Mysterious Betelgeuse, a boiling and magnetic supergiant star
An international research team, lead by French astrophysicists from the Laboratoire d'Astrophysique de Toulouse-Tarbes, has detected a magnetic field at the surface of the supergiant star Betelgeuse. This observational result, published in the journal Astronomy & Astrophysics, demonstrates that, in spite of the theoretical framework usually proposed to account for the magnetism of astrophysical bodies like the Earth or the Sun, the rotation of cosmic objects is not a necessary ingredient to trigger the efficient generation of a magnetic field.
Betelgeuse — the second brightest star in the constellation of Orion (the Hunter) — is a red supergiant, one of the biggest stars known, and almost 1000 times larger than our Sun . It is also one of the most luminous stars known, emitting more light than 100 000 Suns. Such extreme properties foretell the demise of a short-lived stellar king. With an age of only a few million years, Betelgeuse is already nearing the end of its life and is soon doomed to explode as a supernova. When it does, the supernova should be seen easily from Earth, even in broad daylight. According to a scenario elaborated more than half a century ago, the rotation of stars like the Sun produces huge flows of ionized material in their internal layers. These large-scale flows trigger a dynamo mechanism causing the continuous generation of their magnetic field. This process, called a "large-scale dynamo", is generally invoked to describe the solar magnetic cycle, which is particularly spectacular during solar eruptive phases. Yet, even when the Sun is having a temporary respite in its magnetic firework, for instance during the last, unusually long activity minimum that recently ended, our star still hosts a surface magnetic field. The origin of this residual magnetism, which seems to be unaffected by the solar cycle, is still a disputed question among astronomers.
The key of this enigma may be hidden in supergiant stars, a class of objects of which Betelgeuse is one of the most famous members. With about 15 times the solar mass, 1,000 solar radii and a luminosity 100,000 times higher than the Sun's, Betelgeuse is a star reaching the end of its life while burning the last remaining nuclear fuel at its disposal before exploding as a supernova. In addition, another physical parameter of Betelgeuse is differing from the solar case : its rotation is extremely slow. It takes probably several years for Betelgeuse to complete a full rotation, against barely one month for the Sun. This situation seems inadequate to allow for the onset of a large-scale dynamo.
However, observations collected with the NARVAL instrument at Telescope Bernard Lyot (Pic du Midi Observatory, France) reveal a weak polarization level in the light emitted by Betelgeuse : an observational clue unveiling the presence of a weak magnetic field at the surface of the star. This observation is therefore demonstrating that a fast rotation is not a necessary ingredient for the efficient production of a magnetic field. Supergiant stars may use another trick : vigorous convective motions, similar to a continuous boiling, are evacuating the huge amount of energy released in the stellar core by nuclear reactions. Observations obtained at Pic du Midi suggest that this continuous agitation is able, in itself, to generate the stellar magnetic field, through "small-scale" dynamo processes operating on the same scale as the convective cells. Since the Sun itself is exhibiting turbulent motions in its outer layers, it could very well be able to host a similar type of small-scale dynamo, that could be (at least partly) responsible for its residual magnetism during activity minima.
Furthermore, the detection of a magnetic field on Betelgeuse is precious for several reasons. Massive stars reaching the end of their evolution, like Betelgeuse, contribute to spread heavy chemical species in the Galaxy, thanks to a strong wind constituted of ionized particles. Current theoretical models have trouble explaining why the wind ejection is so efficient in supergiants. Here again, the solution is maybe linked to the presence of the magnetic field, due to its known ability to accelerate charged particles.
Boiling and magnetic, supergiant stars therefore seem to constitute perfect cosmic laboratories to test the recent theories developed to explain the generation of magnetic fields in the Universe.
How Supergiant Stars Lose Mass
Red supergiants still hold several unsolved mysteries. One of them is just how these behemoths shed such tremendous quantities of material — about the mass of the Sun — in only 10 000 years. Two teams of astronomers have used ESO’s Very Large Telescope (VLT) and the most advanced technologies to take a closer look at the gigantic star. Their combined work suggests that an answer to the long-open mass-loss question may well be at hand.
The first team used the adaptive optics instrument, NACO, combined with a so-called “lucky imaging” technique, to obtain the sharpest ever image of Betelgeuse, even with Earth’s turbulent, image-distorting atmosphere in the way. With lucky imaging, only the very sharpest exposures are chosen and then combined to form an image much sharper than a single, longer exposure would be.
The resulting NACO images almost reach the theoretical limit of sharpness attainable for an 8-metre telescope. The resolution is as fine as 37 milliarcseconds, which is roughly the size of a tennis ball on the International Space Station (ISS), as seen from the ground.
“Thanks to these outstanding images, we have detected a large plume of gas extending into space from the surface of Betelgeuse,” says Pierre Kervella from the Paris Observatory, who led the team. The plume extends to at least six times the diameter of the star, corresponding to the distance between the Sun and Neptune.
“This is a clear indication that the whole outer shell of the star is not shedding matter evenly in all directions,” adds Kervella. Two mechanisms could explain this asymmetry. One assumes that the mass loss occurs above the polar caps of the giant star, possibly because of its rotation. The other possibility is that such a plume is generated above large-scale gas motions inside the star, known as convection — similar to the circulation of water heated in a pot.
To arrive at a solution, astronomers needed to probe the behemoth in still finer detail. To do this Keiichi Ohnaka from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and his colleagues used interferometry. With the AMBER instrument on ESO’s Very Large Telescope Interferometer, which combines the light from three 1.8-metre Auxiliary Telescopes of the VLT, the astronomers obtained observations as sharp as those of a giant, virtual 48-metre telescope. With such superb resolution, the astronomers were able to detect indirectly details four times finer still than the amazing NACO images had already allowed (in other words, the size of a marble on the ISS, as seen from the ground).
“Our AMBER observations are the sharpest observations of any kind ever made of Betelgeuse. Moreover, we detected how the gas is moving in different areas of Betelgeuse’s surface ― the first time this has been done for a star other than the Sun”, says Ohnaka.
The AMBER observations revealed that the gas in Betelgeuse's atmosphere is moving vigorously up and down, and that these bubbles are as large as the supergiant star itself. Their unrivalled observations have led the astronomers to propose that these large-scale gas motions roiling under Betelgeuse’s red surface are behind the ejection of the massive plume into space.