Initial Mass Function of 25 Orionis down to the Planetary-Mass Domain

The stellar initial mass function (IMF) has great importance in astrophysics but its behavior is still under discussion, especially in the low mass domain. Using new deep optical photometry from DECam with optical and NIR data from the literature, we determine the system IMF of the 25 Orionis stellar group (25 Ori) from the planetary mass domain to the intermediate-mass regime. This is one of the few IMFs across the entire mass range of a stellar association. The resultant system IMF is well-described by a two-part power-law function and by a tapered power-law form. We also report its best log-normal parameterization. This system IMF does not present significant variations within a radius of about 7 pc, which indicates that the substellar and stellar objects in 25 Ori do not have any preferential spatial distribution. We compared the reported system IMF and the BD/star ratio with those of a large diversity of stellar populations and did not find any significant discrepancies, which strongly supports the hypothesis that the star formation mechanism is largely insensitive to environmental conditions. Additionally, comparing the velocity dispersion with the escape velocity of the population, we found that 25 Ori is a gravitationally unbound association.