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IAUC 7859: V838 Mon; C/2000 WM_1

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                                                  Circular No. 7859
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7440/7244/7444 (for emergency use only)

     A. Henden, Universities Space Research Association and U.S.
Naval Observatory (USNO); U. Munari, Padova; and M. Schwartz,
Tenagra Observatory, report that an apparent light echo has been
discovered developing around V838 Mon on images secured with the
USNO Flagstaff Station 1.0-m telescope (scale 0".68/pixel) and
confirmed on images obtained with the 0.81-m Tenagra telescope
(0".87/pixel):  "The light echo is easily visible in the U bandpass
and progressively less visible at the longer B-, V-, R-, and I-band
wavelengths due to the combined effect of a decreasing surface
brightness and increasing output from the central star.  The light
echo seems to originate from a circumstellar ring structure,
detached from the central source.  The light echo was not visible
on Feb. 12 images and first appeared on Feb. 17 images.  Since
then, it has linearly increased in angular diameter at a rate of
0".54/day, reaching 27" by Mar. 23.  Assuming the echo is the
result of scattering by a spherical distribution of circumstellar
dust of the outburst that started around Jan. 1, the expansion rate
suggests a distance to V838 Mon of 700 pc, and first appearance
around Feb. 14 suggests a distance of 7800 AU between the inner
radius of the circumstellar scattering dust and the central star.
If we assume a typical wind velocity of 15 km/s for the material
lost by the asymptotic-giant-branch progenitor, the mass-loss phase
that created the circumstellar material stopped 2500 years ago.
The central star in V838 Mon appears to be engulfed in a few-arcsec-
wide circumstellar cocoon, the possible sign of a recently resumed
phase of mass loss.  The 45-day delay between onset of the outburst
and first detection of the light echo suggests that the sudden
brightening of V838 Mon reported on IAUC 7816 should be reaching
the circumstellar material shortly and should result in an
enhancement of the light echo.  V838 Mon seems to be a close
analogue of FG Sge, V605 Aql, and V4334 Sgr."

     Visual m_1 estimates:  Feb. 6.30 UT, 3.8 (J. G. de S. Aguiar,
Campinas, Brasil, naked eye); 12.74, 4.4 (D. A. J. Seargent, Cowra,
N.S.W., naked eye); 20.52, 5.9 (A. Hale, Cloudcroft, NM, 11x80
binoculars); Mar. 6.83, 7.2 (Y. Nagai, Yamanashi, Japan, 11x80
binoculars); 11.18, 7.6 (W. Hasubick, Buchloe, Germany, 25x100
binoculars); 23.84, 8.9 (A. Pearce, Noble Falls, W. Australia,
20x80 binoculars).

                      (C) Copyright 2002 CBAT
2002 March 25                  (7859)            Daniel W. E. Green

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