Stellar SEDs from 0.3 to 2.5 μm:: Tracing the stellar locus and searching for color outliers in the SDSS and 2MASS

The Sloan Digital Sky Survey (SDSS) and Two Micron All Sky Survey (2MASS) are rich resources for studying stellar astrophysics and the structure and formation history of the Galaxy. As new surveys and instruments adopt similar filter sets, it is increasingly important to understand the properties of the ugrizJHK(s) stellar locus, both to inform studies of "normal'' main-sequence stars and enable robust searches for point sources with unusual colors. Using a sample of similar to 600,000 point sources detected by SDSS and 2MASS, we tabulate the position and width of the ugrizJHK(s) stellar locus as a function of g - i color, and provide accurate polynomial fits. We map the Morgan-Keenan spectral type sequence to the median stellar locus by using synthetic photometry of spectral standards and by analyzing 3000 SDSS stellar spectra with a custom spectral typing pipeline, described in the Appendix to this paper. We develop an algorithm to calculate a point source's minimum separation from the stellar locus in a seven-dimensional color space, and use it to robustly identify objects with unusual colors, as well as spurious SDSS/2MASS matches. Analysis of a final catalog of 2117 color outliers identifies 370 white-dwarf/Mdwarf (WDMD) pairs, 93 QSOs, and 90 M giant/carbon star candidates, and demonstrates that WDMD pairs and QSOs can be distinguished on the basis of their J - K(s) and r - z colors. We also identify a group of objects with correlated offsets in the u - g versus g - r and g - r versus r - i color-color spaces, but subsequent follow-up is required to reveal the nature of these objects. Future applications of this algorithm to a matched SDSS-UKIDSS catalog may well identify additional classes of objects with unusual colors by probing new areas of color-magnitude space.