IV. 2MASS Data Processing

9. Known Asteroid, Comet, Planet, and Satellite Associations

The 2MASS Second Incremental Data Release Point Source Catalog contains 3804 associations with known asteroids, 20 associations with predicted positions of known comets, 3 associations with planets (Uranus and Neptune; Uranus is associated twice), and 9 associations with planetary satellites. With the exception of planetary satellites, identification of known solar system objects takes place as part of pipeline processing.

PSC sources that are associated with known solar system objects have "mp_flg"=1 in their catalog records. The name and orbital data for the objects with which PSC sources are associated are given in the Known Asteroid Detection List, Known Comet Detection List, and Planet and Planetary Satellite Detection List in Section II.6 of this document.

a. Asteroids, Comets and Planets

The strategy used to identify possible detections of known asteroids, comets and planets by 2MASS is to consider the area covered during each survey scan, and the time each point on the sky within the scan was observed. The asteroid and comet ephemerides are then searched to determine which objects may have been within the scan boundaries at the specified epoch. Ephemerides are computed using orbital elements published by the Minor Planet Center for all numbered asteroids, and all multiple-opposition unnumbered asteroids, as well as all periodic comets, and recent nonperiodic comets. The orbits of the planets are included for completeness and are taken from the JPL DE403. The heliocentric position of the Earth is derived from DE403, and topocentric corrections to the two observing sites are included. Although the ephemeris computations are two-body in nature, the database of orbital elements is updated every 100 days to incorporate newly numbered asteroids and improved orbits, and the opportunity is taken to integrate all the orbits to a current epoch of osculation. The ephemeris accuracy is typically 1´´. In addition to the predicted position of the object, the apparent visual magnitude is computed, which can be used to validate proper identification, though a large acceptance window is needed because of unknown lightcurve and color effects, and the line of variation is used to represent the major axis of the error ellipsoid. Distances and phase angle are also computed for purposes of reducing apparent magnitudes to absolute magnitudes.

If a solar system object is predicted to have a position within the observed boundaries of a scan during the time of its observation, a search is made of the extracted 2MASS point source lists for objects that positionally correlate with the predicted position. Candidate 2MASS detections are first screened by searching for infrared sources within a coarse window of 30´´ in RA and DEC around the predicted position. For each 2MASS point source within that window, a two-dimensional ² position parameter is computed using the separation between the 2MASS and predicted positions and the combined position error covariance matrix. If the value of the ² is less than 16.0, the association is acceptable (a threshold of 16.0 corresponds to a completeness error of 0.000335; in other words, one correct match out of every 3000 will be missed in the attempt to avoid false matches). For example, for a predicted asteroid position uncertainty major axis of 3.0´´ and a minor axis of 1.0´´, this threshold just allows a match with a position discrepancy of 8.5´´ along the major axis and 2.8´´ along the minor axis.

2MASS point sources that have a match ² value below the threshold of 16.0 will be flagged as being possible asteroid or comet detections in the PSC by having an mp_flg value set to 1. It is otherwise 0. Objects with mp_flg=1 are not positive solar system object detections, but rather only positional associations. Some percentage of chance associations between predicted asteroid positions and background sources are to be expected, particularly in the Galactic Plane where the background source density is high. Users should examine in detail sources putatively associated with asteroids and comets to determine if the associations are in fact identifications. Information which is useful in determining this includes the 2MASS source color and whether the 2MASS object has a positional association with one of the optical reference catalogs.

Because the astrometric precision of 2MASS point source positions is typically < 0.2´-0.3´ with respect to the ACT (Hipparcos/Tycho) reference system, the dominant uncertainty in matching 2MASS candidate sources to asteroids and comets is the uncertainty in orbital predictions. Typical uncertainties are in the range 1´´-5´´, and as expected, the major axis of the asteroid position uncertainty ellipse is generally parallel to the orbital plane. The astrometric precision of 2MASS also means that every sighting of an asteroid or comet can be used to update orbital data for that object. In a few instances, orbital arclengths for an object are unavailable and the prediction position uncertainty ellipse is not calculated. In the case of some sungrazing comets, the orbital arc may be less than one day, in which case the prediction position uncertainty ellipse is also not calculated.

1186 asteroids were associated with PSC sources the First Incremental Data Release PSC, and the properties of the associated objects are discussed by Sykes et al. (2000a Icarus, in press). 3291 asteroids were associated with PSC sources in the Second Incremental Data Release PSC. 120 of the asteroid associations listed in the First Incremental Release PSC are not included in the Second Incremental Data Release because they fall in the overlap regions between adjacent survey scans that were handled differently in the production of the new PSC (cf., V.4). The following table provides a list of the First Incremental Release asteroids that are not found in the Second Incremental Release PSC:

First Incremental Release Asteroids not in the Second Incremental Release.

There is currently no attempt made during 2MASS data processing to identify previously unknown minor planets or comets. Such a search might be possible using the repeated observations of the small areas in the overlapping regions in adjacent tiles.

b. Planetary Satellites

Planetary satellite associations were made after pipeline processing showed that a planet was serendipitously observed during a scan of a 2MASS Tile. The positions of each satellite at the time of the 2MASS planet observation were derived using the JPL Ephemeris Generator. Each predicted position was then searched in the PSC; 2MASS sources within 5´´ of the predicted positions were flagged as satellite associations. Subsequent review of the 2MASS Atlas Images (see below) indicates that the planetary satellite associations are all high confidence identifications.

Jupiter, Uranus, and Neptune have been observed during survey operations, and detections of Uranus and Neptune are included in the Second Incremental Release PSC. Note that Uranus is associated with two PSC entries, but they are really one detection; one PSC entry reports the J and H detections and the other reports the K_s detection because of a failed bandmerge in this complex source. Figure 1 shows J, H and K_s composite Atlas Images of the fields containing Uranus and Neptune; some of the detected satellites of those planets are indicated. The blue color of Uranus and Neptune is caused by strong atmospheric methane absorption which depresses the observed K_s flux.

Jupiter is bright enough that its glare effectively washes out a large portion of the scan in which it was covered, as well as a large number of surrounding scans, rendering them useless for extracting background source information. Eighteen scans covering and surrounding Jupiter were therefore not included in the Second Incremental Release, and have been reobserved after Jupiter moved out of the field. Although they are not included in the PSC, a number of the satellites of Jupiter were detected in the 2MASS scans; their images can be seen here, and their measurements are presented by Sykes et al. (2000b Icarus, 143, 371).

Figure 1

[Last Update: 2000 March 6, by R. Cutri and M. Sykes]