We present new Spitzer IRAC photometry of twelve very late-type T dwarfs ; nine have [ 3.6 ] , [ 4.5 ] , [ 5.8 ] and [ 8.0 ] photometry and three have [ 3.6 ] and [ 4.5 ] photometry only .
Combining this with previously published photometry , we investigate trends with type and color that are useful for both the planning and interpretation of infrared surveys designed to discover the coldest T or Y dwarfs .
The on-line Appendix provides a collation of MKO-system YJHKL ^ { \prime } M ^ { \prime } and IRAC photometry for a sample of M , L and T dwarfs .
Brown dwarfs with effective temperature ( T _ { eff } ) below 700 K emit more than half their flux at wavelengths longer than 3 \mu m , and the ratio of the mid-infrared flux to the near-infrared flux becomes very sensitive to T _ { eff } at these low temperatures .
We confirm that the color H ( 1.6 \mu m ) - [ 4.5 ] is a good indicator of T _ { eff } with a relatively weak dependence on metallicity and gravity .
Conversely , the colors H - K ( 2.2 \mu m ) and [ 4.5 ] - [ 5.8 ] are sensitive to metallicity and gravity .
Thus near- and mid-infrared photometry provide useful indicators of the fundamental properties of brown dwarfs , and if temperature and gravity are known , then mass and age can be reliably determined from evolutionary models .
There are twelve dwarfs currently known with H - [ 4.5 ] > 3.0 , and 500 \mathrel { \hbox { \hbox to 0.0 pt { \lower 2.15 pt \hbox { $ \sim$ } \hss } \raise 1.72 pt% \hbox { $ < $ } } } T _ { eff } K \mathrel { \hbox { \hbox to 0.0 pt { \lower 2.15 pt \hbox { $ \sim$ } \hss } \raise 1.72 pt% \hbox { $ < $ } } } 800 , which we examine in detail .
The ages of the dwarfs in the sample range from very young ( 0.1 – 1.0 Gyr ) to relatively old ( 3 – 12 Gyr ) .
The mass range is possibly as low as 5 Jupiter masses to up to 70 Jupiter masses , i.e .
near the hydrogen burning limit .
The metallicities also span a large range , from [ m/H ] = -0.3 to [ m/H ] = +0.3 .
The small number of T8 – T9 dwarfs found in the UKIRT Infrared Deep Sky Survey to date appear to be predominantly young low-mass dwarfs .
Accurate mid-infrared photometry of cold brown dwarfs is essentially impossible from the ground , and extensions to the mid-infrared space missions warm- Spitzer and WISE are desirable in order to obtain the vital mid-infrared data for cold brown dwarfs , and to discover more of these rare objects .