Spitzer Space Telescope - General Observer Proposal #60093 3 - 5 micron Photometry of 500 - 800 K Brown Dwarfs Principal Investigator: Sandy Leggett Institution: Gemini Observatory Technical Contact: Sandy Leggett, Gemini Observatory Co-Investigators: Loic Albert, Canada France Hawaii Telescope Etienne Artigau, Gemini Observatory Ben Burningham, Hertfordshire University Xavier Delfosse, Grenoble Observatory Philippe Delorme, St Andrews University Thierry Forveille, Grenoble Observatory Nicolas Lodieu, Inst. Astr. Canarias Phil Lucas, Herftordshire University Mark Marley, NASA-Ames David Pinfield, Herfordshire University Celine Reyle, Besancon Observatory Didier Saumon, Los Alamos National Lab Steve Warren, Imperial College London Science Category: brown dwarfs/very low mass stars Observing Modes: IRAC Post-Cryo Mapping Hours Approved: 9.1 Abstract: We request 9.1hrs to obtain IRAC photometry of 11 cold brown dwarfs. The observations will complete the dataset of near-infrared spectroscopy and 1-5um photometry, for known brown dwarfs with spectral types >T7, and temperatures of 500K to 800K. These rare very late- type T dwarfs are the coolest objects known outside of the solar system. For these the [3.6] band samples a low-flux region of strong CH4 absorption, and the [4.5] band a bright region subject to absorption by dredged-up CO. The [4.5] flux becomes increasingly dominant at low temperatures, and is the best temperature indicator for the latest T dwarfs, for which the near- infrared features are saturated. Combining accurate IRAC photometry with our near-infrared data, and models, will allow us to constrain temperature, metallicity and gravity, as well as vertical mixing in the atmospheres of our targets. Now that objects as cool as ~500K are being found it is vital that we understand their behaviour in the mid-infrared, especially in the WISE era, when the red [3.6]-[4.5] color will be used to find even more extreme objects.