The 2008 Extreme Outburst of the Young Eruptive Variable Star EX Lupi

In early 2008, the young low-mass star EX Lupi, the prototype of the EXor class of eruptive variables, optically brightened by over 5 mag for a period of seven months. The previous time a change of such amplitude had been observed in EX Lup was over 50 years ago. In this Letter, we present new optical and near-IR high-resolution spectroscopy of EX Lup during the 2008 outburst. We investigate the physical characteristics of the outburst both soon after it began and some four months later, and consider the observed energetics and kinematics. Emission line strengths, widths, and profiles significantly changed between the two observations. Also, modeling of the 2.2935 {mu}m CO overtone band head emission suggests that an inner gap in the circumstellar gas disk around the star may be present and that it is from the inner edge of the gas disk that the CO overtone emission probably arises. We derive a mass accretion luminosity and rate during the extreme outburst of {approx}2 {+-} 0.5 L{sub sun} and {approx}(2 {+-} 0.5) x 10{sup -7} M{sub sun} yr{sup -1}, respectively, which suggests that this outburst was indeed one of the strongest witnessed in EX Lup, yet not as strong as those observed in FU Orionis stars.