Demographics of the M-star Multiple Population in the Orion Nebula Cluster

TL;DR

This study analyzes the multiplicity demographics of low-mass stars in the Orion Nebula Cluster using Hubble data, revealing that their companion frequency and mass ratio distribution are similar to the Galactic field and Taurus, suggesting primordial origins.

Contribution

It provides updated constraints on the multiplicity demographics of low-mass stars in the ONC, including new companion detections and a Bayesian analysis of their frequency and mass ratio distribution.

Findings

Companion frequency in ONC is 0.13$^{+0.05}_{-0.03}$.

Mass ratio distribution index is 2.08$^{+1.03}_{-0.85}$.

ONC multiplicity properties are consistent with the Galactic field and Taurus.

Abstract

We present updated results constraining multiplicity demographics for the stellar population of the Orion Nebula Cluster (ONC, a high-mass, high-density star-forming region), across primary masses 0.08-0.7M$_{\odot}$. Our study utilizes archival Hubble Space Telescope data obtained with the Advanced Camera for Surveys using multiple filters (GO-10246). Previous multiplicity surveys in low-mass, low-density associations like Taurus identify an excess of companions to low-mass stars roughly twice that of the Galactic field and find the mass ratio distribution consistent with the field. Previously, we found the companion frequency to low-mass stars in the ONC is consistent with the Galactic field over mass ratios=0.6-1.0 and projected separations=30-160au, without placing constraints on the mass ratio distribution. In this study, we investigate the companion population of the ONC with a double point-spread function (PSF) fitting algorithm sensitive to separations larger than 10au (0.025") using empirical PSF models. We identified 44 companions (14 new), and with a Bayesian analysis, estimate the companion frequency to low-mass stars in the ONC =0.13$^{+0.05}_{-0.03}$ and the power law fit index to the mass ratio distribution =2.08$^{+1.03}_{-0.85}$ over all mass ratios and projected separations of 10-200au. We find the companion frequency in the ONC is consistent with the Galactic field population, likely from high transient stellar density states, and a probability of 0.002 that it is consistent with that of Taurus. We also find the ONC mass ratio distribution is consistent with the field and Taurus, potentially indicative of its primordial nature, a direct outcome of the star formation process.