Description:

In dense stellar environments, stars can physically collide. Stellar collisions play a key role in shaping the stellar population at the centers of stellar clusters and galactic nuclei. Depending on the collision's kinetic energy, different outcomes arise. When the collision kinetic energy is smaller than the binding energy of the colliding stars, the two stars merge into a more massive star, which can manifest as a group of stars appearing younger and bluer than their surroundings — known as blue straggler stars. Conversely, if the collision kinetic energy is larger, as in nuclear stellar clusters around a supermassive black hole, the colliding stars may be partially or completely destroyed, producing expanding ejecta. This process can generate bright flares, making these events promising nuclear transients. In this talk, I will discuss the properties of collision products in these two regimes. In the first part, I will explore magnetic field amplification in low-velocity collisions as a potential solution to the "angular momentum" problem for blue straggler stars. I will also present results for binary mergers and stable mass transfer in the context of blue straggler formation. In the second part, I will discuss the observables of high-velocity collisions involving both giants and main-sequence stars and those of ejecta’ accretion of onto the supermassive black hole. 

Taeho Ryu, MPA Fellow, Max Planck Institute for Astrophysics

Host:  Fred Rasio