Harvard Smithsonian Center for Astrophysics | 2019 Dec 04
The large donor star has a tear-drop shape because it is filling its Roche lobe, donating
mass to two compact stars in a tight binary that correspond to a pair of neutron stars
or black holes. If the mass-gaining binary were simply a single star, mass would flow
steadily through one point, the L1 point. Because the mass gainer is a binary, we see
interesting patterns of mass flow, executing a complex dance. Credit: Sophie Schroeder,
Niels Bohr Institute, and Morgan Macleod, Institute for Theory & Computation, CfA
A scientist at the Center for Astrophysics | Harvard & Smithsonian has announced the discovery that mass in triple star systems takes on the characteristics of recipient stars before mass is actually transferred, which may allow scientists to re-examine previously labeled binary star systems for evidence of a third companion.
"Scientists already knew that the transfer of mass from one star to another is one of the most important processes in astronomy, because it produces events that release tremendous amounts of energy -- from Type Ia supernovae to the merger of black holes. I wanted to extend this to understand what happens if one star transfers mass to a pair of stars," said Rosanne Di Stefano, astronomer at CfA. "To do that I had to generalize the familiar process of star-to-star mass flow."
Di Stefano suggests that the flow of mass is similar to the flow of water through a faucet. "The mass-giving star takes on a tear-drop shape, filling its 'Roche Lobe' and transferring mass through a small region called the L1 point," said Di Stefano. "The Roche Lobe and the L1 point provide a mathematical basis for computing exactly what happens to transferred mass in a binary where both gravity and rotation influence the flow of matter." ...
"In triple-star systems, instead of a fixed position with respect to the rotating stars, like we see in binary systems, the L1 point executes an orbit over a three-dimensional surface, and the shape of the Roche Lobe pulsates in a periodic fashion,” said Di Stefano. "This means that mass is pulled away from the donor star with its motion already imprinted with the periodicity of the inner binary, even though it has yet to travel to that binary, and the visual outcome looks much like an elegant, yet explosive Pas de trois." ...
The Dynamical Roche Lobe in Hierarchical Triples ~ Rosanne Di Stefano
- Monthly Notices of the RAS 491(1):495 (Jan 2020) DOI: 10.1093/mnras/stz2572
- arXiv.org > astro-ph > arXiv:1903.11618 > 27 Mar 2019