Colliding red giants in galactic nuclei: Shocks, jets, impact on the ISM, X- and gamma-rays, neutrinos, fusion ignition and afterglow

Abstract

In galactic nuclei, stellar densities are so high that stars can physically collide with each other. In this work we focus on the collision of red giants and in particular on the formation of non-thermal processes through collisions and their properties. We analytically address these points by evaluating head-on collisions but also take into account scenarios with a deviation from the radial orbit, which we treat in a perturbative fashion. The collisions produce internal shocks with supersonic Mach numbers. Almost immediately, jet-like structures with important Lorentz factors form. The debris from the collision produces another shock wave which, when interacting with the interstellar medium of a galactic nucleus, leads to particle acceleration. We estimate the background flux in X- and gamma rays created by the background of these collisions by deriving the spectral index within a radius of 100 Mpc and find that they are high. Additionally, we make an estimate of the neutrino production and find about $10^{11}$ neutrinos per square meter per second for a collision at 100 Mpc from Earth. Also, we derive that there is a non-negligible chance to ignite fusion during the collision, due to the squeezing of the material. We investigate the possibility that the degenerate cores collide with each other, leading to a high afterglow luminosity, and find that it is non-negligible, although this should be addressed with dedicated numerical simulations. Colliding red giants in galactic nuclei trigger a plethora of high-energy phenomena, and have a particular gravitational wave emission associated, as shown by us, so that their detection will allow us to rule out alternatives.