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

TL;DR

The paper investigates how tides from the natal molecular cloud can rapidly disrupt young star clusters after gas expulsion. By modeling the radial and tangential responses to the cloud's gravitational gradient, it derives a tidal timescale $t_{\rm tidal,ext}$ and analytical solutions for cluster expansion, showing that a few Myr of cloud tides can inflate cluster sizes from a few to tens of parsecs. Observational support is provided via an Orion molecular cloud case and by comparing simulations to Gaia string properties, suggesting that cloud tides can reproduce the elongated, high-velocity-dispersion structures seen in Gaia strings. Overall, cloud tides emerge as a potentially important and efficient mechanism for cluster disruption and a natural explanation for Gaia strings, complementing gas expulsion and Galactic tides in shaping stellar distributions.

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 $t_{\rm tidal,ext} = \sqrt{3/(8πGρ_{\rm mean})}$, where $ρ_{\rm mean}$ 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.

Figures (6)

• Figure 1: An illustration of cloud tide. The sketch shows a cluster with density $\rho_{\rm local}$ different evolution under cloud tide. Befor gas expulsion, the tidal tensor is compressive. After gas expulsion, cluster could be disrupted by its parent cloud.
• Figure 2: Cluster evolves with size-dominated (blue line) and velocity-dominated (orange line) initial preconditions. With a tidal timescale, the cluster is virialized.
• Figure 3: Diagram showing the entire evolutionary process of the cluster. Star clusters initially form within molecular clouds and remain embedded in them. After gas expulsion, the cluster becomes unbound, and gravity from the surrounding residual cloud stretches it into a string-like structure until the cloud disperses.
• Figure 4: Cluster evolution in the absence and presence of cloud tide. The above figure shows the slow evolution without the cloud tide effect, while the bottom figure shows acceleration due to the cloud tide. Cloud tide can lead to significant expansion. In a few Myr, the star cluster can expand from a few parsec to a few tens of parsec.
• Figure 5: A case of a star cluster under cloud tide found in the Orion molecular cloud. The left Planck plot shows the distribution of gas and the star clusters which is under the influence of the cloud tide. Defining this radial direction as the $y$-axis, we calculated velocity dispersion in all directions (right panel), showing significantly higher dispersion along the longer axis, which points to the nearby Orion B cloud.