  We have discovered a luminous light echo around the normal Type II-Plateau
Supernova (SN) 2012aw in Messier 95 (M95; NGC 3351), detected in images
obtained approximately two years after explosion with the Wide Field Channel 3
on-board the Hubble Space Telescope (HST) by the Legacy ExtraGalactic
Ultraviolet Survey (LEGUS). The multi-band observations span from the
near-ultraviolet through the optical (F275W, F336W, F438W, F555W, and F814W).
The apparent brightness of the echo at the time was ~21--22 mag in all of these
bands. The echo appears circular, although less obviously as a ring, with an
inhomogeneous surface brightness, in particular, a prominent enhanced
brightness to the southeast. The SN itself was still detectable, particularly
in the redder bands. We are able to model the light echo as the time-integrated
SN light scattered off of diffuse interstellar dust in the SN environment. We
have assumed that this dust is analogous to that in the Milky Way with R_V=3.1.
The SN light curves that we consider also include models of the unobserved
early burst of light from the SN shock breakout. Our analysis of the echo
suggests that the distance from the SN to the scattering dust elements along
the echo is ~45 pc. The implied visual extinction for the echo-producing dust
is consistent with estimates made previously from the SN itself. Finally, our
estimate of the SN brightness in F814W is fainter than that measured for the
red supergiant star at the precise SN location in pre-SN images, possibly
indicating that the star has vanished and confirming it as the likely SN
progenitor.
