The Substellar Population of the σ Orionis Cluster

摘要

We present a deep I, Z photometric survey (I_(compl)= 22 mag) covering a total area of 1.12deg~2 in the σ Orionis cluster. Using 1,1 - 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 σ = 0.4-1.1.