The Relationship between Accretion Disc Age and Stellar Age and its Consequences for Proto-Stellar Discs

TL;DR

This study investigates the relationship between disc age and stellar age in young stars, finding a discrepancy between models and observations that may be explained by systematic measurement errors in disc mass.

Contribution

The paper compares observational data with theoretical models of proto-stellar discs and suggests that measurement errors in disc mass could resolve existing discrepancies.

Findings

Nominal disc age approximates stellar age within observational scatter.

Models predict disc ages 3-10 times higher than observed.

Systematic underestimation of disc mass could explain the discrepancy.

Abstract

We show that for young stars which are still accreting and for which measurements of stellar age, disc mass and accretion rate are available, nominal disc age (Disc Age = Disc Mass / Accretion Rate) is approximately equal to the stellar age, at least within the considerable observational scatter. We then consider theoretical models of proto-stellar discs through analytic and numerical models. A variety of viscosity prescriptions including empirical power laws, magnetohydrodynamic turbulence and gravitational instability were considered within models describing the disc phenomena of dead zones, photoevaporation and planet formation. These models are generally poor fits to the observational data, showing values of 'Disc Age' which are too high by factors of 3 - 10. We then ask whether a systematic error in the measurement of one of the observational quantities might provide a reasonable explanation for this discrepancy. We show that for the observed systems only disc mass shows a systematic dependence on the value of 'Disc Age / Stellar Age' and we note that a systematic underestimate of the value of disc mass by a factor of around 3 - 5, would account for the discrepancy between theory and observations.