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

It has been theorized that the formation of extremely massive and supermassive stars ($>10^3\ {\rm M}\odot$) could plausibly be the outcome of stellar mergers in low metallicity ($Z<10^{-1}$~Z$\odot$) and dense ($\gtrsim10^3\ {\rm M}\odot\ {\rm pc}^{-3}$) 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. 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. We adapted the 1D hydrodynamic, stellar structure, and evolution code MESA to simulate stellar inspirals. In our simulations, we considered stars of $>1000,\rm M{\odot}$ with inspiraling companions of $<100$ M$_\odot$; 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. We find that the total ejected mass is $\sim$10-30$%$ of the system’s mass. Our results point out that most of the energy deposited by the inspiral is used to eject mass. 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.

💡 Research Summary

This paper investigates how stellar mergers drive mass loss in extremely massive stars (EMS) with masses above 10³ M⊙, a regime that has received little direct hydrodynamic treatment. The authors focus on dense, low‑metallicity stellar environments (Z < 0.1 Z⊙, ρ ≳ 10³ M⊙ pc⁻³) where repeated close encounters can lead to the growth of EMS through successive mergers. Such objects are of great interest because they are potential progenitors of intermediate‑mass black holes (IMBHs) and powerful chemical polluters of early globular clusters and compact star‑forming galaxies.

To explore merger‑induced mass loss, the authors adapt the 1‑D stellar evolution code MESA (release 23.05.1) by adding a new module called StellarInspiral1D. This module couples the orbital evolution of a low‑mass companion (M < 100 M⊙, mass ratio q < 0.1) with the internal response of an EMS modeled as a 1‑D spherical star. The companion is treated as a non‑evolving point‑mass with fixed radius; its gravitational and hydrodynamic drag forces are computed using the Ostriker (1999) and Binney & Tremaine (2008) prescriptions. The drag force depends on the local density of the EMS envelope, the Mach number of the companion, and an effective accretion radius R_acc that combines geometric and Bondi‑Hoyle terms. The orbital equation also includes a 2.5‑order post‑Newtonian term to capture relativistic corrections for very tight orbits.

The EMS models are built by starting from a 0.7 M⊙ pre‑main‑sequence seed and accreting cold gas at a rate ten times higher than the Haemmerlé et al. (2019) star‑formation prescription, with a metallicity of Z = 10⁻⁴ (