Massive stellar cannibals: How stellar mergers drive mass loss in extremely massive stars

Context. It has been theorized that the formation of extremely massive and supermassive stars (> 103 M⊙) could plausibly be the outcome of stellar mergers in low metallicity (Z < 10−1 Z⊙) and dense (≳103 M⊙ pc−3) stellar environments. These objects remain relevant as they can serve as the progenitors of intermediate-mass black holes and they are also formidable chemical polluter candidates, as evidenced by the peculiar abundances seen across cosmic history. Our understanding of the formation of these objects depends on the physical processes involved in their mass accretion and loss, via gas inflows and stellar winds, as well as the stellar mergers themselves. Aims. This work investigates merger-induced mass loss in extremely massive stars within a hydrodynamic framework and provides a prescription derived from the simulations to estimate both the mass loss and the outcome of the interaction. Methods. We adapted the 1D hydrodynamic, stellar structure, and evolution code <ns0:monospace>MESA</ns0:monospace> to simulate stellar inspirals. In our simulations, we considered stars of > 1000 M⊙ with inspiraling companions of < 100 M⊙; hence, with mass ratios of < 0.1. As the inspiral progresses, the orbital energy of the system is lost through the hydrodynamic and gravitational drag forces. This energy gets deposited as thermal energy in the extremely massive star’s envelope. The reaction of the star can be followed by solving Euler’s hydrodynamical equations. Results. The extremely massive star experiences mass loss from pulsations produced by the inspiral. By scaling the mass lost through such pulsations based on the time it takes for the extremely massive star to radiate away the injected thermal energy, we find that the total ejected mass is ∼10–30% of the system’s mass. Our results point out that most of the energy deposited by the inspiral is used to eject mass. This is consistent with the fact that the extremely massive star models that we study are barely bound, as their mean adiabatic exponent is ∼4/3. Conclusions. These findings demonstrate that merger-induced mass loss is non-negligible for the considered configurations. Thus, it is an important process to account for when investigating the formation of extremely massive stars and predicting their possible role throughout cosmic history.