[vsnet-chat 5435] Re: Epsilon Geminorum

[Brian Skiff <Brian.Skiff@lowell.edu>]

     As a fairly finicky photometrist, my reaction is that either the
star is constant at the couple-percent level and the differences you
point out (which are quite small) arise mainly from modest observation/
reduction effects, or that the star could be very slightly variable.
     Among the published results for the star collected at Mermilliod's
Web site (http://obswww.unige.ch/gcpd/gcpd.html), here's what I see:

UBV:  _none_ of the UBV data can be relied upon except the 'final' Johnson
      values from the LPL monograph:  V=2.98, B-V=1.40.  These results
      probably include data from the original tube and filters, so in
      effect cannot be argued with, since they more-or-less define the
      system.  However, it is clear that Johnson's V magnitude zero-point
      was not fixed to any better than 0.02-0.03 mag., so there is plenty
      of room for slop by +/- a few percent.

Geneva:  the Geneva V zero-point has systematic error that is a function
         of Right Ascension.  Maybe their value for epsilon Gem is too
         faint because it is in a low spot in this relation.

uvby:  the values to rely on here are from Erik Olsen and collaborators.
       He used the 1966 Johnson V magnitudes as his reference, and because
       of the large number of observations (many thousands for the 
       standard stars) the V zero-point is accurate.  I have tested Olsen's
       V directly against Landolt equatorial standards and conclude that
       his zero-point is within a few millimag of the Landolt system with
       very small (~0.007 mag.) scatter star-by-star.  Thus his values
       of V=3.002 and 3.008 are probably reliable.

WBVR:  the other place I'd look is the very reliable survey of the 13,000
       brightest northern stars by Kornilov et al.  Internal errors are
       commendably small (7-9 millimag), but there are systematic errors
       in both V and B-V (at least) such that for red stars V is too faint
       and B-V too red.  For epsilon Gem we find V=3.003 and B-V=1.435.
       From my experience I'd expect 'true' V to be something like 2.99.

So from the really reliable photometry, V seems to be something like
2.99 to 3.01.  The 2 percent range is still very much within the slop
expected from small passband 'funnies' in the reductions, choice of
standard stars, etc.  From the data at hand you simply can't decide whether
the star is variable---it could be utterly constant and the published
results could show much wider range.  Probably the only way to decide
such a case would be new _differential_ observations with several
comparison stars.  

\Brian

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