V838 Monocerotis is an unusual variable star that underwent a sudden outburst in 2002. Unlike a classical nova, which quickly evolves to high temperatures, V838 Mon remained an extremely cool, luminous supergiant throughout its eruption. It continues to illuminate a spectacular series of light echoes, as the outburst light is scattered from nearby circumstellar dust. V838 Mon has an unresolved B3 V companion star. During a program of spectroscopic monitoring of V838 Mon, we serendipitously discovered that a neighboring 16th magnitude star is also of type B. We then carried out a spectroscopic survey of other stars in the vicinity, revealing two more B-type stars, all within 45″ of V838 Mon. We have determined the distance to this sparse, young cluster based on spectral classification and photometric main-sequence fitting of the three B stars. The cluster distance is found to be 6.2 ±1.2 kpc, in excellent agreement with the geometric distance to V838 Mon of 6.1 kpc obtained from Hubble Space Telescope polarimetry of the light echoes. An upper limit to the age of the cluster is about 25 Myr, and its reddening is E(B - V) = 0.85. The absolute luminosity of V838 Mon during its outburst, based on our distance measurement, was very similar to that of M31 RV, an object in the bulge of M31 that was also a cool supergiant throughout its eruption in 1988. However, there is no young population at the site of M31 RV. Using our distance determination, we show that the B3 V companion of V838 Mon is sufficient to account for the entire luminosity of the variable star measured on sky-survey photographs before its outburst. The B3 star is currently, however, about 1 mag fainter than before the eruption, suggesting that it is now suffering extinction due to dust ejected from V838 Mon. These results indicate that, whatever the nature of the progenitor object, it was not of high luminosity. Nor does it appear possible to form a nova-like cataclysmic binary system within the young age of the V838 Mon cluster. These considerations appear to leave stellar collision or stellar merger scenarios as one of the remaining viable explanations for the outbursts of V838 Mon and M31 RV.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science