The Magnetic Fields on T Tauri Stars

T Tauri stars are low mass, pre-main sequence stars, many of which are still surrounded by active accretion disks where it is believed planet formation is currently under way. Stellar magnetic fields including a strong dipole component on these newly formed stars are believed to play a critical role in the early evolution of the young star plus disk system. It is currently believed that the stellar magnetic field truncates the accretion disk several stellar radii above the star. This action forces accreting material to flow along the field lines and accrete onto the star at high stellar latitudes. It is also thought that the stellar rotation rate becomes locked to the Keplerian velocity at the radius where the disk is truncated. I will review recent efforts to measure the magnetic field properties of T Tauri stars, focussing on how the observations compare with the theoretical expectations. A picture is emerging indicating that quite strong fields do indeed cover the majority of the surface on young stars; however, the dipole component of the field appears to be alarmingly small. I will also briefly discuss recent work on the origins of magnetic fields in fully convective stars such as T Tauri stars.https://commons.und.edu/ss-colloquium/1011/thumbnail.jp