2008ApJ...688..362L

We have performed a census of circumstellar disks around brown dwarfs in the σ Ori cluster using all available images from the Infrared Array Camera on board the Spitzer Space Telescope. To search for new low-mass cluster members with disks, we have measured photometry for all sources in the Spitzer images and have identified the ones that have red colors that are indicative of disks. We present five promising candidates, which may consist of two brown dwarfs, two stars with edge-on disks, and a low-mass protostar if they are bona fide members. Spectroscopy is needed to verify the nature of these sources. We have also used the Spitzer data to determine which of the previously known probable members of σ Ori are likely to have disks. By doing so, we measure disk fractions of ∼40% and ∼60% for low-mass stars and brown dwarfs, respectively. These results are similar to previous estimates of disk fractions in IC 348 and Chamaeleon I, which have roughly the same median ages as σ Ori (τ∼3 Myr). Finally, we note that our photometric measurements and the sources that we identify as having disks differ significantly from those of other recent studies that analyzed the same Spitzer images. For instance, previous work has suggested that the T dwarf S Ori 70 is redder than typical field dwarfs, which has been cited as possible evidence of youth and cluster membership. However, we find that this object is only slightly redder than the reddest field dwarfs in [3.6]-[4.5] (1.56±0.07 vs. 0.93-1.46). We measure a larger excess in [3.6]-[5.8] (1.75±0.21 vs. 0.87-1.19), but the flux at 5.8 µm may be overestimated because of the low signal-to-noise ratio of the detection. Thus, the Spitzer data do not offer strong evidence of youth and membership for this object, which is the faintest and coolest candidate member of σ Ori that has been identified to date.