The bimodal initial mass function in the Orion Nebula Cloud

TL;DR

This study reveals a bimodal initial mass function in the Orion Nebula Cloud, showing two distinct peaks at 0.25 and 0.025 solar masses, with a significant dip at the hydrogen burning limit, based on deep near-infrared observations.

Contribution

First detailed near-infrared survey of ONC demonstrating a bimodal IMF with two distinct peaks and a pronounced dip at the hydrogen burning limit.

Findings

Bimodal IMF with peaks at 0.25 and 0.025 M_sun

Approximately ten times more substellar candidates than previously known

Substellar IMF peak may be due to ejected brown dwarfs and planetary objects

Abstract

Due to its youth, proximity and richness the Orion Nebula Cloud (ONC) is an ideal testbed to obtain a comprehensive view on the Initial Mass Function (IMF) down to the planetary mass regime. Using the HAWK-I camera at the VLT, we have obtained an unprecedented deep and wide near-infrared JHK mosaic of the ONC (90% completeness at K~19.0mag, 22'x28). Applying the most recent isochrones and accounting for the contamination of background stars and galaxies, we find that ONC's IMF is bimodal with distinct peaks at about 0.25 and 0.025 M_sun separated by a pronounced dip at the hydrogen burning limit (0.08 M_sun), with a depth of about a factor 2-3 below the log-normal distribution. Apart from ~920 low-mass stars (M < 1.4 M_sun) the IMF contains ~760 brown dwarf (BD) candidates and ~160 isolated planetary mass object (IPMO) candidates with M > 0.005 M_sun, hence about ten times more substellar candidates than known before. The substellar IMF peak at 0.025 M_sun could be caused by BDs and IPMOs which have been ejected from multiple systems during the early star-formation process or from circumstellar disks.