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

Abstract

We present a study of 15 new brown dwarfs belonging to the ~7 Myr old 25 Orionis group and Orion OB1a subassociation with spectral types between M6 and M9 and estimated masses between ~0.07 and ~0.01M⊙. By comparing them through a Bayesianmethod with low-mass stars (0.8 ≲M/M⊙ ≲ 0.1) 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 substellar mass limit. Particularly, we found a fraction of 3.9+2.4-1.6 per cent low-mass stars classified as Classic T Tauri star and class II or evolved discs, against a fraction of 33.3+10.8-9.8 per cent in the substellar mass domain. Our results support the suggested scenario in which the dissipation of discs is less efficient for decreasing mass of the central object.