Re: Tycho-2 catalog

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Tom wrote:

> OK, here is the first one on my list:
> 
> RA 81.853
> Dec. -0.486
> refcat I mag 10.03
> tass I mag  7.99
> refcat V mag 10.957

  I looked at this star with SIMBAD/Aladin.  It does not appear
to be a double, or close to another star of similar brightness.

  However, it does appear to be pretty bright in the infrared.
It is very saturated in 2MASS images of the field (so saturated
that there is no measured magnitude for it).  The star is also
an IRAS source.

  My guess is that this is an example of a annoyingly common
phenomenon: a star with an infrared excess, which screws up
the I-band magnitude we extrapolate -- but one which appears
perfectly normal in the Tycho2 B and V bands.  This star's
(Bt-Vt) color is about +0.9 -- nothing special.

  We can _try_ to find such IR-bright stars by cross-matching
the Tycho2 catalog against 2MASS or IRAS, but I suspect that some
(probably many) will slip through.  All we can do is find them
the hard way and toss them from the photometric calibration
process :-(  At least, that's how it looks to me.

                               Michael