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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:

  • errors in the coordinates of a KIC object,
  • errors in the coordinates of a 2MASS object (included because the same object is found with slightly different coordinates on edges of two overlapping 2MASS sky zones), and
  • a visible object in one passband is faint or invisible in the other.

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.

Note that when 2MASS IDs are changed from the KIC on the results page returned to you so are their associated JHK (that is J, H, and/or K) magnitudes.