[vsnet-chat 7099] Re: [baavss-alert] Tycho variables as comparison stars?

[arne]

Martin Nicholson wrote:

>I am sure there is a simple answer to this - at least I hope there is.
>
>Photometric data for Tycho listed stars has been available on the Internet for
>some time now. For example VTmag at maximum (15th percentile)and VTmag at minimum
>(85th percentile)can be obtained quickly and easily.
>
>Many Tycho stars are used on the AAVSO charts as comparison stars. For example on
>the V LYR chart there is a star labelled 100 to the south of this Mira. HOWEVER
>the catalog data gives VTmag at maximum of 9.79 and 11.21 at minimum so this
>comparison star is itself variable.
>
>Just recently the Tycho Epoch Photometry data can also be accessed (from the
>morePhoto column) and this confirms the variability of many Tycho stars.
>
>Does this mean that every AAVSO (and BAA) chart needs to be checked to confirm
>that no Tycho variable is being used as a comparison star? 
>
>
>  
>
Martin,
  Probably baavss-alert is not a good maillist for such questions,
as this is a generic problem and not a time-critical comment.
However, I am responding here as well as your other posting lists
for completeness' sake.
  As Sebastian indicates, not all Tycho "variables" are variable.
At 11th magnitude, a star is barely detected in the Tycho instrument,
with very large errors.  The mean magnitudes are good because they
are the average of hundreds of poor measures.  15th and 85th quartiles
have very little meaning for these stars.
  The AAVSO comp-star database team, headed by Vance Petriew, has gone
through and found every comparison/sequence star used on every AAVSO chart.
These are being cross-indexed with available catalogs, and variability
is one attribute that is being very carefully studied.
  If you think that a particular comp star is variable, I'd highly
recommend first contacting Vance and also looking for other databases
that might contain time-series observations of the suspected variable.
In this case, the two obvious surveys you should check to see if the
star really is variable is the Northern Sky Variability Survey (NSVS) at
  http://skydot.lanl.gov
and the TASS survey (of which you are a member!) at
  http://www.tass-survey.org
Arne
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Arne A. Henden, Director                      ph:    (617) 354-0484
Amer. Assc. Var. Star Obs. (AAVSO)            fax:   (617) 354-0665
25 Birch Street                               email: arne at aavso.org
Cambridge, MA 02138 USA                       www: http://www.aavso.org
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