Observational signatures of runaway mass transfer leading to stellar mergers

One possible evolutionary path to a gravitational wave progenitor involves a phase of unstable mass transfer in a binary star, leading to a phase known as common envelope evolution. An alternative outcome of this unstable phase is the merger of the two stars. During unstable mass transfer, the binary system expels several solar masses of material into its surroundings through equatorial outflows, disk winds, and jets. This mass loss, along with the angular momentum it removes, plays a crucial role in shaping the binary’s subsequent evolution and observable characteristics. However, because this process is only loosely constrained in stellar evolution models, it leads to significant uncertainties in predicting the relative frequencies of various evolutionary pathways.

A major observational breakthrough has been the discovery of a slow, gradual brightening in the months to years leading up to the peak of luminous red novae (LRNe)—a unique type of astronomical transients. Since LRNe are observational manifestation of stellar mergers, this brightening is thought to mark the unstable phase of binary mass transfer and mass loss. Number and quality of observations of this phase will increase substantially once the Vera Rubin Observatory’s LSST begins operations in late 2025.

The goal of this project is to develop a framework for understanding unstable mass transfer in binary stars and its observational signatures. This phase encompasses a rich array of astrophysical processes: outflows, disks, jets, radiation transport, shocks, molecule formation, and dust production. Based on the student’s interests, the project will involve a combination of semi-analytic models, 1D stellar evolution, and multi-dimensional simulations. The project will build on prior work in the group, including 1D modeling (Cehula & Pejcha 2023), moving-mesh radiation hydrodynamics (Calderón et al. 2021, 2024), and 3D magnetohydrodynamics simulations (Gagnier & Pejcha 2023, 2024).

References:

[1] Calderón et al., 2021, Moving-mesh radiation-hydrodynamic simulations of wind-reprocessed transients, https://ui.adsabs.harvard.edu/abs/2021MNRAS.507.1092C/abstract
[2] Calderón et al., 2024, The effect of relativistic precession on light curves of tidal disruption events, https://ui.adsabs.harvard.edu/abs/2024MNRAS.528.2568C/abstract
[3] Cehula & Pejcha, 2023, A theory of mass transfer in binary stars, https://ui.adsabs.harvard.edu/abs/2023MNRAS.524..471C/abstract
[4] Gagnier & Pejcha, 2023, Post-dynamical inspiral phase of common envelope evolution. Binary orbit evolution and angular momentum transport, https://ui.adsabs.harvard.edu/abs/2023A%26A...674A.121G/abstract
[5] Gagnier & Pejcha, 2024, Post-dynamical inspiral phase of common envelope evolution. The role of magnetic fields, https://ui.adsabs.harvard.edu/abs/2024A%26A...683A...4G/abstract