Accretion discs are one of the basic building blocks of modern astrophysics: for instance, scientists think that stars are
formed from a rotating gas cloud that collapses into a disc; in a second step, the disc progressively dissipates to become
a newly born star with its surrounding planetary system. This scenario is inspired from that proposed 200 years ago by the
french mathematician Pierre Simon Laplace to explain the formation of the solar system. Understanding in detail the physics
of accretion discs is thus crucial for disclosing the secrets of how the Sun and the planets of the solar system are born.
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Artist view of a protostellar accretion disc. (©David Darling) |
What was not predicted by Laplace however are the very thin plasma beams (collimated jets) that apparently escape from accretion disc cores, in a direction perpendicular to the plane of the disc. These jets are observed around forming stars and active galaxies, and can reach incredible lengths, of up to several light-years in the particular case of forming stars. Scientists think that these jets are how the disc succeed at dissipating most of its mass and angular momentum before starting to form the protoplanetary clumps that will eventually produce planets. To produce such collimated jets, theoretical models all require the presence of magnetic fields; however, no observational constraint was yet available on the magnetic field in the innermost disc regions, from which jets presumably originate. |
Example of an accretion disc and jet in a protostar (the one showed here is HH30 and not FU Ori) as observed by the Hubble Space Telescope: the jet (in red) is perpendicular to the accretion disc, seen edge-on (and appearing on the bottom of the image, as a dark region between two bright lobes, ©Burrows, STSci/ESA, WFPC2, NASA) |
In some models (called magnetocentrifugal models, and initially proposed in 1976), the rotation of the accretion disc twists the initial magnetic field, assumed to be the large-scale primordial (interstellar) magnetic field, oriented perpendicularly to the disc. The field responds by slowing down the disc plasma and causing it to fall towards the disc central regions. The energy flux produced in this process points away from the disc surface, pushing the surface plasma outwards, leading to a wind from the disc and sometimes a collimated jet. Other models (eg dynamo models) suggest that the field is produced within the disc itself, through processes similar to those generating the magnetic field of the Sun. |
The rotation of the disc (the flat structure in the centre) twists the initially vertical magnetic field (shown here as yellow ropes), leading to the ejection of plasma (shown here as a blue cylinder) perpendicularly to the disc surface, and to the formation of a collimated jet. This result was obtained through a numerical simulation (©Casse & Keppens 2004). |
By detecting the magnetic signatures (through the Zeeman effect) on thousands of spectral absorption lines formed in the inner disc regions (within less than 0.2 astronomical units from disc centre), a team of scientists[1] from the Laboratoire d'Astrophysique de Toulouse-Tarbes (LATT: UMR CNRS, Université Paul Sabatier, Observatoire Midi-Pyrénées) and the Laboratoire d'Astrophysique de Grenoble (LAOG: UMR CNRS, Université Joseph Fourier, Observatoire des Sciences de l'Univers de Grenoble) demonstrated that a strong magnetic field is present, whose intensity is comparable to that emerging from the spots at the surface of the Sun. Moreover, they could establish that the field hosts both a vertical component (perpendicular to the disc) and an azimuthal component (within the disc plane and perpendicular to the disc radius), in agreement with magnetocentrifugal models (and in contradiction with dynamo models). Finally, they find that the field slows down the disc much more than models predict, which may explain why some accretion discs fail at forming collimated jets. |
The polarimetric signal from FU Orionis (V/Ic, upper curve, expanded by 100) detected in absorption lines (I/Ic, lower curve) is 2000 times weaker than the radiation energy (Ic) emitted by the disc (©Donati). |
This discovery could be achieved thanks to ESPaDOnS[2], the new spectropolarimeter built at Observatoire Midi-Pyrénées (by Groupe d'Instrumentation Grands Télescopes of LATT) and recently installed at Canada-France-Hawaii Telescope (CFHT[3]). The technique of spectropolarimetry consists in measuring the polarisation in the light emitted by an astrophysical object, and in particular the variation of the polarisation through the spectral lines of this object. |
The Canada-France-Hawaii Telescope is located atop the Mauna Kea volcano, in the big island of Hawaii (©Cuillandre, CFHT). |
This technique, frequently used in solar physics (and in particular for studying the magnetic field of the Sun) is relatively new and thus very promising in other domains of astrophysics. ESPaDOnS is the most powerful instrument worldwide for this kind of studies, and the only one able to detect the very weak polarisation signals from accretion discs. |
ESPaDOnS consists of two modules, a polarimeter mounted at the telescope focus (left), fibre-feeding a bench-mounted high-resolution spectrograph (right, ©CFHT). |