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IAUC 7892: V838 Mon

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                                                  Circular No. 7892
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)

     M. Orio, National Institute for Astrophysics, Torino, and
University of Wisconsin; D. Harbeck, Max-Planck-Institut fuer
Astronomie, Heidelberg; and J. Gallagher and C. Woodward,
University of Wisconsin, report that images (0".141/pixel) of the
"light echo" around V838 Mon (IAUC 7859), taken with the WIYN 3.5-m
telescope on Apr. 19 (R band, and H_alpha narrow filter) and 21 (V
band, and [O III] narrow filter), show a filled, approximately
circular shell, not limb brightened, with a diameter of 35" on both
nights.  They add:  "The contour of the shell is irregular, with a
broad structure in the shape of an arc and clumps but no small-
scale structure.  There is more structure in V than in R.  Lack of
conspicuous H_alpha and [O III] line emission confirms that it is a
reflection, rather than an ionization, nebula.  An R image obtained
on Apr. 30 at the WIYN 0.90-m telescope (scale 0".60/pixel) still
shows a 35" diameter, so the outer edge of the reflecting material
was reached before Apr. 22."
     H. E. Bond, N. Panagia, and W. B. Sparks, Space Telescope
Science Institute; S. G. Starrfield, Arizona State University; and
R. M. Wagner, University of Arizona, report that the rapidly
evolving light echo around V838 Mon was imaged with the Hubble
Space Telescope on Apr. 30.  The newly installed Advanced Camera
for Surveys was used in its polarimetric mode with a B filter.  The
echo shows significant linear polarization, as expected for
scattering off interstellar grains, but numerical results await
forthcoming calibration observations for the ACS.  The structure of
the echo is complex:  V838 Mon is surrounded by at least 11
separate, approximately circular concentric rings or arcs, out to a
radius of 22".  Most of these arcs correspond to the intersections
of the light-echo paraboloid with sheets of interstellar material
in the line-of-sight (similar to the transient echo rings seen
around SN 1987A), but it is possible that some of the features at
the smallest radii arise in circumstellar material from
hypothetical previous outbursts of V838 Mon.  The echo also
contains several highly structured, non-concentric filamentary
features.  The prominence of the echo strongly suggests that V838
Mon was intrinsically extremely luminous during its prolonged
outburst, and is thus unrelated to the fainter FG Sge-like 'born-
again' red giants.  It may instead be related to the very luminous
extragalactic red variable 'M31 RV' that had a long outburst in
1988.  Analysis of the polarimetric images will yield geometric
constraints on the distance of V838 Mon.  An additional HST
observation will be made during the week starting May 20.
Continued intensive ground-based monitoring of the light curve and
the light echo is urged.

                      (C) Copyright 2002 CBAT
2002 May 6                     (7892)            Daniel W. E. Green

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