Towards a Complete Study of the Initial Mass Function and Early Kinematics Evolution of the 25 Orionis Stellar Group

Young stellar aggregates are the laboratories where we study the process by which almost every star in the Galaxy has formed. Most of the young stellar groups break up in at most a few tens of Myr and only a few of them survive as gravitational bound entities to become open clusters. This phenomenon is usually understood as a rapid evolution of young stellar systems when the parental molecular gas is expelled, however, there is also evidence suggesting that these unbound stellar associations are formed in a structured process. To understand how stars are formed and how young stellar systems evolve, it is essential to estimate the total mass of the association, which can be achieved through the analysis of the stellar initial mass function (IMF). Despite many contributions to the study of the IMF, only a few of them over the whole cluster mass range, it is still unclear how it depends on environmental conditions and/or time. Due to its closeness, low extinction, stellar density and evolutionary status, an excellent place to study the IMF, from planetary-mass objects to intermediate/high-mass stars, and the early evolution of a young stellar system that just emerged from its embedded phase, is the 25 Orionis stellar group (25 Ori).

Combining new deep optical photometry from DECam with optical and NIR data from the literature, we selected 1687 member candidates of 25 Ori with \({I_c}\)-band magnitudes between 5 and 23.3 mag in an area of 1.1\({^\circ}\) radius. With this sample we derived the system IMF of 25 Ori from 0.012 to 13.1 \({M_\odot}\), which 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 lognormal parameterization. This system IMF do 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 as well as the BD/ratio 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.

In order to confirm the membership of each candidate in our sample, we have an ongoing spectroscopic survey using several world-wide facilities. We have obtained high-resolution (\({R\sim22000}\)) spectra of 77 intermediate/high-mass (1.3-11 \({M_\odot}\)) candidates with OAN-SPM/MES and of 1185 intermediate-mass (0.3-5.2 \({M_\odot}\)) candidates with SDSS-IV/APOGEE-2. Additionally, we have low-resolution (\({R\approx1000-2000}\)) spectra of 400 low-mass (0.25-0.8 \({M_\odot}\)) candidates with MMT/Hectospec, of 172 low-mass (0.09-0.7 \({M_\odot}\)) candidates with SDSS-III/BOSS and of 66 brown dwarf (0.01-0.08 \({M_\odot}\)) candidates with GTC/OSIRIS. After applying diverse membership criteria, we have so far confirmed 530 members from the spectroscopic sample, out of which 290 lie inside the 25 Ori area and 208 of them are confirmed for the first time. With this sample of confirmed member, plus those in the literature, we estimated that 25 Ori is a 6.5\({\pm}\)2.5 Myr old population located at 356\({\pm}\)47 pc and presenting a low extinction of 0.29\({\pm}\)0.26 mag. Also, we estimated that the 25 Ori mean radial velocity is 20.9\({\pm}\)2.0 km s\({^{-1}}\). Using these parameters we found that 25 Ori is a dynamically young group without time enough to be relaxed and confirmed that it is, in fact, a gravitationally unbound association that will be part of the Galactic Disk population.

Considering the observed and confirmed members as well as those already confirmed in the literature, the spectroscopic follow-up is \({\sim}\)75% complete, with most of the remaining candidates to be observed with estimated masses around the substellar mass limit. We have ongoing observations to complete the spectroscopic survey of 25 Ori.