The Substellar Population of the σ Orionis Cluster

Abstract

We present a deep I, Z photometric survey (I _compl = 22 mag) covering a total area of 1.12 deg² in the σ Orionis cluster. Using I, I − Z color-magnitude diagrams, we have selected 165 photometric candidates (I = 16–24 mag), which correspond to a mass interval from 0.11 down to 0.006 M _⊙. Using J-band photometry, we have found that 120 of 147 candidates within the completeness of the survey ( ∼ 80%) follow the infrared photometric sequence of the cluster; we will consider them as likely members. Using J, H, K-band data from UKIDSS and mid-infrared data from IRAC/Spitzer, we have found that 6–11% of the bona fide candidates have near-infrared flux excesses, and about 34 ± 8% show mid-infrared flux excesses, probably related to the presence of disks. We have determined that the mass spectrum from 0.11 to 0.013 M _⊙ is a rising function that can be represented by a power-law distribution (dN/dm ∼ m^{− α}) with an exponent α = 0. 4–1.1.