On the distance, reddening and progenitor of V838 Mon

. Extensive optical and infrared photometry as well as low and high resolution spectroscopy are used as inputs in deriving robust estimates of the reddening, distance and nature of the progenitor of V838 Mon, the 2002 outbursting event that produced a most spectacular light-echo. The reddening affecting V838 Mon is found to obey the R V =3.1 law and amounts to ( i ) E B − V =0.86 from the interstellar NaI and KI lines, ( ii ) E B − V =0.88 from the energy distribution of the B3 V component and ( iii ) E B − V =0.87 from the progression of extinction along the line of sight. The adopted E B − V =0.87 ± 0.01 is also the amount required by fitting the progenitor with theoretical isochrones of appropriate metallicity. The distance is estimated from ( a ) the galactic kinematics of the three components of the interstellar lines, ( b ) the amount of extinction vs the HI column density and vs the dust emission through the whole Galaxy in that direction, from ( c ) spectrophotometric parallax to the B3 V companion, from ( d ) comparison of the observed color-magnitude diagram of field stars with 3D stellar population models of the Galaxy, from ( e ) comparison of theoretical isochrones with the components of the binary system in quiescence and found to be around 10 kpc. Pre-outburst optical and IR energy distributions show that the component erupting in 2002 was brighter and hotter than the B3 V companion. The best fit is obtained for a 50 000 K source, 0.5 mag brighter than the B3 V companion. The latter passed unaffected through the outburst, which implies an orbital separation wide enough to avoid mass exchange during the evolution of the binary system, and to allow a safe comparison with theoretical isochrones for single stars. Such a comparison suggests that the progenitor of the outbursting component had an initial mass ∼ 65 M ⊙ , that it was approaching the Carbon ignition stage in its core at the time it erupted in 2002 and that the age of the V838 Mon binary system is close to 4 million yr. The 2002 event is probably just a shell thermonuclear event in the outer envelope of the