Tides in clouds: control of star formation by long-range gravitational force

Abstract: Gravity drives the collapse of molecular clouds through which stars form, yet the exact role of gravity in cloud collapse remains a complex issue. Studies point to a picture where star formation occurs in clusters. In a typical, pc-sized cluster-forming region, the collapse is hierarchical, and the stars should be born from regions of even smaller sizes ($\approx 0.1\;\rm pc$). The origin of this spatial arrangement remains under investigation. Based on a high-quality surface density map towards the Perseus region, we construct a 3D density structure, compute the gravitational potential, and derive eigenvalues of the tidal tensor ($\lambda_{\rm min}$, $\lambda_{\rm mid}$, $\lambda_{\rm max}$, $\lambda_{\rm min} < \lambda_{\rm mid} < \lambda_{\rm max}$), analyze the behavior of gravity at every location and reveal its multiple roles in cloud evolution. We find that fragmentation is limited to several isolated, high-density ``islands''. Surrounding them, is a vast amount of gas ($75 \%$ of the mass, $95 \%$ of the volume) that stays under the influence of extensive tides where fragmentation is suppressed. This gas will be transported towards these regions to fuel star formation. The spatial arrangement of regions under different tides explains the hierarchical and localized pattern of star formation inferred from the observations. Tides were first recognized by Newton, yet this is the first time its dominance in cloud evolution has been revealed. We expect this link between cloud density structure and role gravity to be strengthened by future studies, resulting in a clear view of the star formation process.