We present a census of circumstellar disks in the Chamaeleon I star-forming region .
Using the Infrared Array Camera and the Multiband Imaging Photometer onboard the Spitzer Space Telescope , we have obtained images of Chamaeleon I at 3.6 , 4.5 , 5.8 , 8.0 , and 24 µm .
To search for new disk-bearing members of the cluster , we have performed spectroscopy on objects that have red colors in these data .
Through this work , we have discovered four new members of Chamaeleon I with spectral types of M4 , M6 , M7.5 , and L0 .
The first three objects are highly embedded ( A _ { J } \sim 5 ) and reside near known protostars , indicating that they may be among the youngest low-mass sources in the cluster ( \tau < 1 Myr ) .
The L0 source is the coolest known member of Chamaeleon I .
Its luminosity implies a mass of 0.004-0.01 M _ { \odot } , making it the least massive brown dwarf for which a circumstellar disk has been reliably detected .
To characterize the disk population in Chamaeleon I , we have classified the infrared spectral energy distributions of the 203 known members that are encompassed by the Spitzer images .
Through these classifications , we find that the disk fraction in Chamaeleon I is roughly constant at \sim 50 % from 0.01 to 0.3 M _ { \odot } .
These data are similar to the disk fraction of IC 348 , which is a denser cluster at the same age as Chamaeleon I .
However , the disk fraction at M \gtrsim 1 M _ { \odot } is significantly higher in Chamaeleon I than in IC 348 ( 65 % vs. 20 % ) , indicating longer disk lifetimes in Chamaeleon I for this mass range .
Thus , low-density star-forming regions like Chamaeleon I may offer more time for planet formation around solar-type stars than denser clusters .