2014MNRAS.444.1793D

We present the results of a survey of the low-mass star and brown dwarf population of the 25 Orionis group. Using optical photometry from the CIDA (Centro de Investigaciones de Astronomía `Francisco J. Duarte', Mérida, Venezuela) Deep Survey of Orion, near-IR photometry from the Visible and Infrared Survey Telescope for Astronomy and low-resolution spectroscopy obtained with Hectospec at the MMT telescope, we selected 1246 photometric candidates to low-mass stars and brown dwarfs with estimated masses within 0.02 ≲ M/M_☉ ≲ 0.8 and spectroscopically confirmed a sample of 77 low-mass stars as new members of the cluster with a mean age of ∼ 7 Myr. We have obtained a system initial mass function of the group that can be well described by either a Kroupa power-law function with indices α3 = -1.73±0.31 and α2 = 0.68±0.41 in the mass ranges 0.03 ≤ M/M_☉ ≤ 0.08 and 0.08 ≤ M/M_☉ ≤ 0.5, respectively, or a Scalo lognormal function with coefficients m_c=0.21^+0.02_-0.02 and σ = 0.36±0.03 in the mass range 0.03 ≤ M/M_☉ ≤ 0.8. From the analysis of the spatial distribution of this numerous candidate sample, we have confirmed the east-west elongation of the 25 Orionis group observed in previous works, and rule out a possible southern extension of the group. We find that the spatial distributions of low-mass stars and brown dwarfs in 25 Orionis are statistically indistinguishable. Finally, we found that the fraction of brown dwarfs showing IR excesses is higher than for low-mass stars, supporting the scenario in which the evolution of circumstellar discs around the least massive objects could be more prolonged.