April 2026
Estimating ionization fractions in SILCC simulations (Lennart Buhlmann)
GAS EXPULSION FROM YOUNG STAR CLUSTERS
Star clusters form from infrared dark clouds as embedded objects. When massive star formation occurs within the embedded cluster, the feedback from these stars impacts its surrounding gas.
The ensuing interplay between self-gravity and stellar feedback results either in continuing star formation, or cleaning the cluster of its natal gas in which case star formation is quenched. Depending on the gas expulsion time-scale and the star formation efficiency, gas expulsion can have a profound impact on star cluster dynamics, possibly unbinding a significant fraction of stars from the cluster.
Using hydrodynamic simulations including feedback in the from of stellar winds and ionising radiation, and self-gravity with a realistic sink particle integration, we find that the approximation of the source of the feedback strongly influences the gaseous dynamics. In one model, we represent all the massive stars by a single source located at the cluster centre. In the other model, we resolve the cluster to individual stars and follow their dynamics. While the stellar feedback drives a single shell out of the cluster in the former model, it forms a complicated network of sheets and filaments in the latter model, which slows down the process of gas expulsion.
For example, an embedded cluster of total mass 3×103M⊙ expels its natal gas on a timescale tge of 0.2Myr in the former model, while tge reaches 0.9 Myr in the latter model. The value of tge of the latter model is closer to observations, which provide an estimate of tge of the order of 1 to 2 Myr. Another finding which is in agreement with observations is that tge slightly decreases for more massive clusters.
In our models, star clusters with central escape speed larger than ≈10 km/s, which corresponds to the sound speed in an HII region, cannot overcome their self-gravity by their stellar feedback. This indicates that these clusters are formed either with higher star formation efficiency (SFE) than adopted in this work (we use SFE=1/3), or that the clusters can form with relatively low SFEs, but their gas is expelled by another feedback mechanism (e.g, radiation pressure), which is not included in the present work.
Video: Gas expulsion from an embedded star cluster of total mass 3×103M⊙. The cluster features the model which is resolved to individual stars: massive stars are shown by blue asterisks, lower mass stars are represented by circles. The panels show: density (upper left); pressure (upper right); temperature (lower left); and velocity (lower right). Feedback sweeps the cloud to a network of sheets, filaments and semi-shells, which are gradually eroded and pushed from the cluster centre, clearing the cluster of gas on a time-scale of 1 Myr.
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