Disks around Brown Dwarfs in the Sigma Orionis Cluster

TL;DR

This study uses Spitzer data to identify and analyze disks around brown dwarfs in Sigma Orionis, estimating disk fractions and challenging previous interpretations of certain objects' youth and membership.

Contribution

It provides a new photometric analysis of disks around brown dwarfs in Sigma Orionis, identifying candidates and comparing disk fractions with similar clusters.

Findings

Disk fractions are approximately 40% for low-mass stars and 60% for brown dwarfs.

The study finds discrepancies with previous analyses regarding the object S Ori 70.

Spitzer data do not strongly support youth or membership for S Ori 70.

Abstract

We have performed a census of disks around brown dwarfs in the Sigma Ori cluster using all available images from IRAC onboard 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 5 promising candidates, which may consist of 2 brown dwarfs, 2 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 Sigma 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 Cha I, which have roughly the same median ages as Sigma Ori (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 5.8um 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 Sigma Ori that has been identified to date.