Modelling the evolution of the Galactic disc scale height traced by open clusters

TL;DR

This study models how the disruption of open clusters by giant molecular clouds near the galactic plane explains the observed increase in their scale height with age, challenging the idea that dynamical heating is the sole cause.

Contribution

It introduces a computational model demonstrating that selective disruption by GMCs can account for the scale height evolution of open clusters, offering an alternative to dynamical heating explanations.

Findings

Model reproduces the increase in scale height with age.

Sample completeness affects observed distributions.

Disruption by GMCs explains scale height evolution.

Abstract

Context. The scale height of the spatial distribution of open clusters (OCs) in the Milky Way exhibits a well known increase with age which is usually interpreted as evidence for dynamical heating of the disc or of the disc having been thicker in the past. Aims. We address the increase of the scale height with age of the OC population from a different angle. We propose that the apparent thickening of the disc can be largely explained as a consequence of a stronger disruption of OCs near the Galactic plane by disc phenomena, namely encounters with giant molecular clouds (GMCs). Methods. We present a computational model that forms OCs with different initial masses and follows their orbits while subjecting them to different disruption mechanisms. To setup the model and infer its parameters, we use and analyse a Gaia-based OC catalogue (Dias et al. 2021). We investigate both the spatial and age distributions of the OC population and discuss the completeness of the sample. The simulation results are then compared to the observations. Results. Consistent with previous studies, the observations reveal that the SH of the spatial distribution of OCs increases with age. We find that it is very likely that the OC sample is incomplete even for the solar neighbourhood. The model simulations successfully reproduce the SH increase with age and the total number of OCs that survive with age up to 1 Gyr. For older OCs, the predicted SH from the model starts deviating from the observations, although remaining within the uncertainties of the observations. This can be related with effects of incompleteness and/or simplifications in the model. Conclusions. A selective disruption of OCs near the galactic plane through GMC encounters is able to explain the SH evolution of the OC population.