Strong biases in estimating the time dependence of mass accretion rates in young stars

Abstract

The temporal decay of mass accretion in young stars is a fundamental tracer of the early evolution of circumstellar discs. Through population syntheses, we study how correlated uncertainties between the estimated parameters of young stars (luminosity, temperature, mass and age) and mass accretion rates Ṁacc, as well as observational selection effects, can bias the temporal decay of mass accretion rates ( Ṁacc α t -η) inferred from a comparison of measured Ṁ acc with isochronal ages in young stellar clusters. We find that the presence of realistic uncertainties reduces the measured value of η by up to a factor of 3, leading to the inference of shallower decays than the true value. This suggests a much faster temporal decay of Ṁ acc than generally assumed. When considering the minimum uncertainties in ages affecting the Orion Nebula Cluster, the observed value η ~ 1.4, typical of Galactic star-forming regions, can only be reproduced if the real decay exponent is η ≳ 4. This effect becomes more severe if one assumes that observational uncertainties are larger, as required by some fast star formation scenarios. Our analysis shows that while selection effects due to sample incompleteness do bias η, they cannot alter this main result and strengthen it in many cases. A remaining uncertainty in our work is that it applies to the most commonly used and simple relationship between Ṁ acc the accretion luminosity and the stellar parameters. We briefly explore how a more complex interplay between these quantities might change the results. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.