The stellar initial mass function ( IMF ) is an essential input for many astrophysical studies but only in a few cases it has been determined over the whole cluster mass range , limiting the conclusions about its nature .
The 25 Orionis group ( 25 Ori ) is an excellent laboratory to investigate the IMF across the entire mass range of the population , from planetary-mass objects to intermediate/high-mass stars .
We combine new deep optical photometry with optical and near-infrared data from the literature to select 1687 member candidates covering a 1.1 ^ { \circ } radius area in 25 Ori .
With this sample we derived the 25 Ori system IMF from 0.012 to 13.1 M _ { \odot } .
This system IMF is well described by a two-segment power-law with \Gamma = -0.74 \pm 0.04 for m < 0.4 M _ { \odot } and \Gamma = 1.50 \pm 0.11 for m \geq 0.4 M _ { \odot } .
It is also well described over the whole mass range by a tapered power-law function with \Gamma = 1.10 \pm 0.09 , m _ { p } = 0.31 \pm 0.03 and \beta = 2.11 \pm 0.09 .
The best lognormal representation of the system IMF has m _ { c } = 0.31 \pm 0.04 and \sigma = 0.46 \pm 0.05 for m < 1 M _ { \odot } .
This system IMF does not present significant variations with the radii .
We compared the resultant system IMF as well as the BD/star ratio of 0.16 \pm 0.03 we estimated for 25 Ori with that of other stellar regions with diverse conditions and found no significant discrepancies .
These results support the idea that general star formation mechanisms are probably not strongly dependent to environmental conditions .
We found that the substellar and stellar objects in 25 Ori have similar spatial distributions and confirmed that 25 Ori is a gravitationally unbound stellar association .