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Notes on Kepler/2MASS Cross Match
In its matching between KIC objects that fall on one of the Kepler CCD
detectors and the
2MASS catalog (see also
Skrutskie, M. F. et al. 2006, AJ, 131, 1163S),
MAST used a positional search radius of 5 arcsec for each KIC object and
identified the 2MASS object closest to the KIC object's coordinates. The
2MASS identifiers in the KIC were taken from other catalogs and no
cross matching was performed with the 2MASS catalog itself (D. Monet, priv.
comm.). While investigating the differences between MAST's and the KIC
identifications, MAST found that the KIC coordinates were assembled
from disparate catalogs, each having its own search radius limit.
Evidence of this heterogenity can be seen in a histogram of Kepler objects and their nearest
neighbors. This histogram shows the distribution of KIC objects cross matched
against other KIC objects (but not themselves!).
The jagged distribution of this plot is caused by the differing
positional criteria of the source catalogs to obtain object coordinates.
There are a number of reasons why cross matches between two catalogs, or even objects in a single catalog, are imperfect. In the matching between the KIC and 2MASS catalogs we noticed the following:
For any of these reasons, errors in cross matchings can occur because of a chance association with an "interloping" object, that is one that happens to be within the 5 arcsec cone search area. MAST does not adjust its matches on the basis of optical or IR colors. However, as a point or information of information, we note from a study of a subset of objects with twoMass conflict flag value of 3, typical hybrid (g-H) colors are ~ +3-4.