No wide spread of stellar ages in the Orion Nebula Cluster

The wide luminosity dispersion seen for stars at a given effective temperature in the Hertzsprung–Russell diagrams of young clusters and star-forming regions is often interpreted as due to significant (∼10 Myr) spreads in stellar contraction age. In the scenario where most stars are born with circumstellar discs, and that disc signatures decay monotonically (on average) over time-scales of only a few Myr, any such age spread should lead to clear differences in the age distributions of stars with and without discs. We have investigated large samples of stars in the Orion Nebula Cluster (ONC) using three methods to diagnose disc presence from infrared measurements. We find no significant difference in the mean ages or age distributions of stars with and without discs, consistent with expectations for a coeval population. Using a simple quantitative model, we show that any real age spread must be smaller than the median disc lifetime. For a lognormal age distribution, there is an upper limit of <0.14 dex (at 99 per cent confidence) to any real age dispersion, compared to the ≃0.4 dex implied by the Hertzsprung–Russell diagram. If the mean age of the ONC is 2.5 Myr, this would mean at least 95 per cent of its low-mass stellar population have ages between 1.3–4.8 Myr. We suggest that the observed luminosity dispersion is caused by a combination of observational uncertainties and physical mechanisms that disorder the conventional relationship between luminosity and age for pre-main-sequence stars. This means that individual stellar ages from the Hertzsprung–Russell diagram are unreliable and cannot be used to directly infer a star formation history. Irrespective of what causes the wide luminosity dispersion, the finding that any real age dispersion is less than the median disc lifetime argues strongly against star formation scenarios for the ONC lasting longer than a few Myr.