System IMF of the 25 Ori Group from Planetary-Mass Objects to
  Intermediate/High-Mass Stars
  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 $\Gamma=-0.74\pm0.04$ for
$m<0.4\ M_\odot$ and $\Gamma=1.50\pm0.11$ for $m\ge0.4\ M_\odot$. It is also
well described over the whole mass range by a tapered power-law function with
$\Gamma=1.10\pm0.09$, $m_p=0.31\pm0.03$ and $\beta=2.11\pm0.09$. The best
lognormal representation of the system IMF has $m_c=0.31\pm0.04$ and
$\sigma=0.46\pm0.05$ 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 $0.16\pm0.03$ 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.
