Hydrodynamical simulations of massive stars collisions

Since the detection of gravitational waves traveling through our universe, a new field of astrophysics has opened up, and with it many hypotheses and theoretical predictions have been put to the test. This is the case of the pair-instability mass gap (60 and 120 Msun. Gravitational wave signals have been detected whose primary and secondary black holes fall within the mass range where it was previously thought unthinkable that they could exist as a product of stellar evolution. Different mechanisms could explain these observations, among which the gravitational collapse of a particular type of remnant from a collision between two predecessor stars stands out. Multiple studies support this hypothesis, however, it is necessary to determine the incidence of the collision parameters on the main characteristics of the post-coalescence star, especially to corroborate that its final mass and chemical composition made it a suitable remnant to produce these special black holes. To achieve this goal, the hydrodynamic simulations of stellar collisions are a perfect tool. In this thesis, we have analyzed a set of simulations produced with the smoothed particle hydrodynamics code StarSmasher. This set explores different configurations of mass, radius, stellar evolution time, velocity at infinity and impact parameter for the two colliding stars. We have been able to analyze the temporal evolution of the percentage of unbound mass for all cases, obtaining that collisions produce a range of mass loss between 9.7 % and 19.3 %, with the impact parameter being the most influential value in the increase of this percentage. We have also visualized that the relaxation of the final remnant occurs quite quickly after the impact as long as the collision is radial. Only two days are necessary for the remnant mass to remain stable, as well as its velocity. In the case of non-radial collisions, the relaxation of the post-coalescence star is much more complex, since it is altered by successive collisions between the cores of the two primary stars. Finally, we also found that non-radial collisions are the only ones that result in a remnant with tangential rotational velocity, a fact that would favor the chemical enrichment of its constituent layers and modify its subsequent evolution.