apply the moving-mesh code AREPO to follow the interaction of a $1M_\odot$ compact star with a $2M_\odot$ red giant possessing a $0.4M_\odot$ core. The nearly Lagrangian scheme combines advantages of smoothed particle hydrodynamics and traditional grid-based hydrodynamic codes and allows us to capture also small flow features at high spatial resolution. Our simulations reproduce the initial transfer of energy and angular momentum from the binary core to the envelope by spiral shocks seen in previous studies, but after about 20 orbits a new phenomenon is observed. Large-scale flow instabilities are triggered by shear flows between adjacent shock layers. These indicate the onset of turbulent convection in the common envelope, thus altering the transport of energy on longer time scales. At the end of our simulation, only 8% of the envelope mass is ejected. The failure to unbind the envelope completely may be caused by processes on thermal time scales or unresolved microphysics.

• The paper demonstrates that moving-mesh simulations capture spiral shock energy transfer and reveal large-scale turbulent instabilities during the common envelope phase.
• The study finds that only 8% of the envelope mass becomes unbound, highlighting the challenges in achieving complete envelope ejection.
• The methodology using the Arepo code ensures superior angular momentum conservation and fine-structure resolution, bolstering the simulation's reliability.

Hydrodynamic Moving-Mesh Simulations of the Common Envelope Phase in Binary Stellar Systems

The paper "Hydrodynamic moving-mesh simulations of the common envelope phase in binary stellar systems" by S. T. Ohlmann et al. details significant advancements in modeling the complex interactions during the common envelope (CE) phase of binary stellar evolution using the moving-mesh code Arepo. This research additionally employs hydrodynamic simulations to enhance understanding of the CE phase, a critical yet poorly understood period in the evolution of binary systems with compact objects, which drives the formation of close binaries pertinent to phenomena such as cataclysmic variables, Type Ia supernova progenitors, and X-ray binaries.

Methodology and Simulation Setup

The authors employ the Arepo code, renowned for its nearly Lagrangian moving-mesh methodology, enabling the conservation of angular momentum and resolution of small-scale hydrodynamic features. The simulation involves a binary system consisting of a $1M_\odot$ compact star and a $2M_\odot$ red giant with a $0.4M_\odot$ core. The focus is on the dynamic spiral-in and subsequent CE phase, involving the transfer of energy and angular momentum necessary for envelope ejection—a prominent challenge in previous modeling attempts.

Key Findings

1. In-depth Hydrodynamic Analysis: The simulation confirms initial energy transfer from the core to the envelope via spiral shocks—a phenomenon observed in prior works. However, the authors document an unprecedented observation: large-scale flow instabilities initiated by shear flows, potentially leading to turbulent convection within the envelope and impacting longer-term energy transport.
2. Envelope Ejection Evaluation: Despite detailed modeling, only 8% of the envelope mass is unbound by the simulation's conclusion, highlighting the challenges in achieving complete envelope ejection in present models. The authors suggest that additional long-term processes or unresolved microphysics, such as recombination energy, may play roles in successful envelope ejection.
3. Resolution and Conservation: The Arepo code's ability to simulate using an adaptive moving mesh results in better conservation of angular momentum and resolution of fine structures compared to adaptive mesh refinement and smoothed particle hydrodynamics methods. The total energy is conserved within 3% during the run, showcasing the simulation's reliability and precision.

Implications and Future Directions

The findings underscore the importance of detailed hydrodynamic modeling to unravel the complexities of CE interactions, which are pivotal in the evolution of close binary systems. The observed large-scale instabilities could necessitate revised models incorporating turbulence physics and convection processes potentially operating on long time scales. This paper suggests a need for further enhancements, such as incorporating additional physical effects like recombination energy, to replicate real systems where the envelope is successfully ejected.

Future simulations could extend the parameter space exploration, considering various initial orbital parameters and mass combinations to robustly link simulation outcomes to observational CE systems. By improving microphysical treatments and computational techniques, we can expect a deeper theoretical understanding and predictive capability for the diverse outcomes of the CE phase in binary systems.

In conclusion, the research presents a step forward in simulating the intricate interactions during the CE phase, using enhanced numerical methods to capture dynamic instabilities and offering insights to drive future computational and theoretical advancements in the stellar astrophysics community.