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

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.