[vsnet-campaign-v838mon 363] HST observations of V838 Mon light echo

["Howard E. Bond" <bond@stsci.edu>]

The rapidly evolving light echo around V838 Mon was imaged with the Hubble
Space Telescope on 2002 April 30, using Discretionary Time allocated by the
STScI Director.  We used the newly installed Advanced Camera for Surveys with a
Johnson B filter and 3 different polarizers.  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 extremely complex: the variable star is surrounded
by at least 11 separate and approximately circular concentric rings or arcs,
extending out from the star to a radius of about 22 arcsec.  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 this light 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 our polarimetric images will yield constraints on the distance of
V838 Mon and the luminosity of its nova-like event.

An additional HST observation will be made during the week starting 2002 May
20. Continued intensive coverage of the light curve by vsnet observers will be
crucial in interpreting the light echo, and we acknowledge their extremely
important contributions.

Howard E. Bond, Nino Panagia, and William B. Sparks (Space Telescope Science
Institute); Sumner G. Starrfield (Arizona State University; Principal
Investigator); R. Mark Wagner (University of Arizona)

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