How External Medium outside Prestellar Cores Affects Protostellar Growth: Variations in Accretion Rate and Evolution of Disks and Outflows

Abstract

We investigate how the external medium surrounding prestellar cores affects the star formation process by conducting three-dimensional resistive magnetohydrodynamic simulations. The initial cores follow critical Bonnor-Ebert profiles and are embedded in environments with different ambient densities. The simulations follow the evolution at least until the envelope mass within a radius equal to twice the critical Bonnor-Ebert radius drops to 35% of the initial cloud mass. We reveal that in environments with higher external density, enhanced mass inflow from the envelope leads to Bondi-like accretion as the protostellar mass increases. The continued inflow substantially increases the final stellar mass, resulting in star formation efficiencies that appear to exceed unity in dense environments. The external medium also influences the evolution of circumstellar disks and protostellar outflows: with the high-density external medium, disks grow rapidly but their mass becomes smaller relative to the protostellar mass, and the outflow is sustained over a long duration. However, the ratio of angular momentum removed by outflows and magnetic braking to that introduced by inflowing gas decreases with increasing external density. These results suggest that the density of the external medium regulates not only protostellar mass growth but also the inflow-outflow balance and angular momentum transport in magnetized, rotating star-forming cores.

Figures (10)

• Figure 1: The initial density profile in the radial direction for each model. The gray dotted and dash-dotted lines represent one and two times the critical Bonnor-Ebert radius, corresponding to $R_{\rm 1BE} = 2.9 \times 10^{-2}\, \rm{pc}$ and $R_{\rm 2BE} = 5.7 \times 10^{-2}\, \rm{pc}$. The color maps are the 2D initial density slice in the $y=0$ plane for the model B2H and B2L. The white dotted and dash-dotted circles in the color maps indicate the positions of $r = R_{\rm 1BE}$ and $r = R_{\rm 2BE}$.
• Figure 2: Density distributions on $z=0$ (top) and $y=0$ (bottom) planes for models B2H (left), B2 (middle) and B2L (right). Each panel shows the snapshot at $M_{\rm env}/M_{\rm 2BE,0}=0.35$. The white dotted and dash-dotted circles in the bottom panels represent the positions of $r = R_{\rm 1BE}$ and $r = R_{\rm 2BE}$.
• Figure 3: Mass fractions of the protostar ($M_{\rm ps}$, red), disk ($M_{\rm disk}$, yellow), outflow ($M_{\rm out}$, blue), ejected gas ($M_{\rm eject}$, gray), and envelope ($M_{\rm env}$, green), relative to the initial cloud mass ($M_{\rm 2BE,0}$) are plotted against protostellar mass $M_{\rm ps}$. The results are plotted up to the point where $M_{\rm mps} = 0.89$, $0.52$, and $0.27\,{\rm M}_\odot$ for models B2H, B2, and B2L, corresponding to the stage each simulation has reached. For all models, the evolution after the envelope mass $M_{\rm env}$ decreases to $35\,\%$ of the initial cloud mass $M_{\rm 2BE, 0}$ is shown with lighter colors.
• Figure 4: Mass accretion rate onto the protostar, $\dot{M}_{\rm ps}$, for models B2H (red), B2 (black), and B2L (blue). The left panel shows $\dot{M}_{\rm ps}$ plotted against protostellar mass $M_{\rm ps}$, and the right panel against time since protostar formation. Thin lines indicate the mass accretion rate onto the protostars at each time. Thick lines are moving averages along the horizontal axis. The white-edged circles mark the epochs at which the envelope mass $M_{\rm env}$ has decreased to $50\,\%$ of the initial cloud mass $M_{\rm 2BE,0}$ for each model, corresponding to protostellar masses of approximately $0.32\,{\rm M}_\odot$ (B2H), $0.12\,{\rm M}_\odot$ (B2), and $0.11\,{\rm M}_\odot$ (B2L). The white-edged squares mark the epochs when $M_{\rm env}$ has decreased to $35\,\%$ of $M_{\rm 2BE,0}$, corresponding to protostellar masses of approximately $0.85\,{\rm M}_\odot$ (B2H), $0.24\,{\rm M}_\odot$ (B2), and $0.16\,{\rm M}_\odot$ (B2L). The subsequent evolution beyond these epochs is shown with lighter colors.
• Figure 5: Mass accretion rates onto the protostar plotted against protostellar mass for models B2H (red), B2 (black), and B2L (blue). The shaded regions indicate the range of Bondi accretion rates estimated for each model, using the gas density at $r = R_{\rm 1BE}$ (upper bound) and $r = R_{\rm 2BE}$ (lower bound).