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

¹ CENTRA, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
² Laboratório de Instrumentação e Física Experimental de Partículas (LIP), Av. Prof. Gama Pinto 2, 1649-003 Lisboa, Portugal

^★ Corresponding authors; sandro@sim.ul.pt; andre@sim.ul.pt; duarte.almeida@sim.ul.pt

Received: 15 April 2024
Accepted: 22 December 2024

Abstract

Context. The scale height (SH) of the spatial distribution of open clusters (OCs) in the Milky Way exhibits a well-known increase with age that is usually interpreted as evidence for dynamical heating of the disc or for the disc having been thicker in the past.

Aims. We address the increase in the SH 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 encounters with giant molecular clouds (GMCs).

Methods. We present a computational model that forms OCs with initial masses and follows their orbits, while subjecting them to different disruption mechanisms. To set up the model and infer its parameters, we used and analysed a Gaia-based OC catalogue. We investigate both the spatial and age distributions of the OC population and discuss the sample completeness. The simulation results are 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 likely that the OC sample is incomplete even for the solar neighbourhood. The simulations successfully reproduce the SH evolution and the total number of OCs that survive with age up to 1 Gyr. For older OCs, the model-predicted SH starts deviating from observations, although it remains within the uncertainties of the observations. This can be related to the effects of incompleteness and/or simplifications in the model.

Conclusions. The OC encounters with GMCs effectively explain the SH evolution of the OC population. An interesting result is that the average time for an object with a Sun-like orbit to encounter a GMC is approximately 700 Myr, aligning well with previous estimates for the Sun obtained through different methods.