A Young Open Cluster Surrounding V838 Monocerotis

Abstract

During a program of spectroscopic monitoring of V838 Mon, we serendipitously discovered that a neighboring 16th-mag star is 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 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. The cluster's age is less than 25 Myr. 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. It does not appear possible to form a nova-like cataclysmic binary system within the young age of the V838 Mon cluster, and the lack of a young population surrounding M31 RV suggests that the outburst mechanism does not require a massive progenitor. These considerations appear to leave stellar-collision or merger scenarios as one of the remaining viable explanations for the outbursts of V838 Mon and M31 RV.