Disc evolution and the relationship between $L_{\rm acc}$ and $L_\ast$ in T Tauri stars

TL;DR

This study models the evolution of accretion and stellar luminosities in T Tauri stars, showing that observed luminosity ratios can be explained by initial disc mass fractions, but data degeneracies limit constraining disc-stellar relations.

Contribution

The paper introduces an approximate stellar evolution model for accreting pre-mainsequence stars that explains observed luminosity ratios and highlights limitations in constraining disc properties.

Findings

Observed $L_{acc}/L_*$ ratios are consistent with initial disc mass fractions of 0.01-0.2.

The upper bound $L_{acc} ightarrow L_*$ is a generic feature of disc accretion.

Degeneracies in data hinder constraining disc-stellar mass relations.

Abstract

We investigate the evolution of accretion luminosity $L_{\rm acc}$ and stellar luminosity ${L_\ast}$ in pre-mainsequence stars. We make the assumption that when the star appears as a Class II object, the major phase of accretion is long past, and the accretion disc has entered its asymptotic phase. We use an approximate stellar evolution scheme for accreting pre-mainsequence stars based on Hartmann, Cassen & Kenyon, 1997. We show that the observed range of values $k = L_{\rm acc}/L_\ast$ between 0.01 and 1 can be reproduced if the values of the disc mass fraction $M_{\rm disc}/M_*$ at the start of the T Tauri phase lie in the range 0.01 -- 0.2, independent of stellar mass. We also show that the observed upper bound of $L_{\rm acc} \sim L_\ast$ is a generic feature of such disc accretion. We conclude that as long as the data uniformly fills the region between this upper bound and observational detection thresholds, then the degeneracies between age, mass and accretion history severely limit the use of this data for constraining possible scalings between disc properties and stellar mass.