V838 Monocerotis

  The 2002 eruptions of V838 Mon has been one of the most amazing phenomena in stellar astrophysics of recent years. Our continuing HST multi-band imaging and polarimetric monitoring (Bond et al. 2003, Nature 422, 405) of its huge and expanding light-echo (see picture), the first observed in the Galaxy since 70 years, still is a favourite headline topic, from the front cover of Nature to popular magazines and planetarium shows. The light-echo has expanded so much that it is currently larger than the field of view of the HST instruments. Our models of its polarimetric evolution (Sparks et al. 2007, ApJ, in press) has provided a robust distance measurement. It is a direct application of a geometrical distance method that may also have important applications in Local Cosmology. Because of the rapid rise and decay of the central star in outburst, which illuminated the circumstellar nebula, the light-echo image probes different thin parabolic slices of the nebula, offering the unique opportunity of a real 3D tomographic reconstruction of the mass loss history and nebular formation. At the peak of the outburst, the central star of V838 Mon became the brightest object in the whole Local Group. It was also the first observed brown-dwarf-type L "supergiant". The star's atmosphere remained optically thick while it was expanding to a radius well in excess of 1000 Sun radii, and the effective temperature dropped to the level of brown-dwarfs. Spectacular molecular absorptions dominated the spectrum. The L-type supergiant has a normal B3V hot and massive star as companion. It has been just observed undergoing a complete eclipse (Munari et al. 2007, A&A, in press), proving the binary nature, young age and large mass of the V838 Mon progenitor. There are only three other similar objects known: RV, that exploded in M31 in 1988, M85OT2006-1 in eruption in M85 in 2006, and V4332 Sgr that appeared in the Bulge of our Galaxy in 1992. We organized a conference on the V838 Mon phenomenon (Corradi, R., Munari, U. editors, 2007, "The Nature of V838 Mon and Its Light Echo", ASP Conf. Ser 363). In addition to large effort done for imaging, with essentially every ground-based telescope worldwide, the continuous and frequent spectroscopic monitoring of V838 Mon carried out since its 2002 outburst with the Asiago 1.82m telescope is the most complete. Our spectra have allowed many investigations (including photometric and polarimetric ones; e.g. Munari et al. 2002, A&A 389, L51; 2005, A&A 434, 1107; 2007 ASPC 363, 13; Desidera et al. 2004, A&A 414, 591) during the six years of the continuing outburst.

People:  U. Munari, H. Navasardyan, A. Vallenari, S. Desidera, E. Giro

Collaboration:H. Bond (STScI), B. Sparcs (STScI), A. Henden (USNO Flagstaff), R. Corradi (ING La Palma), S. Starrfield (Arizona Univ.), L. Crause (SAAO Cape Town)

Recent Publications: Bond et al. (2003), Nature 422,405; Munari et al. (2005), A&A 434,1107; Corradi, R., Munari, U. editors, (2007), The Nature of V838 Mon and Its Light Echo, ASP Conf. Ser 363