System initial mass function of the 25 Ori group from planetary-mass objects to intermediate/high-mass stars

Abstract

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 • radius area in 25 Ori. With this sample we derived the 25 Ori system IMF from 0.012 to 13.1 M . This system IMF is well described by a two-segment power-law with Γ = −0.74 ± 0.04 for m < 0.4 M and Γ = 1.50 ± 0.11 for m ≥ 0.4 M . It is also well described over the whole mass range by a tapered power-law function with Γ = 1.10 ± 0.09, m p = 0.31 ± 0.03 and β = 2.11 ± 0.09. The best lognormal representation of the system IMF has m c = 0.31 ± 0.04 and σ = 0.46 ± 0.05 for m < 1 M . 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±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.

A multiple power-law distribution for initial mass functions

“…This agrees with the generation of outflows at the onset of nuclear fusion, as suggested by Shu et al (1987). Interestingly, such an IMF with two power laws has been suggested by observations of the ONC by Da Rio et al (2012) and of the 25 Orionis group by Suárez et al (2019).…”

The GALAH survey: Chemical homogeneity of the Orion complex

“…their errorbars as seen in Figure 13 to the region where our data are complete and obtained a slope α = 2.49 ± 0.15. This is a bit steeper than the traditional Kroupa slope of α = 2.3 (Kroupa 2001) or α = 2.27 ± 0.08 measured in the λ Ori association (Barrado y Navascues et al 2004), α = 2.40 ± 0.09 in the 25 Ori cluster (Suárez et al 2019), and α = 2.4± in the ONC (De Marchi et al 2005), α = 2.21±0.18 in the Trapezium cluster (Muench et al 2002), but flatter than α = 2.7 in the ONC and the Trapezium cluster These estimates do not include any runaway/ejected stars into the IMF, so the actual number is a fraction higher. Assuming a steeper IMF, sometimes quoted in the literature listed above, the number of SNe drops to essentially zero.…”

A Core Mass Function Indistinguishable from the Salpeter Stellar Initial Mass Function Using 1000 au Resolution ALMA Observations

“…We compared our high-mass CMF to the Salpeter IMF because of its validity for field stars in the solar neighborhood and in a variety of stellar associations (e.g. ; Bastian et al 2010;Suárez et al 2019;Mužić et al 2019). This has been interpreted as a suggestion of a universal IMF (Bastian et al 2010;Offner et al 2014).…”