Researchers from Heidelberg University, including D. Gagnier, G. Leidi, M. Vetter, R. Andrassy, and F. K. Röpke, have delved into the complex dynamics of common envelope evolution (CEE), a critical phase in the life cycle of binary star systems. Their work, published in the journal Astronomy & Astrophysics, offers new insights into the small-scale gas dynamics and forces at play during this process, with potential implications for our understanding of stellar evolution and, by extension, the energy sector’s reliance on nuclear fusion processes.
Common envelope evolution occurs when two stars in a binary system orbit so closely that one star expands and engulfs the other, creating a shared envelope of gas. This phase is crucial for understanding the ultimate fate of binary systems, as it can lead to the formation of exotic objects like neutron stars and black holes. However, simulating this process is challenging due to the vast differences in scale between the stars and the complex dynamics involved.
The researchers employed local, three-dimensional hydrodynamic simulations to study the small-scale dynamics around a compact companion star as it plunges into the envelope of a red giant star. They focused on the effects of rotation, accretion, and stratification—the variation in density and temperature within the envelope. Their simulations revealed that stratification generates an inward-directed force on the companion, partially counteracted by an outward lift force induced by rotation. Both forces significantly influence the evolution of the binary separation, a critical factor in determining the system’s future.
The study also found that without accretion and with small gravitational softening radii, a quasi-hydrostatic bubble forms around the companion. However, accretion prevents this bubble’s formation and converts the companion’s kinetic energy into heat, which could contribute to the ejection of the envelope. Interestingly, the researchers noted that accretion only marginally affects the drag and lift forces acting on the companion.
Moreover, the companion’s spin-up rate—the rate at which it gains angular momentum—varies non-monotonically over time, first increasing and then decreasing as it plunges deeper into the envelope. This finding could have implications for the final angular momentum and spin of the compact object, which is crucial for understanding the formation and evolution of neutron stars and black holes.
The researchers propose revised semi-analytical prescriptions for both drag and lift forces, which could improve future simulations of common envelope evolution. They also suggest that future magnetohydrodynamic simulations should investigate how accretion, rotation, and stratification affect magnetic amplification and how magnetic fields, in turn, influence mass and angular momentum accretion rates, as well as the drag and lift force exerted on the companion.
For the energy sector, a deeper understanding of stellar evolution, including common envelope evolution, can provide valuable insights into the life cycles of stars and the processes that drive nuclear fusion. This knowledge can contribute to the development of more accurate models of stellar energy production and the eventual fate of stars, which are essential for predicting the availability of fusion-based energy sources in the universe. Additionally, understanding the dynamics of binary star systems can help in the search for and characterization of exoplanets, some of which may harbor conditions suitable for life, further expanding the potential for energy exploration and utilization in the cosmos.
Reference(s):
Gagnier, D., Leidi, G., Vetter, M., Andrassy, R., & Röpke, F. K. (2023). Local simulations of common-envelope dynamical inspiral. Impact of rotation, accretion, and stratification. Astronomy & Astrophysics, 671, A10.
This article is based on research available at arXiv.