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

Downes, Juan José; Román-Zúñiga, Carlos; Ballesteros-Paredes, Javier; Mateu, Cecilia; Briceño, César; Hernández, Jesús; Petr-Gotzens, Monika G.; Calvet, Nuria; Hartmann, Lee; Mauco, Karina
Monthly Notices of the Royal Astronomical Society, Volume 450, Issue 4, p.3490-3502 (2015).


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 ˜0.01 M. By comparing them through a Bayesian method 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.