Probing the Milky Way stellar and brown dwarf initial mass function with modern microlensing observations

Abstract

We use recent microlensing observations toward the central bulge of the Galaxy to probe the overall stellar plus brown dwarf initial mass function (IMF) in these regions well within the brown dwarf domain. We find that the IMF is consistent with the same Chabrier (2005) IMF characteristic of the Galactic disk. In contrast, other IMFs suggested in the literature overpredict the number of shorttime events, thus of very-low mass stars and brown dwarfs, compared with observations. This, again, supports the suggestion that brown dwarfs and stars form predominantly via the same mechanism. We show that claims for different IMFs in the stellar and substellar domains rather arise from an incorrect parameterization of the IMF. Furthermore, we show that the IMF in the central regions of the bulge seems to be bottom-heavy, as illustrated by the large number of short-time events compared with the other regions. This recalls our previous analysis of the IMF in massive early type galaxies and suggests the same kind of two-phase formation scenario, with the central bulge initially formed under more violent, burst-like conditions than the rest of the Galaxy.

A rich population of free-floating planets in the Upper Scorpius young stellar association

“…To establish the fraction of FFPs to stars, large samples with robust uncertainties and low contamination rates are needed to provide a statistically significant answer. Recently, some works have measured the mass function of the field population reaching planetary mass objects (Kirkpatrick et al, 2019(Kirkpatrick et al, , 2021Bardalez Gagliuffi et al, 2019;Chabrier and Lenoble, 2023). However, these mass functions are the combination of different star formation events which happened over several Gyr and thus, might differ from the initial mass function reported by simulations of star formation.…”

The origin of free-floating planets

“…To establish the fraction of FFPs to stars, large samples with robust uncertainties and low contamination rates are needed to provide a statistically significant answer. Recently, some works have measured the mass function of the field population reaching planetary mass objects (Kirkpatrick et al 2019(Kirkpatrick et al , 2021Bardalez Gagliuffi et al 2019;Chabrier and Lenoble 2023). However, these mass functions are the combination of different star formation events which happened over several Gyr and thus, might differ from the initial mass function reported by simulations of star formation.…”