Pre-main sequence binaries: Formation, evolution, and interaction with disks

Recent observations suggest that the majority of young low mass stars have companion stars. Over the separation range from 1 to 150 AU, this abundance of companions is most pronounced, with the frequency of young companions stars similar to 2-3 times greater than that of their older counterparts on the main sequence. Possible explanations for this difference in companion star frequency are discussed as well as the effects these companion stars may have on circumstellar and circumbinary material. The preponderance of young binary systems makes the study of binary formation and evolution critical to understanding the origins of both stars and planets. This review discusses recent contributions to the observations of young companions and the theories which describe their formation.