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.
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 : http://arxiv.org/abs/1301.1804 /
Contact:
- E. Alecian
Observatoire de Paris - LESIA - CNRS - C. Neiner
Observatoire de Paris - LESIA - CNRS
[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).
