Tides from the cloud can induce the fast disruption of star clusters and offer an explanation for Gaia strings

Abstract

Young stars form in clusters within molecular clouds, but older stars are evenly distributed across the galactic disk, necessitating an explanation for cluster dissolution. We analytically study tidal forces from cold molecular clouds as a key mechanism for accelerated cluster disruption. Cloud tides, caused by the gravitational pull of the parent cloud along the radial direction, arise from the spatial gradient of gravitational acceleration and drive cluster disruption. This mechanism activates after gas expulsion and remains effective until the cloud is disrupted by stellar feedback or the cluster moves away. Cloud tides act on gas-deprived clusters, causing exponential expansion on a tidal timescale of $t_{\rm tidal,ext} = \sqrt{3/(8\pi G\rho_{\rm mean})}$, where $\rho_{\rm mean}$ is the cloud's density at the cluster's location. With a duration of a few Myr, cloud tides can lead to a 10 times increase of the cluster size, producing bar-like elongated stellar aggregations resembling Gaia strings. These results establish cloud tides as a potentially important mechanism for star cluster disruption.