Mid-infrared data , including Spitzer warm-IRAC [ 3.6 ] and [ 4.5 ] photometry , is critical for understanding the cold population of brown dwarfs now being found , objects which have more in common with planets than stars .
As effective temperature ( T _ { eff } ) drops from 800 K to 400 K , the fraction of flux emitted beyond 3 \mu m increases rapidly , from about 40 % to > 75 % .
This rapid increase makes a color like H - [ 4.5 ] a very sensitive temperature indicator , and it can be combined with a gravity- and metallicity-sensitive color like H - K to constrain all three of these fundamental properties , which in turn gives us mass and age for these slowly cooling objects .
Determination of mid-infrared color trends also allows better exploitation of the WISE mission by the community .
We use new Spitzer Cycle 6 IRAC photometry , together with published data , to present trends of color with type for L0 to T10 dwarfs .
We also use the atmospheric and evolutionary models of Saumon & Marley to investigate the masses and ages of 13 very late-type T dwarfs , which have H - [ 4.5 ] > 3.2 and T _ { eff } \approx 500 K to 750 K .

Note : This is an updated version of ; a photometry compilation is available at www.gemini.edu/staff/sleggett .