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We present the results of a survey of the low mass star and brown dwarfpopulation of the 25 Orionis group. Using optical photometry from the CIDA DeepSurvey of Orion, near IR photometry from the Visible and Infrared SurveyTelescope for Astronomy and low resolution spectroscopy obtained with Hectospecat the MMT, we selected 1246 photometric candidates to low mass stars and browndwarfs with estimated masses within $0.02 \lesssim M/M_\odot \lesssim 0.8$ andspectroscopically confirmed a sample of 77 low mass stars as new members of thecluster with a mean age of $\sim$7 Myr. We have obtained a system initial massfunction of the group that can be well described by either a Kroupa power-lawfunction with indices $\alpha_3=-1.73\pm0.31$ and $\alpha_2=0.68\pm0.41$ in themass ranges $0.03\leq M/M_\odot\leq 0.08$ and $0.08\leq M/M_\odot\leq0.5$respectively, or a Scalo log-normal function with coefficients$m_c=0.21^{+0.02}_{-0.02}$ and $\sigma=0.36\pm0.03$ in the mass range $0.03\leqM/M_\odot\leq0.8$. From the analysis of the spatial distribution of thisnumerous candidate sample, we have confirmed the East-West elongation of the 25Orionis group observed in previous works, and rule out a possible southernextension of the group. We find that the spatial distributions of low massstars and brown dwarfs in 25 Orionis are statistically indistinguishable.Finally, we found that the fraction of brown dwarfs showing IR excesses ishigher than for low mass stars, supporting the scenario in which the evolutionof circumstellar discs around the least massive objects could be moreprolonged.

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