Optical and Infrared Colors of Stars Observed by the Two Micron All Sky Survey and the Sloan Digital Sky Survey

We discuss optical and infrared photometric properties of stars matched inthe Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS)commissioning data for 50 deg2 of sky. About 98% (63,000) of objects listed inthe 2MASS Point Source Catalog in the analyzed area are matched within 2 arcsecto an SDSS source. The matched sources represent 8% of the about 800,000 SDSSsources in this area, and 15% of them are resolved in SDSS imaging data,although they are detected as point sources in 2MASS data. For about 14,000 stars with the smallest photometric errors (< 10%) in bothsurveys, we present optical and infrared color-magnitude and color-colordiagrams. We use optical (SDSS) colors to identify the stellar spectralsequence and show that stars of different spectral types can have similarinfrared colors, thus making the classification of stars based on only 2MASSdata very difficult. However, a broad separation into ``early'' and ``late''spectral types (relative to type K0) is possible with a reliability of about95% even with 2MASS colors alone. The distributions of matched sources in color-magnitude and color-colordiagrams are compared to the predictions of a stellar population synthesiscode. We find that the models are in fair overall agreement with the data. Themost significant discrepancies are found for the number ratio of ``early'' to``late'' type stars (by about a factor of 2) and in the colors of M stars (upto 0.2 mag).Comment: 31 pages, 8 figures, accepted to A