Paper detail

Strong Biases in Estimating the Time Dependence of Mass Accretion Rates in Young Stars

The temporal decay of mass accretion in young stars is a fundamental tracer of the early evolution of circumstellar disks. Through population syntheses, we study how correlated uncertainties between the estimated parameters of young stars (luminosity, temperature, mass, age) and mass accretion rates Mdot, as well as observational selection effects, can bias the temporal decay of mass accretion rates (Mdot t^-eta) inferred from a comparison of measured Mdot with isochronal ages in young stellar clusters. We find that the presence of realistic uncertainties reduces the measured value of eta 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 Mdot than generally assumed. When considering the minimum uncertainties in ages affecting the Orion Nebula Cluster, the observed value eta~1.4, typical of Galactic star forming regions, can only be reproduced if the real decay exponent is eta>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 eta, they can not 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 Mdot, the accretion luminosity and the stellar parameters. We briefly explore how a more complex interplay between these quantities might change the results.