Hungry Young Stars: A New Explanation for the FU Ori Outbursts Shantanu Basu, Eduard I. Vorobyov Department of Physics and Astronomy The University of Western Ontario 207 th AAS Meeting, January 11, 2006

Computer Simulations of Star Formation Our calculations reveal the explanation for colossal bursts of luminosity in young stars. This explains the remarkable transient brightening (by factors of hundreds) of a class of young stars known as the FUOr’s. We solve the equations of gas dynamics including the effect of a surrounding nebular cloud that feeds a gas disk and protostar (central yellow circle in image). The gas disk forms spiral arms and protoplanetary embryos (see arrows). Interactions with the spiral arms in the disk drives the embryos into the central star, resulting in colossal bursts of luminosity. This process repeats as long as nebular material rains onto the disk. protoplanetary embryo Black region contains low-density nebular material

The FU Ori phenomenon The Orion Constellation Betelgeuse (  Ori) FU Ori – 3 0 NW of Betelgeuse, the prototype for a class of stars Rigel (  Ori) Orion Nebula (M42) Negative image – C. Briceno FU Ori brightened by a factor of ~ 250 in the course of one year, and is currently declining in brightness with a decay timescale of years. The surrounding material reveals that this is an infant star. It is believed that all young stars undergo multiple eruptions.

Disk Evolution and Stellar Mass Accumulation E. I. Vorobyov & S. Basu Image of gas density in a disk surrounding a new protostar. Rate at which mass falls onto the protostar.

Spiral Arms  Embryos  Luminosity Bursts At the peak of the outburst, the star is at least several hundred times more luminous than our Sun. The star devours the equivalent of one Earth mass every ten days during the century-long outburst. Luminous bursts due to episodes of embryo consumption E. I. Vorobyov & S. Basu

Summary We provide an explanation for the origin of FU Ori bursts. A young star devours embryos that form in the disk, resulting in colossal bursts of luminosity. This process repeats as long as nebular material rains onto the disk. The new feature in our model is the self-consistent formation and evolution of a gas disk due to matter falling in from the surrounding nebula. The last generation of embryos (formed when nebular gas is gone) may escape consumption and form giant planets, brown dwarfs, or companion stars, depending on their mass. Future calculations will reveal the answer. This research has been performed by Dr. Eduard I. Vorobyov and Prof. Shantanu Basu of The University of Western Ontario, and funded by the Natural Sciences and Engineering Research Council of Canada, the North Atlantic Treaty Organization, and the Canadian Institute for Theoretical Astrophysics. All press conference materials can be downloaded from Some CD’s are available upon request.