Open cluster dissolution rate and the initial cluster mass function in the solar neighbourhood

Modelling the age and mass distributions of clusters observed by Gaia

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; duarte.almeida@sim.ul.pt; andre@sim.ul.pt; sandro@sim.ul.pt

Received: 9 August 2024
Accepted: 10 December 2024

Abstract

Context. The dissolution rate of open clusters (OCs) and the integration of their stars into the Milky Way’s field population have been explored using their age distribution. With the advent of the Gaia mission, there is an exceptional opportunity to revisit and enhance studies covering these aspects of OCs with ages and masses from high-quality data.

Aims. Our aim is to build a comprehensive Gaia-based OC mass catalogue that, combined with the age distribution, allows for deeper investigation of the disruption experienced by OCs within the solar neighbourhood.

Methods. We determined masses by comparing luminosity distributions to theoretical luminosity functions. The limiting and core radii of the clusters were obtained by fitting the King function to their observed density profiles. We examined the disruption process by performing simulations of the build-up and mass evolution of a population of OCs that we compared to the observed mass and age distributions.

Results. Our analysis yielded an OC mass distribution with a peak at log(M) = 2.7 dex (∼500 M_⊙) as well as radii for 1724 OCs. Our simulations showed that when using a power-law initial cluster mass function (ICMF), no parameters are able to reproduce the observed mass distribution. Moreover, we find that a skew log-normal ICMF provides a good match to the observations and that the disruption time of a 10⁴ M_⊙ OC is t₄^tot = 2.9 ± 0.4 Gyr.

Conclusions. Our results indicate that the OC disruption time t₄^tot is about two times longer than previous estimates based solely on OC age distributions. We find that the shape of the ICMF for bound OCs differs from that of embedded clusters, which could imply a low typical star formation efficiency of ≤20% in OCs. Our results also suggest a lower limit of ~60 M_⊙ for bound OCs in the solar neighbourhood.