Dr. Ágnes Kóspál

Konkoly Observatory, Hungary

Magnetic field and accretion in the young eruptive star EX Lupi

Abstract: While the Sun is a quiet and well-balanced star nowadays, during its first few million years it possessed a strong magnetic field and accreted actively. Theoretical models predict that under certain circumstances the interaction of a strongly magnetic star and its circumstellar disk may lead to short busts of increased accretion onto the star. The observable examples of this phenomenon may be a group of young stars called EXors, named after the prototype EX Lupi, which show irregular brightenings due to elevated accretion. EX Lupi had its historically largest outburst in 2008. Our group observed the system during this event, and discovered the crystallisation of amorphous silicate grains in the inner disk by the heat of the outburst. Spectroscopic evidence from the quiescent and outburst periods suggests that the mass accretion proceeds through the same magnetospheric accretion channels in both periods but with different mass flux. However, no information on the magnetic field of EX Lup can be found in the literature. Here, we explore the magnetic field structure of EX Lup using spectropolarimetric monitoring taken during the quiescent period. We detected strong and largely poloidal topology with a prominent cool polar cap and an accretion spot above it. If EX Lup is a good proxy for the proto-Sun, similar magnetic field-disk interactions and the resulting outbursts might have happened during the early evolution of the Solar System as well, significantly affecting the material available for planet formation.