Substellar Objects in Nearby Young Clusters (SONYC) III: Chamaeleon-I

TL;DR

This study conducts a deep optical and infrared survey of the Chamaeleon-I star-forming region to identify and characterize substellar objects, including new candidates and members, refining the understanding of the cluster's low-mass population.

Contribution

It provides new spectroscopic and photometric data on substellar objects in Chamaeleon-I, identifying two new objects above the substellar limit and estimating the number of missing low-mass members.

Findings

Identified 13 objects with M spectral types, including 2 above the substellar limit.

Estimated that fewer than 7 very-low-mass members are missing from the current census.

Discovered two new candidate members using combined JK and Spitzer data.

Abstract

SONYC - Substellar Objects in Nearby Young Clusters - is a survey program to investigate the frequency and properties of substellar objects with masses down to a few times that of Jupiter in nearby star-forming regions. In this third paper, we present our recent results in the Chamaeleon-I star forming region. We have carried out deep optical and near-infrared imaging in four bands (I, z, J, Ks) using VIMOS on the ESO Very Large Telescope and SOFI on the New Technology Telescope, and combined our data with mid-infrared data from the Spitzer Space Telescope. The survey covers ~0.25 sqdeg on the sky, and reaches completeness limits of 23.0 in the I-band, 18.3 in the J-band, and 16.7 in Ks-band. Follow-up spectroscopy of the candidates selected from the optical photometry (I <~ 21) was carried out using the multi-object spectrograph VIMOS on the VLT. We identify 13 objects consistent with M spectral types, 11 of which are previously known M-dwarfs with confirmed membership in the cluster. The 2 newly reported objects have effective temperatures above the substellar limit. We also present two new candidate members of Chamaeleon-I, selected from our JK photometry combined with the Spitzer data. Based on the results of our survey, we estimate that the number of missing very-low-mass members down to ~0.008 MSun and Av <= 5 is <= 7, i.e. <= 3% of the total cluster population according to the current census.