MaTYSSE¹⁾ is a Large observing Program carried out with the spectropolarimeters ESPaDOnS@CFHT, NARVAL@TBL and HARPS-Pol@ESO, and complemented with photometric observations. It addresses major unsolved issues regarding the formation of Sun-like stars and their planetary systems, and in particular about the strong impact of magnetic fields on these initial steps so critical for our understanding of the early life of a star like our Sun.

More specifically, MaTYSSE aims at studying the large-scale magnetic topologies of a sample of low-mass protostars that have mostly dissipated their accretion disc already (called weak-line T Tauri stars / wTTSs) to investigate how different they are from those of protostars that are still surrounded by their accretion discs (called classical T Tauri stars / cTTSs), and from those of mature main-sequence stars; being the missing link in our knowledge of magnetic topologies of low-mass stars, wTTSs should reveal the kind of magnetospheres with which Sun-like stars initiate their unleashed spin-up as they contract towards the main-sequence.

Through this survey, MaTYSSE will also be able to assess whether close-in giant planets (called hot Jupiters / hJs) are significantly more frequent around low-mass protostars than around mature stars and whether magnetospheric gaps can explain the survival of hJs around Sun-like stars. MaTYSSE also aims at monitoring a few selected cTTSs to document the long-term variation of their magnetic large-scale topologies and investigate how these variations are likely to affect magnetospheric gaps and the survival of hJs.

By coupling together studies of magnetic fields of protostars and searches for young exoplanets, MaTYSSE will also ensure that scientists from the CFHT community are well prepared for exploiting SPIRou when the instrument comes on-line. MaTYSSE will also contribute to the MagIcS spectropolarimetric LEGACY survey.