Monocerotis: A Geometric Distance from Hubble Space Telescope Polarimetric Imaging of its Light Echo

Following the outburst of the unusual variable star V838 Monocerotis in 2002, a spectacular light echo appeared. A light echo provides the possibility of direct geometric distance determination, because it should contain a ring of highly linearly polarized light at a linear radius of ct, where t is the time since the outburst. We present imaging polarimetry of the V838 Mon light echo, obtained in 2002 and 2005 with the Advanced Camera for Surveys onboard the Hubble Space Telescope, which confirms the presence of the highly polarized ring. Based on detailed modeling that takes into account the outburst light curve, the paraboloidal echo geometry, and the physics of dust scattering and polarization, we find a distance Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; [email protected], [email protected], [email protected], [email protected], [email protected] South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa RSAA, Australian National University, Cotter Road, Weston Creek ACT 2611, Australia AAVSO, 49 Bay State Road, Cambridge, MA 02138 USA; [email protected] INAF Osservatorio Astronomico di Padova, via dell’Osservatorio 8, 36012 Asiago, Italy; [email protected] INAF/Osservatorio Astrofisico di Catania, Via S. Sofia 78, I-95123 Catania, Italy, and Supernova Ltd., OYV #131, Northsound Road, Virgin Gorda, British Virgin Islands; [email protected] School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287; [email protected] Dept. of Physics & Astronomy, Goucher College, 1021 Dulaney Valley Rd., Baltimore, MD 21204; [email protected] Large Binocular Telescope Observatory, 933 North Cherry Avenue, Tucson, AZ 85721; [email protected]