The Initial Mass Function of the Orion Nebula Cluster across the H-burning limit

TL;DR

This study provides a detailed census of the Orion Nebula Cluster, employing a novel photometric technique to accurately determine stellar properties and derive the initial mass function down to substellar masses, revealing a decline in brown dwarf formation.

Contribution

Introduces a new photometric method to determine effective temperatures of stars in the ONC, improving accuracy and completeness of the initial mass function analysis.

Findings

The initial mass function declines steeply with decreasing mass.

The technique accurately measures effective temperatures independent of extinction.

The survey increases known cluster members and extends to substellar masses.

Abstract

We present a new census of the Orion Nebula Cluster (ONC) over a large field of view (>30'x30'), significantly increasing the known population of stellar and substellar cluster members with precisely determined properties. We develop and exploit a technique to determine stellar effective temperatures from optical colors, nearly doubling the previously available number of objects with effective temperature determinations in this benchmark cluster. Our technique utilizes colors from deep photometry in the I-band and in two medium-band filters at lambda~753 and 770nm, which accurately measure the depth of a molecular feature present in the spectra of cool stars. From these colors we can derive effective temperatures with a precision corresponding to better than one-half spectral subtype, and importantly this precision is independent of the extinction to the individual stars. Also, because this technique utilizes only photometry redward of 750nm, the results are only mildly sensitive to optical veiling produced by accretion. Completing our census with previously available data, we place some 1750 sources in the Hertzsprung-Russel diagram and assign masses and ages down to 0.02 solar masses. At faint luminosities, we detect a large population of background sources which is easily separated in our photometry from the bona fide cluster members. The resulting initial mass function of the cluster has good completeness well into the substellar mass range, and we find that it declines steeply with decreasing mass. This suggests a deficiency of newly formed brown dwarfs in the cluster compared to the Galactic disk population.