The Star Formation Factory revisited I. The impact of metallicity on collapsing star-forming clouds

Abstract

Context. Stellar feedback regulates star formation and shapes the interstellar medium, yet its role during the collapse of molecular clouds remains uncertain over a wide range of initial conditions. Aims. We explore how stellar winds and supernovae influence star formation in collapsing gas clouds that span a broad parameter space in mass, size, and metallicity. Methods. Using a one-dimensional numerical model, we follow the evolution of feedback-driven bubbles produced by embedded clusters, incorporating time-dependent energy and mass injection, self-gravity, integrated cloud collapse, radiative cooling, shell instabilities, and triggered star formation. Our treatment of gas cooling in the hot bubble explicitly accounts for heat transfer across the bubble-shell interface. Results. We find that metallicity acts as a key regulator of feedback, comparable in importance to cloud mass and radius. In low-metallicity clouds, reduced radiative cooling is offset by weaker stellar winds, leading to prolonged star formation and higher efficiencies. Across a substantial portion of parameter space, the expanding shell undergoes a stalling phase that further enhances the star formation efficiency, an outcome that is not observed at higher metallicities. Conclusions. Our results suggest that the diverse properties of star clusters across cosmic time may arise from the metallicity-dependent interplay between stellar feedback and gas cooling.

Figures (20)

• Figure 1: Schematic of the model setup of this work. We model the evolution of feedback-driven bubbles, formed by the winds and supernovae from the star cluster forming at the center. The scheme presents a cone of the otherwise spherically symmetric bubble structure, showing the location of the reverse ($R_{\rm{rs}}$) and forward shocks ($R_{\rm{sh}}$), and the contact discontinuity ($R_{\rm{cd}}$) which separates shocked wind from ambient swept-up gas. Furthermore, the dashed arrows indicate the directions of the different forces that determine the evolution of the shell radius: the inward gravitational force ($F_{\rm{g}}$) and ram pressure from the cloud ($F_{\rm{cl}}$), and the outward force produced by the thermal energy of the hot bubble ($F_{\rm{th}}$). This term is calculated by considering radiative cooling within the bubble ($Q_{ \rm{w}}$), which includes the effect of mass evaporation from the cold shell. The evaporated material ($\dot{M}_{\rm{ev}}$) mass-loads the bubble interior, thus modifying its density and temperature structure and, consequently, the cooling rate. In addition, our model setup includes star formation in the shell (not included for clarity of the diagram). See the text for details.
• Figure 2: Evolution of the mechanical power (top) and mass input rate (bottom) per unit solar mass of a stellar population with MW (dashed lines), dwarfA (dotted lines) and IZw18 (solid lines) metallicities as obtained using the Bonn Optimized Stellar Tracks BoOST2022.
• Figure 3: Time evolution of the shell radius (top left), shell mass (top right), stellar mass (bottom left) and thermal energy (bottom right) for the low metallicity (IZw18) models, that were performed for three different values of $\epsilon_{ff}$ (the first three models listed in Table \ref{['tab:sim_params']}). The color map shows the average initial gas number density of the clouds, $\bar{n}_{\rm{cl}}=3 M_{\rm{gas,0}}/(4 \pi \mu m_{\rm H} R_{\rm{cl,0}}^3)$.
• Figure 4: Model evolution for a case with $\log[M_{\rm{gas,0}} (M_{\odot})]=6.3$, $R_{\rm{cl}}=25.7$ pc, $\epsilon_{\rm{ff}}=0.3$ and for the IZw18 metallicity. First plot from top to bottom: shell radius (left y-axis), shell velocity (right y-axis). Second: shell mass ($M_{\mathrm{sh}}$), stellar mass formed from core star formation ($M_{\mathrm{sc,c}}$), and total stellar mass ($M_{\mathrm{sc}}$). Third: the outward force produced by the thermal energy ($\dot{U}_{\rm{th}}$, dashed) and the total inward forces ($\dot{U}_{\rm{in}}$, solid). Fourth, bottom right panel: Mechanical power (solid) produced by the stellar feedback and the cooling rate (dashed).
• Figure 5: Same as Fig. \ref{['single_SSF']}, but for two cases with different cloud gas masses and radii, as shown in the inset ($\epsilon_{\mathrm{ff}}=0.3$ in both cases). Upper panel: time evolution of $L_{\rm{w}}$ (dashed lines) and $Q_{\rm{w}}$ (solid lines). Bottom panel: Evolution of the shell radius with time for the same models, using the same color as in the top panel.