Teen TITANS simulations -- I. Inefficient intermediate-mass black hole seeding via stellar collisions in young massive clusters

Abstract

Young massive clusters (YMCs) provide favorable environments for frequent stellar collisions, potentially leading to the formation of very massive stars (VMSs) and seeds of intermediate-mass black holes (IMBHs). We investigate the role of repeated stellar collisions in YMCs using TITANS, a new suite of 18 direct $N$-body simulations. Our models span cluster masses $10^5 - 10^6\,\rm M_\odot$, half-mass densities $ρ_{\rm h}=100 - 10^5\,\rm M_\odot\,pc^{-3}$, and include high primordial binary fractions, consistent with observations of massive stars in young clusters. Overall, our simulations assume cluster properties that are typical of YMCs in the low-redshift Universe. We find that repeated stellar collisions are efficient only in the densest clusters with short relaxation times and are absent in systems with $ρ_{\rm h}<500\,\rm M_\odot\,pc^{-3}$ and $t_{\rm rh}>1.3\,\rm Gyr$. Rapid mass segregation allows massive stars to sink to the cluster center, merge, and undergo subsequent collisions, even in clusters with long core-collapse times. However, collision chains are typically triggered by primordial binary mergers and usually involve only two collisions. In our simulations, only three VMSs form through repeated collisions and reach $m_*>330\,\rm M_\odot$, while most VMSs have $m_*<300\,\rm M_\odot$ and form through primordial binary mergers. None constitute viable IMBH seeds, as their helium cores fall in the (pulsational) pair-instability regime. We form five IMBHs from stellar collisions involving stars at different evolutionary stages, while the dominant channel is the merger of stellar-mass black holes, producing twelve IMBHs. For properties typical of local YMCs, stellar collision chains are therefore inefficient in producing IMBHs more massive than $140\,\rm M_\odot$, as most collisionally formed VMSs attain masses that fall in the pair-instability regime.

Figures (12)

• Figure 1: Initial binary fraction (top row) and density at half-mass radius (bottom row) as a function of the initial number of stars, for Monte Carlo (green squares), direct $N$-body (violet circles) and hybrid $N$-body simulations (brown stars). The titans simulations are represented by a red star.
• Figure 2: Schematic representation of a stellar collision chain. We show on the left a chain initiated by the merger of the components of a binary (either primordial or dynamically formed). On the right we show instead a chain initiated by an hyperbolic stellar collision.
• Figure 3: Efficiency of repeated stellar collisions $\eta_{\rm rep}$ as a function of the half-mass density $\rho_{\rm h}$, normalized by $\rho_{\rm h,3}$ and $M_{\rm cl,5}$. The points are colored as a function of the cluster initial relaxation time $t_{\rm rh}$.
• Figure 4: Number of stellar collision chains $N_{\rm ch,rep}$ as a function of the normalized density at half-mass radius $\rho_{\rm h}/\rho_{\rm h,3}$. In violet we represent the number of chains containing two collisions, in blue the number of chains containing three collisions and in four the number of chains containing four or more collisions. The dashed lines and annotations represent the linear fits of the points.
• Figure 5: Primary stellar type of stars involved in repeated collisions. The $x$ axis shows the simulated models. The stellar phases on the $y$ axis are: MS, shell hydrogen burning (sHB), core helium burning (cHeB), shell helium burning (sHeB) and Wolf-Rayet star (WR). The color represents the relaxation time at half-mass radius $t_{\rm rh}$.