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We present a statistical method for the photometric search of rareastronomical sources based on the weighted k-NN method. A metric is defined ina multi-dimensional color-magnitude space based only on the photometricproperties of template sources and the photometric uncertainties of bothtemplates and data, without the need to define ad-hoc color and magnitude cutswhich could bias the search. The metric is defined as a function of twoparameters, the number of neighbors k and a threshold distance D_th that can beoptimized for maximum selection efficiency and completeness. We apply themethod to the search of L and T dwarfs in the Spitzer Extragalactic First LookSurvey and the Bootes field of the Spitzer Shallow Survey, as well as to thesearch of sub-stellar mass companions around nearby stars. With high level ofcompleteness, we confirm the absence of late-T dwarfs detected in at least twobands in the First Look Survey, and only one in the Shallow Survey (previouslydiscovered by Stern et al. 2007). This result is in agreement with the expectedstatistics for late-T dwarfs. One L/early-T candidate is found in the FirstLook Survey, and 3 in the Shallow Surveys, currently undergoing follow-upspectroscopic verification. Finally, we discuss the potential for brown dwarfsearches with this method in the Spitzer warm mission Exploration Scienceprograms.

A k-NN Method to Classify Rare Astronomical Sources: Photometric Search of Brown Dwarfs with Spitzer/IRAC