VI. Analysis of the 2MASS Second Incremental Release Catalogs

7. Comparison to External Catalogs and Models

Stars Showing Relatively Large Discrepencies between the PSC and Truth Photometry

Some of the standards showed discrepencies between their magnitudes in the 2nd Incremental Data Release and their adopted standard values. These included the Persson standard BRI0021 and 4 secondary standards listed below. The secondary standards may be identified by their coordinates -- they are not the primary Persson standards.

Summary

Star RA Dec Problem Reason Concern Status

BRI0021 6.102500000 -1.972260000 J bad Smudge in Image OK: Bad ² Keep as standard

S860D (1) 185.458520000 -0.010540000 K_s bad 2 + calibration? OK: only 2 Keep as standard

S860D (2) 185.468250000 -0.141250000 K_s bad 2 + calibration? OK: only 2 Keep as standard

OphN9b 246.826430000 -24.914810000 J,H,K_s bad Variable OK: Variable Remove as standard

S813D 310.249660000 -5.211750000 J,H,K_s bad Obvious double OK: Bad ² Remove as standard

Star	RA	Dec	Problem	Reason	Concern	Status
BRI0021	6.102500000	-1.972260000	J bad	Smudge in Image	OK: Bad ²	Keep as standard
S860D (1)	185.458520000	-0.010540000	K_s bad	2 + calibration?	OK: only 2	Keep as standard
S860D (2)	185.468250000	-0.141250000	K_s bad	2 + calibration?	OK: only 2	Keep as standard
OphN9b	246.826430000	-24.914810000	J,H,K_s bad	Variable	OK: Variable	Remove as standard
S813D	310.249660000	-5.211750000	J,H,K_s bad	Obvious double	OK: Bad ²	Remove as standard

Comments

BRI0021:
The M9.5 red dwarf BRI0021 has a bad J magnitude. Ironically, the resulting erroneously red color coupled with bookkeeping mistakes led to it being observed at Keck, which resulting in the discovery of a powerful flare in this supposedly magnetically inactive object ("BRI0021-0214: Another Surprise at the Bottom of the Main Sequence," Reid, I.N., Kirkpatrick, J.D., Gizis, J.E., & Liebert, J. 1999, ApJ, 527, L105)

The reason for the erroneous magnitude is that there is a faint J smudge in the data next to the star, as seen in the J and K_s images, shown in Figures 1 and 2. One would expect such an artifact (the cause of which is unknown) to throw off the photometry. Indeed, the observed H and K_s magnitudes are in good agreement (H: 11.081 vs "true" 11.079; K_s: 10.560 vs. 10.538), but the J magnitude is too faint (J: 12.019 vs. 11.862). This is clearly indicated in the database, where the J PSF ² value is 7.0 for J as opposed to 0.7 and 0.6 for H and K_s. Furthermore, the J aperture value is 11.903, which is in reasonable agreement with the "true" value.

We note also that last year we looked at the ~1000 observations of BRI0021 then avaiable and there was no obvious evidence of true variability.

Figure 1 Figure 2

S860D secondaries:
The two S860D secondary standards, which have 0.1 mag discrepencies at K_s, are in the same scan: 990124s scan 083. Both are fairly faint (K_s=13.326 and and 13.288, respectively) and have of 0.049 and 0.044, which means that the problem is at the 2- and 3- levels, respectively. Curiously, in both cases the K_s aperture magnitudes are 1 closer to the "truth" (although the corresponding H aperture magnitudes become bad by 1 ). The image, shown in Figure 3, looks fine for both.

The behavior of nearby standards suggests that K_s may be slightly off at this time, compared to the nightly solution. That may be contributing 1 of the discrepancy.

Thus, we suggest that we have successfully selected a scan with faint sources with a zero-point that is slightly off -- leading to a 2 event, which is not a big surprise that these are encountered.

Figure 3

OphN9b:
This secondary standard in the Ophn9b field is too faint by 0.4 mag at J, 0.2 mag at H, and 0.2 mag at K_s. The aperture and PSF magnitudes agree, and the ² values are fine. Furthermore, the J and K_s images, shown in Figures 4 and 5, look fine.

M. Skrutskie has found that this source is variable, as seen in Figure 6. Thus, it is not suitable as a standard star.

Figure 4 Figure 5 Figure 6

S813D:
This secondary standard in the S813D field is obviously extended in the images, as seen in Figure 7. Note that other stars in the field do not look double. It is therefore no surprise that the photometry is not good, and in fact the aperture and PSF magnitudes wildly differ and the PSF ² values are 47, 23, and 5.8 for J, H, and K_s, respectively! So, the Catalog values are correctly marked.

We do not know why this source is extended. The fact it occurs in all three bands indicates it is NOT just due to a bad pixel. There is no known asteroid in the scan.

Further examination confirmed that this source is a double, and it was removed from the standard stars list.

Figure 7

Return to Section VI.7c.