Slice through the three-dimensional simulation volume after 105 days in the common envelope. In the orbital plane, the companion star and the red giant core are circling around each other. Credit: Ohlmann /HITS
HITS astrophysicists use new methods to simulate the common-envelope phase of binary stars, discovering dynamic irregularities that may help to explain how supernovae evolve. More than half the stars have a companion that can have a major impact on their primary companions. The interplay within these so-called binary star systems is particularly intensive when the 2 stars involved are going through a phase in which they are surrounded by a common envelope. Compared to the overall time taken by stars to evolve, this phase is extremely short, so astronomers have great difficulty observing and understanding it. This is where theoretical models with highly compute-intensive simulations come in to understand stellar events such as supernovae.
Scientists have successfully used simulations to discover dynamic irregularities that occur during the common-envelope phase and are crucial for the subsequent existence of binary star systems. These so-called instabilities change the flow of matter inside the envelope, thus influencing the stars’ distance from one another and determining, for example, whether a supernova will ensue and, if so, what kind it will be.
The study was a collaboration between Physics of Stellar Objects (PSO) group and Theoretical Astrophysics group (TAP). Prof. Volker Springel’s Arepo code for hydrodynamic simulations was used and adapted for the modeling. It solves the equations on a moving mesh that follows the mass flow, and thus enhances the accuracy of the model.
Binary star luminosity comes from the nuclear fusion of hydrogen at the core of the stars. As soon as the H is exhausted in the heavier of the two stars, the star core shrinks. At the same time, a highly extended stellar envelope evolves of H and He. The star becomes a red giant. As the envelope of the red giant goes on expanding, the companion star draws the envelope to itself via gravity, and part of the envelope flows towards it. In the course of this process the 2 stars come closer to one another. Finally, the companion star may fall into the envelope of the red giant and both stars are then surrounded by a common envelope. As the core of the red giant and the companion draw closer together, the gravity between them releases energy that passes into the common envelope. As a result, the envelope is ejected and mixes with interstellar matter in the galaxy, leaving behind it a close binary star system consisting of the core of the giant and the companion star.
Sebastian Ohlmann of the PSO group explains why this common-envelope phase is important for our understanding of the way various star systems evolve: “Depending on what the system of the common envelope looks like initially, very different phenomena may ensue in the aftermath, such as thermonuclear supernovae.” Note the core of the giant is anything between 1000 and 10000 times smaller than the envelope, so that spatial and temporal scale differences complicate the modeling process and make approximations necessary. The innovative simulations are a first step towards a better understanding of this phase. http://www.h-its.org/scientific-news/turbulent-times/
Recent Comments