HD190073, a star with a magnetic personality!

In the diagram temperature-luminosity (Hertzprung-Russell diagram), for each step, the structure of the star and the type of the associated magnetic field are schematized. The star HD 190073 is located at the transition between steps 3 and 4.

Researchers from the Observatoire de Paris and of the Service d’Astrophysique-Laboratoire AIM du CEA-Irfu have observed an unexpected change of direction of the fossil magnetic dipole of a young massive star, the Herbig star HD 190073.

Magnetic fields are in all aspects of our lives. They are used in objects as common as a car, a television set or a particle accelerator. Planets and stars have magentic fields too. The aurora borealis, a phenomena often observed at high latitudes, is directly caused by the coupling between the Earth and the Sun's magnetic field.

Magnetic fields are routinely measured in stars other than the Sun thanks to spectropolarimeters like NARVAL at the Bernard Lyot Telescope at the Pic du Midi in France and ESPaDOnS at the CFHT. In massive stars, the magnetic field is thought to be created by a different mechanism than the mechanism that creates the magnetic field around the Sun or the Earth. The field of these stars is thought to be a Fossil field. This means that the field of the star is either of Galactic origin or that it was formed by a Dynamo generated at the birth of the star.

A fossil field has the characteristic of being very stable and can last for hundreds of thousands and probably millions of years. The stability of the magnetic field of HD190073 was well established with observations coming from the MiMeS[1] large program at CFHT. These particular measurements were a very strong case for the Fossil field theory. A team of researchers from France, Sweden, the UK and from Canada in collaboration with the MiMeS large program at CFHT measured a strong variation in the magnetic field of HD190073 using NARVAL at the Pic du Midi and HARPSpol at the 3.6m-ESO Telescope in Chile. Such a dramatic change had never been measured before in any star and is thought to be caused by the birth of a convection zone, a zone where the material of the star literally boils.

To test whether the new magnetic configuration has really stabilized and to determine the precise parameters of this new configuration, additional observations will be required. HD 190073 represents a unique opportunity to witness the appearance of a convective core in a star.

For more Information, please see the official press release.

Reference :

"The dramatic change of the fossil magnetic field of HD 190073: evidence of the birth of the convective core in a Herbig star ?"
E. Alecian, C. Neiner, S. Mathis, C. Catala, O. Kochukhov, J. Landstreet, the MiMeS collaboration, Astronomy & Astrophysics, vol. 549, L8.
Get the paper here from the arXiv.
E. Alecian
Observatoire de Paris - LESIA - CNRS

C. Neiner
Observatoire de Paris - LESIA - CNRS

S. Mathis

[1] MiMeS (for Magnetism in Massive Stars) is an international project led by France and Canada to study the magnetism of massive stars, in particular with 3 large observational programs and theoretical developments.