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IAUC 5691: 1987A; 1992x

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                                                  Circular No. 5691
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM     EASYLINK 62794505

     A. P. S. Crotts, Columbia University; and W. E. Kunkel,
Carnegie Observatories, write:  "The light echo from the circumstellar
nebula, discovered in 1989 March extending some 2" from SN
1987A (IAUC 4791), has disappeared sometime between 1992 Jan. 15
and Mar. 20 UT (consistent with Crotts 1990, Proc. ESO/EIPC Workshop
on 'Supernova 1987A and Other Supernovae', eds. Danziger and
Kjar, p. 559; and somewhat at variance with several other models).
The echo was seen clearly as a bridge extending approximately from
star 2 to star 3 and passing just south of the supernova on 1991
Jan. 26.  By 1992 Jan. 15, it had faded considerably to a barely
perceptible patch in the vicinity of the supernova, and by 1992
Mar. 20 was no longer visible.  This disappearance is below the
level of 2 x 10E-15 erg sE-1 cmE-2 AE-1 arcsecE-2, as determined in
the continuum at 612.0 nm.  It has not reappeared in observations
at several epochs since then.  This behavior was similar in several
other continuum bands, and corresponds to a maximal extent of the
nebula behind the supernova of about 2.2 lyr.  The nebulosity is
still visible in recombination lines, although dimming.  This
disappearance allows the photometry of SN 1987A to be determined more
accurately in the continuum, where it is very faint, corresponding
to V = 19.9, with r.m.s. errors of about 0.2 mag, on 1992 Nov. 19.
This sets limits on the presence of a stellar source, such as a
companion star, corresponding to about 2.5 solar masses on the main
sequence.  The disappearance of the circumstellar echo allows
precise subtraction of the flux from the remaining stars.  This will
allow an accurate determination of the evolution of the echo, hence
the three-dimensional geometry of the bipolar nebula.  We are
preparing these data for publication."

     Total visual magnitude estimates: 1992 Nov. 24.80 UT, 14.3 (A.
Nakamura, Kuma, Japan, 0.60-m reflector); 17.26, 12.7 (A. Hale, Las
Cruces, NM, 0.41-m reflector); 24.88, 12.2 (F. Van Loo, Mont
Ventoux, France, 0.20-m reflector); 27.07, 12.0 (J. E. Bortle,
Stormville, NY, 0.50-m reflector); 29.84, 12.5 (T. Vanmunster,
Landen, Belgium, 0.35-m reflector); 1993 Jan. 12.12, 10.7 (C.
Spratt, Victoria, BC, 0.20-m reflector).

1993 January 15                (5691)            Daniel W. E. Green

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