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.
