Letter to the Editor

The star cluster age function in the Galactic disc with Gaia DR2

Fewer old clusters and a low cluster formation efficiency

¹ Dept. FQA, Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
e-mail: fanders@fqa.ub.edu, tcantat@fqa.ub.edu
² ICREA, Passeig Lluis Companys 23, 08010 Barcelona, Spain

Received: 29 May 2020
Accepted: 27 November 2020

Abstract

We perform a systematic reanalysis of the age distribution of Galactic open star clusters. Using a catalogue of homogeneously determined ages for 834 open clusters contained in a 2 kpc cylinder around the Sun and characterised with astrometric and photometric data from the Gaia satellite, we find that it is necessary to revise earlier works that relied on data from the Milky Way Star Cluster survey. After establishing age-dependent completeness limits for our sample, we find that the cluster age function in the range 6.5 <  log t <  10 is compatible with Schechter-type or broken power-law functions. Our best-fit values indicate an earlier drop of the age function (by a factor of 2−3) with respect to the results obtained in the last five years, and are instead more compatible with results obtained in the early 2000s along with radio observations of inner-disc clusters. Furthermore, we find a typical destruction timescale of ∼1.5 Gyr for a 10⁴ M_⊙ cluster and a present-day cluster formation rate of 0.55_−0.15^+0.19 Myr⁻¹ kpc⁻², suggesting that only 16₋₈⁺¹¹% of all stars born in the solar neighbourhood form in bound clusters. Accurate cluster-mass measurements are now needed to place more precise constraints on open-cluster formation and evolution models.