Mass and Spin Growth of Very Massive Stars in Star Clusters Potentially Associated with Little Red Dots

Ataru Tanikawa, Masaru Shibata, Kunihito Ioka

First listed 2026-06-02 | Last updated 2026-06-01

Abstract

Using gravitational $N$-body simulations, we investigate the evolution of mass and spin for very massive stars (VMSs) in dense star clusters, which may be potentially associated with Little Red Dots (LRDs). Our results show that VMS masses can reach $10^3$--$10^4\,M_\odot$, depending on the initial conditions of the host clusters. Notably, the VMS mass increases by up to a factor of three when accounting for the bloated state at the Hayashi track induced by stellar collisions, provided that this state is maintained at accretion rates exceeding $3 \times 10^{-2}\,M_\odot\,{\rm yr}^{-1}$. In all cases, the spin of the VMS, when normalized to the dimensionless black hole (BH) spin parameter, exceeds $10$. While our model may overestimate VMS masses and spins due to the omission of post-main-sequence evolution and the loss of mass and angular momentum during collisions, we nonetheless demonstrate that VMSs formed in dense star clusters can be highly spinning. Such a rapidly spinning VMS is expected to collapse into an intermediate-mass BH surrounded by a massive accretion disk. This BH-disk system could trigger powerful explosions and emit burst gravitational waves, similar to those observed in GW190521 and GW231123, for which the remnant BH masses are estimated to be $\gtrsim 100\,M_\odot$.

Short digest

This paper uses gravitational N-body simulations of four very dense cluster models motivated as the central regions of LRD progenitors to follow runaway stellar collisions and the coupled growth of very massive star mass and spin. Across Kroupa and top-heavy initial mass functions, the runaway product reaches roughly 10^3 to 10^4 solar masses, and including a collision-induced bloated Hayashi-track state can raise the final VMS mass by up to a factor of three when the accretion rate stays above 3 x 10^-2 solar masses per year. In every model, the VMS angular momentum corresponds to a black-hole-like dimensionless spin parameter above 10, implying collapse to a rapidly rotating intermediate-mass black hole plus a massive disk rather than a slowly rotating remnant. That makes dense-cluster VMS formation a concrete route from LRD-like stellar systems to explosive electromagnetic and burst gravitational-wave transients, with the authors noting that neglected post-main-sequence evolution and collision-driven mass and angular-momentum loss likely make their masses and spins upper estimates.

Key figures to inspect

• Figure 1. This figure defines the paper's central physical assumption by showing how much larger the stellar radius becomes in the bloated Hayashi-track state than in the unbloated case. It is the setup figure to use if you want readers to understand why collision cross-sections, runaway growth efficiency, and the final VMS mass can change so strongly once repeated collisions keep the star puffed up.
• Figure 2. This is the main result figure because it tracks, for every cluster model, the time evolution of VMS mass, accretion rate, spin, and collision spacing, while directly comparing bloated and unbloated runs. It lets the reader see the quantitative core claims in one place: masses reaching 10^3 to 10^4 solar masses, mass boosts of up to about three in the bloated models, spins staying extremely large, and the link between sustaining the bloated state, Kelvin-Helmholtz timescales, and the accretion-rate threshold of 3 x 10^-2 solar masses per year.
• Figure 3. This later diagnostic figure is the best synthesis panel for the spin result because it compares the simulated VMS spin with the analytic estimate from Eq. 17. It matters because the paper's most distinctive conclusion is not just that runaway collisions make very massive stars, but that they make them generically and robustly highly spinning, which underpins the proposed black-hole-plus-disk collapse channel and its transient consequences.