[vsnet-alert 11033] Re: re CSS090201:090210-113032 : problematic 2MASS colours

[Brian Skiff]

     Another option is look at the DENIS database, which is another
useful catalogue available through VizieR.  For this object, DENIS
shows:

Ic = 16.50 +/- 0.07

J = 15.75 +/- 0.19  (consistent with 2MASS within their mutual errors)

...but no detection at K, all on JD 2450516.609 (about two years
prior to the 2MASS observation date).  If in fact the 2MASS
Ks = 14.7, then I'm a little surprised it doesn't show up at that
wavelength in DENIS.  So one could suspect that the object is
quite a bit fainter than the 2MASS value.  I note that amongst
the 2MASS data-quality flags is 'Ndet' (number of 3-sigma detections 
and number of frames), which for this star is _zero_ in K despite
6 frames on which it should appear.  Thus it seems the magnitude
and errors are suspicious since none of the detections is especially
reliable.  A Malmquist-type of bias thus does apply here, since
there are six scans across the field (not just one), and the bias 
works in the sense of picking up only on the statistically-spurious 
high values.

\Brian