Early-Forming Massive Stars Suppress Star Formation and Hierarchical Cluster Assembly

TL;DR

This study investigates how the timing of very massive star formation shapes star formation and cluster assembly by running four high-resolution simulations of identical $10^4\,M_{\odot}$ GMCs with Torch within AMUSE. By forcing the first massive star to be $50$, $70$, or $100\,M_{\odot}$ (vs. a fiducial case where it forms stochastically), the authors show that early massive-star feedback globally unbinds gas about $2\ \mathrm{Myr}$ earlier, reduces the total stellar mass formed by up to a factor of three, and lowers the star-formation efficiency per free-fall time by up to a factor of seven. This early feedback also fragments the forming stellar population into spatially separated, energetically unbound subclusters, hindering hierarchical assembly into a single young massive cluster. In contrast, the fiducial run forms a single, centrally concentrated cluster that contains the majority of stars and a substantial fraction of the initial gas. The results underscore the critical role of massive-star timing in regulating gas dynamics, star formation, and cluster architecture, with implications for interpreting observed embedded clusters and their assembly histories.

Abstract

Feedback from massive stars plays an important role in the formation of star clusters. Whether a very massive star is born early or late in the cluster formation timeline has profound implications for the star cluster formation and assembly processes. We carry out a controlled experiment to characterize the effects of early-forming massive stars on star cluster formation. We use the star formation software suite \texttt{Torch}, combining self-gravitating magnetohydrodynamics, ray-tracing radiative transfer, $N$-body dynamics, and stellar feedback to model four initially identical $10^4$ M$_\odot$ giant molecular clouds with a Gaussian density profile peaking at $521.5 \mbox{ cm}^{-3}$. Using the \texttt{Torch} software suite through the \texttt{AMUSE} framework we modify three of the models to ensure that the first star that forms is very massive (50, 70, 100 M$_\odot$). Early-forming massive stars disrupt the natal gas structure, resulting in fast evacuation of the gas from the star forming region. The star formation rate is suppressed, reducing the total mass of stars formed. Our fiducial control model without an early massive star has a larger star formation rate and total efficiency by up to a factor of three and a higher average star formation efficiency per free-fall time by up to a factor of seven. Early-forming massive stars promote the buildup of spatially separate and gravitationally unbound subclusters, while the control model forms a single massive cluster.

Figures (7)

• Figure 1: Snapshot of the fiducial run and early-forming massive star runs 50M, 70M, and 100M at $2\tau_{\rm ff}$. All runs start from identical initial conditions. The gas column density as well as stars are plotted. Stars denoted by red circles have masses $<7$ M$_\odot$ and do not produce any form of feedback in our model. Stars denoted by blue circles have masses $>7$ M$_\odot$ and produce ionizing radiation, winds, and supernovae. Plotted star sizes are scaled by the star's mass. The orange arrows highlight the forced massive star in each of the forced runs.
• Figure 2: The same snapshot as in Figure \ref{['fig:sim_grid_plot']} at $2\tau_{\rm ff}$ but as a slice at z=0 pc of ionized Hydrogen density. To highlight the morphology of the ionized regions, a gas cell is determined to be fully ionized if its ionization fraction exceeds 0.99. Gas cells that do not meet this threshold do not have their densities represented and appear as the dark maroon color. The stars are shown but note that they may lie above or below the slice plane. The orange arrows highlight the forced massive star in each of the forced runs.
• Figure 3: Total energy of gas in the computational domain, comprising kinetic, thermal, and magnetic energy of the gas, energy from gas self-gravity, and gravitational energy due to stars acting on the gas for the fiducial and forced runs. The $y$-axis is truncated close to zero to reveal detail at times before and after the transition from negative to positive total energy. In each case, the energy is first dominated by gravitational potential energy, and then becomes dominated by gas kinetic energy and gas thermal energy once the massive star forms in the forced runs or several $>$50 M$_{\odot}$ stars form in the fiducial run.
• Figure 4: (Left) Total gas mass in computational domain. Reduction in gas is due to gas leaving the domain or accreting onto sinks and ultimately becoming stars. (Right) Cumulative ZAMS stellar mass for the fiducial and three forced massive star runs. The dotted vertical line marks $2\tau_{\rm ff}$, the focus of Sect. \ref{['sec:Hierarchical']}.
• Figure 5: Total mass of gas in the computational domain that satisfies the Jeans criterion, one of the six criteria necessary for sinks to form and one of four sink accretion criteria.