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Date: Fri, 16 Apr 93 11:18:38 EDT
From: aavso@cfa0.harvard.edu (Janet Mattei)
Subject: Comparison star magnitudes
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CONSISTENT COMPARISON STAR MAGNITUDE MEASUREMENTS DESPERATELY NEEDED
for SN 1993J in NGC 3031
Photoelectric (V) and preferably also B, R, I measurements of the comparison
stars shown on the Thompson-Bryan and AAVSO preliminary charts are
desperately needed in order to obtain homogeneous brightness estimates of
Supernova 1993J in NGC 3031. This need was also pointed out by Dr. Gerard
de Vaucouleurs earlier.
Presently there are the following finder charts of the NGC 3031 field:
1. AAVSO PRELIMINARY CHART, indicated with designation 0947+69. Made
in 1979 by T. Fetterman, an active AAVSO observer and supernova searcher.
In the absence of a chart of this galaxy for visual observers, it was
developed to aid observers identify the field stars and to provide approximate
brightness estimates for these stars. The bright magnitudes (brighter than
about 9.0 to 9.5) are from the SAO Catalog, in which magnitudes come from
various sources and so are often not homogeneous. The faint magnitudes are
visual eye estimates by T. Fetterman. These visual eye estimates may have a
zero point problem and/or inconsistancies. Due to uncertainties in the
magnitudes of the comparison stars, this chart was indicated by the AAVSO as
preliminary and thus subject to revision. The circulation of this chart was
very limited.
2. THOMPSON-BRYAN CHART. Published by Cambridge University Press,
well-drafted field of NGC 3031, printed with white stars on black background.
The comparison star magnitudes come from the following sources:
a. Photoelectric sequences
i. One set [underlined] is from A. R. Sandage (Astron. Journ. 89, 621,
1984), and includes mostly stars northeast of NGC 3031. Observers have
reported that these magnitudes are consistent within themselves.
ii. One set [overlined] is from H. G. Corwin, and includes stars in
the field of the galaxy and close to the supernova. Observers have reported
brightness differences for some of the stars in this set.
b. Visual magnitudes
From AAVSO preliminary chart 0947+69 [double-underlined]. Not all
the stars on the preliminary chart are entered on the Thompson-Bryan chart.
c. Source(s) unknown
Also on this Thompson-Bryan chart are magnitudes for which the source
is not indicated. These stars do not have any markings (e.g., underlining or
overlining).
Thompson and Bryan circulated a limited number of an early edition of their
finder chart, with stars indicated as black dots on a white background. The
chart of SN 1993J distributed with AAVSO Alert Notice 169 is this chart.
Regrettably, we noticed after distribution that this early version is
different from the published one, in that some comparison star magnitudes in
the early version are different from the ones in the published chart. Why
these magnitudes were changed and what their source is are not known to me.
3. GUIDE STAR CATALOG CHART. Magnitudes were independently obtained by the
Space Telescope Science Institute. For some stars shown in this field, the
GSC magnitudes are quite different from photoelectric magnitudes mentioned
above.
Thus, in the finder charts mentioned above, there are at least five sources
of comparison star magnitudes. These magnitudes are not always consistent
within and between sources.
In order to revise the brightness estimates reported by visual observers and
those doing differential photometry, and to follow the optical behavior of
this supernova, photoelectric measurements of ALL comparison stars with
magnitudes on the Thompson-Bryan chart need to be made as soon as possible.
Also, it is essential that all observers report what comparison stars they
have used and are using in making magnitude estimates.
Janet A. Mattei
AAVSO Director
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