Properties of Star Clusters - II: Scale Height Evolution of Clusters

TL;DR

This study introduces a new method to measure star cluster scale height evolution beyond 1Gyr, revealing a linear increase with age up to 3.5Gyr and a weak dependence on galactocentric distance.

Contribution

We developed a novel approach using modelled distributions and statistical tests to determine cluster scale height with 25% accuracy, enabling the first observational insights into long-term evolution.

Findings

Scale height increases from 40pc at 1Myr to 75pc at 1Gyr.

Beyond 1Gyr, scale height grows linearly to about 550pc at 3.5Gyr.

Weak trend of increasing scale height with galactocentric distance.

Abstract

Until now it has been impossible to observationally measure how star cluster scale height evolves beyond 1Gyr as only small samples have been available. Here we establish a novel method to determine the scale height of a cluster sample using modelled distributions and Kolmogorov-Smirnov tests. This allows us to determine the scale height with a 25% accuracy for samples of 38 clusters or more. We apply our method to investigate the temporal evolution of cluster scale height, using homogeneously selected sub-samples of Kharchenko et al. (MWSC), Dias et al. (DAML02), WEBDA, and Froebrich et al. (FSR). We identify a linear relationship between scale height and log(age/yr) of clusters, considerably different from field stars. The scale height increases from about 40pc at 1Myr to 75pc at 1Gyr, most likely due to internal evolution and external scattering events. After 1Gyr, there is a marked change of the behaviour, with the scale height linearly increasing with log(age/yr) to about 550pc at 3.5Gyr. The most likely interpretation is that the surviving clusters are only observable because they have been scattered away from the mid-plane in their past. A detailed understanding of this observational evidence can only be achieved with numerical simulations of the evolution of cluster samples in the Galactic Disk. Furthermore, we find a weak trend of an age-independent increase in scale height with galactocentric distance. There are no significant temporal or spatial variations of the cluster distribution zero point. We determine the Sun's vertical displacement from the Galactic Plane as $Z_\odot=18.5\pm1.2$pc.