The 2MASS position reconstruction algorithms (cf. IV.6) required positionally correlating infrared detections with objects in the primary astrometric reference catalog, Tycho 2. Infrared sources were also correlated with the higher density USNO-A2.0 catalog to monitor camera distortion. 2MASS PSC sources that have optical catalog counterparts within approximately 5´´ of the PSC position have listed for convenience the optical catalog identifier (a="T" (Tycho) or "U" (USNOA)), blue and visual-or-red magnitudes (b_m_opt and vr_m_opt), the 2MASS/optical position separation (dist_opt) and position angle (phi_opt) information listed in the PSC source record, for convenience.
It is stressed that optical data listed for the PSC sources are derived from positional associations, and not necessarily identifications. Chance alignment of infrared and optical sources is possible, especially in high source density regions. The association reported is the closest optical source to the 2MASS source position, within a maximum separation of approximately 5´´, with a match to a Tycho 2 source taking precedence over a USNO-A2.0 match. No association data is listed if there is no Tycho 2 or USNO-A2.0 source within the association radius. No attempt is made to find the best pairings of 2MASS and optical sources, and an optical source can be matched to more than one 2MASS source. A measure of possible confusion in the association process is given by the nopt_mchs parameter in the PSC, which specifies the number of possible optical matches for each 2MASS point source within the ~5´´ search radius. The relative accuracy of the PSC and the optical comparison catalogs are sufficiently high, though, that separations larger than ~2´´ are good indications of chance associations, high proper motion, or confusion. Figure 1 shows the distribution of dist_opt values for the 1,608,024 PSC sources at |b|>75o that have optical associations (84.4% of all high latitude sources); >95% have dist_opt <2´´.
For Tycho 2 associations (a="T"), dist_opt was computed using the mean Tycho 2 position (mRAdeg, mDEdeg), corrected for proper motion to the time of the 2MASS observation if proper motions were available. For Tycho 2 associations that do not have mean positions, or for which the Tycho 2 posflg was "D" (double star treatment) or "P" (photo-centre treatment), the separation was computed using the observed Tycho 2 catalog position (RAdeg, DEdeg).
For USNO-A2.0 matches (a="U"), dist_opt gives the simple separation between PSC and USNO-A2.0 reported positions. Because USNO-A2.0 was derived from plate material taken at a much earlier epoch than 2MASS, proper motions can lead to larger separations and possibly missed associations. Conversely, the large epoch difference between USNO-A2.0 and 2MASS makes possible statistical studies utilizing systematic motions of large samples of objects (e.g., Adams et al. 2001 AJ, 121, 2053).
A software error in setting the PSC optical catalog identifier, a, resulted in an incorrect value of a="0" being assigned to optical associations that have dist_opt5´´. There are 361,663 sources in the full PSC with this error. These objects can be identified as having a="0" and non-null values in the other optical association fields: dist_opt, phi_opt, b_m_opt, and vr_m_opt. When searching for PSC sources with our without optical associations, the value of the optical catalog identifier, a, cannot be used alone as a discriminant.
The optical magnitudes listed for the optical associations in the PSC are derived from the Tycho 2 and USNO-A2.0 values. In the case of Tycho 2 associations, the optical magnitudes listed are Johnson B and V magnitudes and are derived from the Tycho blue (BT) and visual (VT) magnitudes using the transformations given by (Høg et al. 2000, "Guide to the Tycho-2 Catalog"):
V = VT - 0.090 * (BT - VT)
B-V = 0.850 * (BT - VT)
For USNO-A2.0 associations, the optical magnitudes are the photographic blue and red magnitudes taken explicitly from the USNO-A2.0 Catalog.
Missed Optical Associations Near the Equatorial Poles
An error in the positional correlation procedure between 2MASS and USNO-A2.0 sources resulted in associations being systematically missed in the vicinity of the north and south equatorial poles. In Figure 2 is shown the fraction of bright, three-band 2MASS PSC sources without optical counterparts (nopt_mchs=0) plotted as a function of declination within 20° of the north equatorial pole. The fraction of PSC sources without reported counterparts increases rapidly and monotonically with proximity to the pole for declinations > 86°. Optical associations for PSC sources near the south equatorial pole exhibit identical behavior.
An independent correlation between 2MASS and USNO-A2.0 sources in the vicinity of the poles shows that most PSC sources do have USNO-A2.0 counterparts within 5´´. Furthermore, good agreement between 2MASS and USNO-A2.0 positions near the poles, and the analysis of 2MASS astrometric accuracy (e.g., VI.6.a.ii) indicate that there is no evidence for systematic problem with 2MASS or USNO-A2.0 positions near the poles. Therefore, the apparent increase in fraction of sources without optical counterparts shown in Figure 2 is an artifact of the correlation processing.
For sources with |dec|>86°, the optical association information is not reliable. The absence of optical association data in the PSC record does not necessarily mean that there is not an optical source within 5´´ of the 2MASS position.
|Figure 1||Figure 2|
[Last Updated: 2003 July 1, by R. Cutri and H.McCallon]