Common Envelope Evolution: Where we stand and how we can move forward (1209.4302v2)

Abstract: This work aims to present our current best physical understanding of common-envelope evolution (CEE). We highlight areas of consensus and disagreement, and stress ideas which should point the way forward for progress in this important but long-standing and largely unconquered problem. Unusually for CEE-related work, we mostly try to avoid relying on results from population synthesis or observations, in order to avoid potentially being misled by previous misunderstandings. As far as possible we debate all the relevant issues starting from physics alone, all the way from the evolution of the binary system immediately before CEE begins to the processes which might occur just after the ejection of the envelope. In particular, we include extensive discussion about the energy sources and sinks operating in CEE, and hence examine the foundations of the standard energy formalism. Special attention is also given to comparing the results of hydrodynamic simulations from different groups and to discussing the potential effect of initial conditions on the differences in the outcomes. We compare current numerical techniques for the problem of CEE and also whether more appropriate tools could and should be produced (including new formulations of computational hydrodynamics, and attempts to include 3D processes within 1D codes). Finally we explore new ways to link CEE with observations. We compare previous simulations of CEE to the recent outburst from V1309 Sco, and discuss to what extent post-common-envelope binaries and nebulae can provide information, e.g. from binary eccentricities, which is not currently being fully exploited.

• The paper analyzes energy formalism uncertainties, highlighting limitations of the αCE efficiency parameter in modeling envelope ejection.
• The paper explores angular momentum conservation via the γ‐formalism and cautions against its blanket application in all CEE scenarios.
• The paper compares recent hydrodynamic simulations with observational data, advocating refined initial conditions for future CEE research.

Overview of Common Envelope Evolution Research

The paper "Common Envelope Evolution: Where we stand and how we can move forward" presents an in-depth exploration of the theoretical and computational aspects related to common envelope evolution (CEE) in binary star systems. Authored by a diverse group of researchers, including N. Ivanova, S. Justham, and several other experts, the paper thoroughly examines the current understanding of CEE and outlines potential pathways for future research.

Common envelope (CE) phases are critical in the evolution of many binary systems, particularly those involving close binary configurations. The outcomes of these phases can lead to a range of post-CE objects such as compact binaries, Type Ia supernovae progenitors, and the nearest stellar-mass black holes. The paper effectively reviews existing theoretical frameworks, such as energy and angular momentum conservation laws, and discusses their applicability to understanding CEE scenarios.

Energy Budget and Its Complexities

The paper provides a rigorous analysis of the energy formalism commonly used to predict CEE outcomes, where the orbital energy reduction is equated to the envelope's binding energy. An essential parameter, $\alpha_{\rm CE}$, represents the efficiency of energy conversion during CEE. However, the authors highlight that this formalism often oversimplifies complex energy interactions, such as internal energy contributions, recombination energy, and potential additional sources like accretion energy. The paper critically evaluates these assumptions and suggests that the standard energy approach may not always feasibly capture the full scope of energetic dynamics within CEE.

Angular Momentum Considerations

Angular momentum conservation is another pivotal aspect explored in this paper. The authors discuss the $\gamma$-formalism, which is used as an alternative to the traditional energy prescription, especially in cases where the energy formalism fails, such as in the formation of double white dwarfs. While this angular momentum-based parametrization offers insights into certain binary configurations, the paper warns against its blanket application across all CEE events. The sensitivity of outcomes to specific parameter values and the necessary conditions for post-CE systems make generalized assumptions about $\gamma$ potentially misleading.

Computational Progress and Simulation Insights

The paper explores contemporary advancements in computational simulations of CEE, comparing results from recent hydrodynamic studies using grid-based and smoothed particle hydrodynamics (SPH) methodologies. The findings from simulations highlight significant variability in final separations and mass ejections, influenced by initial conditions such as corotational spin states and mass ratios.

Practical and Theoretical Implications

One of the core contributions of this paper is its exploration of the implications of CEE on astrophysical phenomena and observations. The research presents potential observational signatures, such as the role of CEE in shaping planetary nebulae and influencing post-CE binary eccentricities, which could provide empirical constraints on theoretical models. Furthermore, V1309 Sco's transient outburst is examined as an exemplary CE-related event, providing valuable empirical data to compare against simulation predictions.

Future Directions

The paper outlines several strategic directions for future research in CEE. These include refining theoretical models to account for multi-dimensional hydrodynamics and incorporating additional physical processes such as recombination energy. The paper calls for improved initial conditions in simulations, better resolution of angular momentum and energy transfer, and a holistic integration of observational data to inform and validate theoretical advancements.

In conclusion, the paper significantly contributes to the field of astrophysics by integrating current knowledge with innovative hypotheses and sophisticated computational approaches. This comprehensive examination of CEE not only underscores existing challenges but also charts a path forward for resolving one of the most intricate problems in binary stellar evolution.