The extreme outburst of the young eruptive star EX Lupi in 2008
EX Lupi is the prototype of EXors, a spectacular group of low-mass pre-main sequence stars characterized by repetitive optical outbursts of 1-5 magnitude lasting for a few months-to-few years. The eruptions are attributed to enhanced accretion from the circumstellar disk to the stellar surface, probably triggered by an instability in the disk. The historically brightest outburst of EX Lupi in 2008 offered a unique opportunity to better understand the physics of the EXor eruptions. In my talk I will summarize the main results of our coordinated observing campaign, in which we used a large set of telescopes to observe and characterize the path of accretion from the disk onto the stellar surface, with the aim of comparing the different models of outburst physics. Our measurements demonstrated that the brief episodes of highly increased accretion, apart from contributing to the build-up of the final stellar mass, have substantial effect on the structure and composition of the circumstellar material as well. We discovered episodic crystallization of silicate grains in the disk surface due to the increased luminosity during the 2008 outburst, resulting in material that forms the building blocks of comets and planets.