[vsnet-chat 5167] Re: Curious observations

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

Re: [vsnet-chat 5127] Curious observations

> 3- Observers using different sequences and charts- This is one area where
> vast improvements could be made if all the VS organizations got together and
> agreed on the sequences used for observing stars. Steps are already being
> taken in this direction. Through a cooperative effort we have documented all
> the charts and sequences for program stars common to the AAVSO, BAAVSS and
> RASNZ. In some cases there are widely divergent magnitudes assigned to the
> same comp stars. Work has begun on revising these sequences.

    I have a strong concern about the progress of this, since VSNET was the
one of the earliest organizations (as early as in 1995 using CCD V band
photometry, in 1997 using Hipparcos/Tycho magnitudes) which undertook
the homogenizing efforts of comparison stars using the modern standard
photometric scales.

    However, a look at the AAVSO website, I must regret that even the
"standard" chart of U Gem still uses a different scale, in spite of the
fact there is a footnote they partly empolyed Hipparcos and PEP(V)
magnitudes.

    The largest deviations from the modern scale are seen in bright
comparison stars:

    GSC number    Hipparcos   Ground-based V    AAVSO
                  or Tycho-2
  =======================================================
    1375 0427       8.64          8.650          8.5
    1375 0223       9.28           -             9.3
    1375 1114       9.51      9.250 (NSV 03816)  9.0
    1375 0641       9.27           -             9.1

    The AAVSO magnitudes are systematically brighter by a few tenths
of a magnitude.  Nor they show suspected variable stars (NSV stars).
This discrepancy of magnitudes seem to have (at least to some extent)
bewildered researchers working on visual observations of U Gem
(cf. E. Kuulkers (1999) in "Disk Instabilities in Close Binary Systems",
p.169).  Given the importance of the object, and the wide availablity
of modern photometric sequence (CCD and Hipparcos/Tycho), such a discrepancy
should be removed as soon as possible.  At any time, a new observer,
fully equipped with modern catalogs, may report systematically different
magnitudes on international standard systems.

    I wish to think that we are witnessing an intermediate step toward
a revision (even five years after the release of Hipparcos/Tycho
catalogs...), but I am simply wondering whether the AAVSO has an intention
to revise the charts of such well-known variables using modern photometry.

Regards,
Taichi Kato

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