The number fraction of discs around brown dwarfs in Orion OB1a and the 25 Orionis group

TL;DR

This study investigates the fraction of discs around brown dwarfs in the Orion OB1a and 25 Orionis group, revealing that disc dissipation is less efficient for lower mass objects, with significant differences across the sub-stellar boundary.

Contribution

It provides new measurements of disc fractions around brown dwarfs in Orion, highlighting mass-dependent disc evolution with a Bayesian comparison to low-mass stars.

Findings

Brown dwarfs have a higher disc fraction than low-mass stars.

Disc dissipation is less efficient in lower mass objects.

Significant differences in disc types across the sub-stellar boundary.

Abstract

We present a study of 15 new brown dwarfs belonging to the $\sim7$ Myr old 25 Orionis group and Orion OB1a sub-association with spectral types between M6 and M9 and estimated masses between $\sim0.07$M$_\odot$ and $\sim0.01$ M$_\odot$. By comparing them through a Bayesian method with low mass stars ($0.8\lesssim$ M/M$_\odot\lesssim0.1$) from previous works in the 25 Orionis group, we found statistically significant differences in the number fraction of classical T Tauri stars, weak T Tauri stars, class II, evolved discs and purely photospheric emitters at both sides of the sub-stellar mass limit. Particularly we found a fraction of $3.9^{+2.4}_{-1.6}~\%$ low mass stars classified as CTTS and class II or evolved discs, against a fraction of $33.3^{+10.8}_{-9.8}~\%$ in the sub-stellar mass domain. Our results support the suggested scenario in which the dissipation of discs is less efficient for decreasing mass of the central object.