The low-mass star and sub-stellar populations of the 25 Orionis group

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 Astronomia 'Francisco J. Duarte', Merida, 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 less than or similar to M/M-circle dot less than or similar to 0.8 and spectroscopically confirmed a sample of 77 low-mass stars as new members of the cluster with a mean age of similar to 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 alpha(3) = -1.73 +/- 0.31 and alpha(2) = 0.68 +/- 0.41 in the mass ranges 0.03 <= M/M-circle dot <= 0.08 and 0.08 <= M/M-circle dot <= 0.5, respectively, or a Scalo lognormal function with coefficients m(c) = 0.21(-0.02)(+0.02) and sigma = 0.36 +/- 0.03 in the mass range 0.03 <= M/M-circle dot <= 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.