Evolving magnetism in intermediate mass PMS stars

On the main sequence, only 5-10\% of intermediate mass stars ($>1.2 M_\odot$) host strong large-scale surface magnetic fields. These magnetic field topologies tend to be relatively simple dipolar fields that are stable over decades. However, all stars with masses between $1.2-4 M_\odot$ undergo a T Tauri star stage during their pre-main sequence evolution, during which they possess outer convective envelopes and (presumably) dynamo activity. We present the results of our spectropolarimetric study of a sample of 38 intermediate-mass PMS stars. Magnetic fields are detected in the cooler half of our sample, strongly suggesting the magnetic incidence drops very quickly (within 0.1 Myr) as the radiative core exceeds 0.75$R_*$ (or $0.98 M_*$). The properties of the spectropolarimetric signatures also imply an increase in the field complexity as the radiative core develops; this is the first indication that intermediate-mass T Tauri stars show similar behaviour to their low-mass counterparts and has a corresponding implication for their early angular momentum evolution