A Young Stellar Cluster Surrounding the Peculiar Eruptive Variable V838 Monocerotis

Abstract

The unusual variable star V838 Monocerotis underwent an eruption in 2002. It continues to illuminate a spectacular series of light echoes, as the outburst light is scattered from circumstellar dust. V838 Mon has an unresolved B3 V companion star. We serendipitously discovered that a neighboring 16th-mag star is also of type B. We then carried out a survey of other stars in the vicinity, revealing two more B-type stars 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 5.9 kpc obtained from Hubble Space Telescope polarimetry of the light echoes. Using our distance determination, we show that the B3 V companion of V838 Mon is sufficient to account for the entire luminosity of the star measured on 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. Considerations of the pre-outburst luminosity and cluster age appear to leave stellar-collision or -merger scenarios as one of the remaining viable explanations for the outburst of V838 Mon.