Monday, March 11, 2013

Diminish font size Enlarge the font Save this article in PDF impression facebook googleplus twitter The birth of a convective core in a young star modifies its magnetic field

Researchers from the Observatoire de Paris and of the Service d’Astrophysique-Laboratoire AIM du CEA-Irfu have just observed an unexpected change of direction of the fossil magnetic dipole of a young massive star, the Herbig star HD 190073. Massive stars are known to possess fossil magnetic fields stable on tens of years and according to theorists these fossil fields could even be stable over millions of years, i.e. over the life of a star. However in 2011 and 2012, as part of a large program of observation of the magnetism of massive stars, the scientists found a global change in the orientation of the magnetic field observed at the surface of the star HD 190073. The only currently plausible hypothesis is that the failover of the magnetic axis of the star is due to the birth of convective movements in the core of the star. Such an event had never been observed before and is the subject of an article which has just been published in the journal Astronomy & Astrophysics.
Credits : Degroote / KU-Leuven
Evolution of a massive star
After its birth, a young massive star, as the stars of lower mass, is first fully convective. As in boiling water, movements of material on large scales in the star then help to evacuate the energy outward from the star (step 1). Thus, from an initial magnetic flux, the star generates a magnetic field by the motion of electric charges, through a «dynamo» mechanism. Later, it forms a "radiative" stable core where convection is absent, and energy is drained by the radiation of the star only (step 2). This radiative core grows until the outer convective envelope has completely disappeared (step 3). The magnetic field generated in the initial convective regions, is then transformed, into a "fossil" magnetic field, stable and usually dipolar, that is to say with two poles like the Earth’s magnetic field.

Finally, when the star is about to reach the "main sequence" where she spends most of its life, it establishes a stable system in which hydrogen is converted into helium by thermonuclear reactions and a convective core is created again in its centre because of the strongly exothermic nuclear reactions that take place (step 4). The appearance of a dynamo magnetic field generated in this convective central region can then disrupt the initial configuration and result in a tilting of the initial magnetic field.

Steps in the evolution of a massive young star.
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.

The magnetic tilt of HD 190073

The first observations of the star HD 190073 obtained in the Canada-France-Hawaii Telescope (CFHT) in Hawaii (USA), between 2004 and 2009 showed that it possessed a dipolar magnetic field, stable over this period, in agreement with the theory of the fossil field in massive stars. The surprise came in 2011 and 2012, when this star is observed once again under the large programs MiMeS [1] at the CFHT telescope, at the Bernard Lyot (TBL) at the Pic du Midi (France) and at the European Southern Observatory ESO. The variation of the surface magnetic field seems to indicate that the magnetic field axis has moved. Its axis is no longer coincident with the axis of rotation of the star, but tilted, causing a periodic variation of the observed field. According to the researchers, this change can be interpreted by the disruption brought by the appearance of the dynamo field produced by the formation of a convective core in the star.

Indeed, according to recent numeric simulations [2], the dynamo magnetic field generated in the convective core of massive stars and intermediate-mass stars couples with the fossil magnetic field of their radiative envelope. This coupling occurs at the boundary between the core and envelope, and causes a change of orientation of the axis of the fossil field.

For the first time, this transition appears to have been observed in a star, HD 190073.

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.
see the arXiv : /

[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.
[2] " Effects of Fossil Magnetic Fields on Convective Core Dynamos in A-type Stars", Featherstone, N., Browning, M., Brun, A., Toomre J., The Astrophysical Journal, Volume 705, Issue 1, pp. 1000-1018 (2009).