Hubble Space Telescope Observations of the Light Echo around V838 Monocerotis

The outburst of V838 Monocerotis in early 2002, and the subsequent appearance of its light echoes, occurred just before the installation of the Advanced Camera for Surveys into the Hubble Space Telescope (HST). This fortunate sequence of events has allowed us to obtain spectacular HST images of the echoes, yielding not only pictures of extraordinary beauty, but also providing unique scientific information. Our team has used the HST images to provide a direct geometrical distance to V838 Mon, based on polarimetric imaging, and limits on the distance based on the apparent angular expansion rates. Several morphological features seen in the HST images strongly suggest that the illuminated dust was ejected from the star in a previous outburst, similar to the current one. In particular, a "double-helix" feature points exactly back to the star. Moreover, three-dimensional mapping of the outer edges of the dust suggests an overall ellipsoidal shape, centered on V838 Mon itself. And the appearance of the light echo in the most recent Hubble images is remarkably similar to that of a well-known planetary nebula, M27. Future work on the HST images will include an analysis of interstellar-dust physics, in a situation where the scattering angle and illumination are unambiguously known, and visualization of a fully three-dimensional map of the dust distribution