The star cluster age function in the Galactic disc with Gaia DR2: Fewer old clusters and a low cluster formation efficiency

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. ; Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work was partially supported by the Spanish Ministry of Science, Innovation and University (MICIU/FEDER, UE) through grant RTI2018-095076-B-C21, and the Institute of Cosmos Sciences University of Barcelona (ICCUB, Unidad de Excelencia "María de Maeztu") through grant CEX2019-000918-M. FA is grateful for funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 800502.