14 juin - 17 juin 2016


S05 R��union des utilisateurs des t��lescopes fran��ais (TBL/OHP193)

A new family of magnetic stars: the ultra-weakly magnetic Am stars

Aurore Blazère

Magnetic field in hot stars play a important role e. G. for the stellar evolution, rotation or environment. However the magnetic properties in these stars are not well knowns. About 7% of hot stars host a dipolar magnetic field above 100 G. In these stars the magnetic field is of fossil origin. The configuration of the magnetic field is usually a simple dipole and the magnetic field is usually a stable.

Recently, ultra weak magnetic fields (less than 1 Gauss) have been discovered in the normal A stars Vega and in 4 Am stars (Sirius A, beta UMa, theta Leo and Alhena). Theories to explain the dichotomy between “strong” fields and ultra weak magnetic field are based on the stability of the field. However, the shape of the Zeeman signatures observed in three of the Am stars are not expected in the normal Zeeman effect: these signatures show a positive prominent lobe without negative lobe.

I the frame of the BRITE spectropolarimetric survey, we discovered an ultra weak magnetic field in the subgiant Am star: Alhena A(gamma Gem). This is the first detection of a normal Zeeman signature in an Am stars, similar to the one of Vega.

I will presented the magnetic properties of the Am stars and the lastest results that we obtained in this domain.