[vsnet-alert 11030] re CSS090201:090210-113032 : problematic 2MASS colours

[varposts at Safe-mail.net]

Kato Taichi elucidates :-

"2MASS 090210.248 -113031.68 (2000.0) 15.550 14.932 14.713 

   The secondary star appears to contribute the 2MASS colors, further supporting the classification as a long-Porb SS Cyg-type object.  This region of sky is void of GCVS variables, probably somehow neglected in past variable star surveys."

I knew I'd forgotten something, I forgot to comment on why I hadn't commented upon the 2MASS colours of this object, as in tandem with optical and UV bluishness, the apparent 2MASS redness does indeed favour UGSSness.

The papers and online documentation make clear that the safe working lower threshold for 2MASS is magnitude 15.5 for J and H.  Whilst for Ks it is one whole magnitude brighter, ie there the threshold is 14.5.  This can be quite irrespective of quoted errors, precision can be high, accuracy low.

The errors for the above objects (just looked up in the IRSA IPAC server via GATOR) are 0.059 for J and 0.125 for Ks.  The signal to noise ratio for J is 23.3, for Ks 9.6.

So on top of the greater error for Ks there's also less surety of inherent accuracy.

Having said that, these "Malmquit Effect"-esque type errors at photometric instrumentation faint limits tend to have a bias towards making objects seem brighter than true, which'd of course make this setup potentially redder in J-Ks than the current apparent case, _if_ J has some usable level of accuracy.

But there's no way of knowing (ie needs more measures at quiescence with 'scopes of fainter limiting magnitude in the relevant passbands), and I had forgot to comment on why I hadn't commented on the 2MASS colours.

Most CRTS CV candidates are going to have this trouble in 2MASS, after all this is one is relatively bright in all passbands at quiescence compared to most of 'em.

Cheers

John