The Light Echoes around V838 Monocerotis

The Light Echoes around V838 Monocerotis V838 Monocerotis, which burst upon the astronomical scene in early 2002, is a completely unanticipated new object. It underwent a large-amplitude and very luminous outburst, during which its spectrum remained that of an extremely cool supergiant. A rapidly evolving set of light echoes around V838 Mon was discovered soon after the outburst, and quickly became the most spectacular display of the phenomenon ever seen. These light echoes provide the means to accomplish four unique types of measurements based on continued HST imaging during the event: (1) Study effects of MHD turbulence at high resolution and in 3 dimensions; (2) Construct the first unambiguous and fully 3-D map of a circumstellar dust envelope in the Milky Way; (3) Study dust physics in a unique setting where the spectrum and light curve of the illumination, and the scattering angle, are unambiguously known; and (4) Determine the distance to V838 Mon through direct geometric techniques. Because of the extreme rarity of light echoes, this is almost certainly the only opportunity to achieve such results during the lifetime of HST. We propose two visits during Cycle 15, in order to continue the mapping of the circumstellar dust and to achieve the other goals listed above.

This program contains two visits, 4 orbits each, for imaging of the light echo around V838 Mon with the ACS/WFC. Images will be taken in V (F606W) and I (F814W). We will use a 3-point dither to remove the gap between the chips, with 2 exposures in each filter at each of the 3 points. The I exposures are always 2x409 sec at each dither position, and the V exposure times are then adjusted to fill each orbit. The total exposure times achieve the desired result of a ratio of 2.7 between the total V and total I exposure times, based on the average surface brightnesses seen in previous HST images of the light echo. The two visits during Cycle 15 are placed as far apart as possible, at the beginning and end of the target visibility season. One visit will be in September 2006, and the other in February 2007. ISM AND CIRCUMSTELLAR MATTER 12 false 0.0 false false false false false false SUPPORTED ISM EJECTA PLANETARY NEBULA REFLECTION NEBULA WIND Geometric center of light echo, based on data from Cycle 14. 6/14/06: updated to ICRS reference frame, using galex website. J2000 07 04 4.8000 -03 50 50.00 J2000 ISM EJECTA PLANETARY NEBULA REFLECTION NEBULA WIND Geometric center of light echo, based on data from Cycle 14. 6/14/06: updated to ICRS reference frame, using galex website. J2000 07 04 4.8000 -03 50 50.00 J2000 ACS/WFC Imaging No No F435W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W POS-TARG DITHER 2.994 LINE false true 3 85.269997 ACS-WFC-DITHER-LINE 1

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