THE CONTINUED OPTICAL TO MID-INFRARED EVOLUTION OF V838 MONOCEROTIS*

ABSTRACT The eruptive variable V838 Monocerotis (V838 Mon) gained notoriety in 2002 when it brightened nine magnitudes in a series of three outbursts and then rapidly evolved into an extremely cool supergiant. We present optical, near-infrared (near-IR), and mid-IR spectroscopic and photometric observations of V838 Mon obtained between 2008 and 2012 at the Apache Point Observatory 3.5 m, NASA IRTF 3 m, and Gemini South 8 m telescopes. We contemporaneously analyze the optical and IR spectroscopic properties of V838 Mon to arrive at a revised spectral type L3 supergiant and effective temperature –2200 K. Because there are no existing optical observational data for L supergiants, we speculate that V838 Mon may represent the prototype for L supergiants in this wavelength regime. We find a low level of Hα emission present in the system, consistent with interaction between V838 Mon and its B3V binary; however, we cannot rule out a stellar collision as the genesis event, which could result in the observed Hα activity. Based upon a two-component blackbody fit to all wavelengths of our data, we conclude that, as of 2009, a shell of ejecta surrounded V838 Mon at a radius of AU with a temperature of K. This result is consistent with IR interferometric observations from the same era and predictions from the Lynch et al. model of the expanding system, which provides a simple framework for understanding this complicated system.