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On the distance, reddening and progenitor of V838 Mon
Extensive optical and infrared photometry as well as low and highresolution spectroscopy are used as inputs in deriving robust estimatesof the reddening, distance and nature of the progenitor of V838 Mon, the2002 outbursting event that produced a most spectacular light-echo. Thereddening affecting V838 Mon is found to obey the R_V=3.1 law andamounts to (i) EB-V=0.86 from the interstellar NaI and KIlines; (ii) EB-V=0.88 from the energy distribution of the B3V component; and (iii) EB-V=0.87 from the progression ofextinction along the line of sight. The adoptedEB-V=0.87±0.01 is also the amount required by fittingthe progenitor with theoretical isochrones of appropriate metallicity.The distance is estimated from (a) the galactic kinematics of the threecomponents of the interstellar lines; (b) the amount of extinction vs.the HI column density and vs. the dust emission through the whole Galaxyin that direction; from (c) spectrophotometric parallax to the B3 Vcompanion; from (d) comparison of the observed color-magnitude diagramof field stars with 3D stellar population models of the Galaxy; from (e)comparison of theoretical isochrones with the components of the binarysystem in quiescence and found to be around 10 kpc. Pre-outburst opticaland IR energy distributions show that the component erupting in 2002 wasbrighter and hotter than the B3 V companion. The best fit is obtainedfor a 50 000 K source, 0.5 mag brighter than the B3 V companion. Thelatter passed unaffected through the outburst, which implies an orbitalseparation wide enough to avoid mass exchange during the evolution ofthe binary system, and to allow a safe comparison with theoreticalisochrones for single stars. Such a comparison suggests that theprogenitor of the outbursting component had an initial mass ~65M_ȯ, that it was approaching the carbon ignition stage in its coreat the time it erupted in 2002 and that the age of the V838 Mon binarysystem is close to 4 million yr. The 2002 event is probably just a shellthermonuclear event in the outer envelope of the star.
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