First detection of a magnetic field on the surface of a Mira-type star

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An international team of astronomers, including researchers at the IRAP (CNRS / Université Paul Sabatier-Toulouse III) has detected for the first time a magnetic field at the surface of a Mira-type star (1), the star χ Cygni (in Cygnus). These results, obtained with observations carried out at the Pic du Midi, demonstrate for the first time the existence of a magnetic field on the surface of these pulsating giant stars and shed new light on the important weight loss experienced by these end of life stars. This work is published on January 7, 2014 in the journal Astronomy & Astrophysics.

Schematic representation of a Mira star surrounded by its atmosphere and its extensive circumstellar envelope. The solar disk and the orbits of the planets Mars and Jupiter in our solar system are plotted on this representation for comparison (dashed lines). Earlier detections of a magnetic field were made, through the study of molecular emission (Maser SiO) in the circumstellar envelope of Miras (ie compared with our solar system, at the orbit of Jupiter). The observations made here by spectropolarimetry now impose constraints on the detectable magnetic field at the surface of a Mira star. Credits: C. Mercier, LUPM (design and construction); B. Freytag (center image)

Miras stars are giant, cool and evolved stars. They furthermore are variable stars. Their periodic brightness variation over a year is linked to a pulsating phenomenon that occurs in the internal areas of the Mira star. In the case of these cool stars, stellar pulsations will generate shock waves that will sweep the stellar environment (atmosphere and circumstellar envelope). They will also cause large movements of material, the atmospheric gas being periodically driven outwardly in the wake of these waves, then falling onto the surface of the star.

The team of researchers has observed, in spring 2012, a Mira star in the Cygnus constellation, the star χ Cyg, while it was at its maximum brightness. These observations were made from the Pic du Midi, using the Bernard Lyot Telescope equipped with the Narval spectropolarimeter. This instrument provides both the spectrum of the star across the visible domain and the state of polarization of this starlight which can then reveal the presence of a magnetic field at the surface of the star.

The analysis of a set of 175 spectra on the star χ Cyg, one of the brightest Miras stars observable from the northern hemisphere, has lead to note the signature of a very weak stellar magnetic field on the surface. This discovery was made possible thanks to the presence, at the time of the observations, of one of these shock waves generated by stellar pulsation.

The origin of this surface magnetism of a Mira star is also an enigma. It could be produced and maintained by turbulent convective motions on the surface of the cold giant stars, and moreover undergo a compressive amplification due to the shock wave, as the interpretation of observations of χ Cyg suggests.

For these Miras stars, the existence of a magnetic field was only known in the external areas of their environment, through observations of molecular emission of SiO masers (2) made with radiotelescopes better able to probe the coldest stellar regions. The first detection of a magnetic field on the surface of a Mira star, complete the magnetic overview of these stars from the surface (1 stellar radius) to the ends of the circumstellar envelope (several thousand stellar radii). It also reopens the debate on the role of a magnetic field, even of low intensity, could play in the process of mass loss suffered by the star at the end of its life, as well as its contribution to shaping the environments of planetary nebulae (with various, highly asymmetric morphologies) which result from the final evolution of a Mira star.

Notes :

  1. Miras stars are cold and evolved stars. Cold because their surface temperature is very low (about 3000-3500 K, which is twice lower than that of the Sun), and evolved because they are at a stage of life that foreshadows what will be that of the Sun in more than 5 billion years, when it will have consumed all its fuel hydrogen that it is currently burning in his innermost zones. Then the Sun will evolve as a Red Giant, its radius greatly increasing and its surface temperature gradually lowering.
  2. SiO Masers: The Maser (Microwave Amplification by Stimulated for Emission of Radiation) is a (experimental or natural) device emitting a coherent beam of microwaves (so it is the equivalent of a Laser in the radio portion of the electromagnetic spectrum). Different molecular species (eg, SiO, H2O, OH) may have masers emissions and, in astrophysics, masers occur naturally in stellar and planetary environments, in the interstellar medium, etc..

Source :

  • A. Lèbre et al. “Search for surface magnetic fields in Mira stars. First detection in chi Cygni”, Astronomy and Astrophysics, 01/2014. Link towards the article

IRAP Contact :

  • Michel Aurière, IRAP, michel.auriere@irap.omp.eu |T 05 62 56 60 19

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