ESPaDOnS
first scientific results


Since first offered to the CFHT community (in Feb 2005), ESPaDOnS obtained a significant number of scientific results, some of them representing a major advance in the field of stellar magnetism. After less than one year of official operation, six papers are already in press or submitted to refereed journals. Some of them are advertised below. An ADS list of selected published papers about ESPaDOnS and ESPaDOnS results is available here.

Artist view of a Sun-like baby star weaving magnetic links to its accretion disc © Chandra
Nature's recipe for building new worlds: a zest of magnetic fields?

How do stars like the Sun form? How are planetary systems born? To answer these questions, astrophysicists need to find out Nature's recipe to turn vast cosmic clouds of gas into accretion discs and into stars and planets. One of the crucial ingredients in this recipe is likely magnetic fields.
The authors (JF Donati et al) just succeeded at mapping the large arches and funnels that magnetic fields weave between baby stars and their accretion discs. These observations should yield more accurate models of how new-born stars interact with their accretion disc to form planetary systems such as our own.
These results are published in the Monthly Notices of the Royal Astronomical Society. Learn more about this discovery (full paper available here).

A giant planet embedded in the magnetosphere of its star

Artist view of the giant exoplanet orbitng tau Bootis, through the star's magnetic archs (credit David Aguilar, CfA). This image was produced using an image of the Sun obtained by the TRACE solar spacecraft (© TRACE operation team, Lockheed Martin).
The catalogue of extrasolar planets is growing continuously, containing today more than 200 objects, and the detection of these exoplanets has almost become a routine. But what are the characteristics of the stellar hosts, how can we explain the formation of these planetary systems, or why are some of these giant exoplanets, which are called 'hot jupiters', migrating down to very close-in orbits? Astrophysicists suspect the magnetic field to play a crucial role in some of these questions. However, although indirect effects of magnetic fields have already been detected on stars hosting giant extrasolar planets, no direct measurement had ever been done... until now!
The authors (C Catala et al) detected the magnetic field of tau Bootis, a one billion year old star, having a mass of one and a half solar masses and located at nearly 50 light years from the Earth. This cool and weakly active star, orbited by a giant planet with 4.4 Jupiter masses on a very close-in orbit at 0.049 AU (i.e. 5% of the Sun-Earth distance), possesses a magnetic field of a few gauss, just a little more than the Sun's, but showing a more complex structure.
These results will be published in the Monthly Notices of the Royal Astronomical Society. Learn more about this discovery (full paper available here).

The surprisingly complex magnetic field of tau Scorpii. © Jardine & Donati


The surprising magnetic field of young massive star

Our Sun has its flares and spots and wind, but it's a placid star compared to some. Stars that are much more massive live fast and die young, with blue-white, intensely hot surfaces that emit energy at a rate millions of times greater than that of the Sun. These stars are so bright that their light alone propels outflowing stellar winds - up to a billion times stronger than the solar wind - at speeds of up to one per cent of the speed of light.
The authors (JF Donati et al) discovered that one such star, the naked-eye tau Scorpii, unexpectedly hosts a complex network of magnetic field lines over its surface. Tau Sco has been known for some time to emit X-rays at an unusually high rate and to rotate slower than most otherwise similar stars. The newly-discovered magnetic field, presumably a relic from the star's formation stage, goes some way to explaining both characteristics, although the mechanism by which the magnetic field slowed down tau Scorpii's rotation so strongly remains mysterious.
These results will be published in the Monthly Notices of the Royal Astronomical Society. Learn more about this discovery (full paper available here).

The simple magnetic field structure of V374 Pegasi. © Jardine & Donati
The very simple magnetic field of an ultra-cool fully-convective dwarf

Understanding how cool stars produce magnetic fields within their interiors is crucial for predicting the impact of such fields, like the activity cycle of the Sun. In this respect, studying fully-convective stars enables to investigate how convective zones participate by themselves to magnetic field generation. The authors (JF Donati and collaborators) present in this paper a magnetic map of a rapidly-rotating very-low-mass fully-convective dwarf obtained through tomographic imaging from time series of spectropolarimetric data. Their results, demonstrating that fully-convective stars are able to trigger axisymmetric large-scale poloidal fields without differential rotation, challenge existing theoretical models of field generation in cool stars.
This paper is published in Science (2006 Feb 03 issue).
Learn more about this discovery, and read the full paper on astro-ph/0602069.

Direct detection of a magnetic field in the innermost regions of an accretion disk

Models predict that magnetic fields play a crucial role in the physics of astrophysical accretion discs and associated winds and jets. While disc rotation progressively twists the initially vertical field lines around the rotation axis, the field retroacts on the disc plasma by slowing it down and forcing it to fall towards the central star. The magnetic energy flux produced in this process, pointing away from the disc, pushes the surface plasma outwards. As field lines quickly open above the disc as a result of accretion, the ejected plasma is centrifugally accelerated along field lines, giving rise to a disc wind and sometimes even to a collimated jet. This paper (by J-F Donati and collaborators) reports a direct magnetic field detection in the core of the protostellar accretion disc FU Orionis using the new high-efficiency high-resolution spectropolarimeter ESPaDOnS. Zeeman signatures of FU Ori (upper curve, right panel) are found to coincide with spectral lines from the accretion disc (lower curve); they indicate that the surface field reaches a strength of about 1 kG close to disc centre and includes a significant azimuthal component, in good agreement with recent disc/jet theoretical models. The authors find that the field is very filamentary and slows down the disc plasma much more than what models expect, which may explain why FU Ori fails at collimating its wind into a jet.
This paper is published in Nature (2005 Nov 24 issue).
Learn more about this discovery, and read the full paper on astro-ph/0511695.


Discovery of pre-main sequence progenitors of magnetic Ap/Bp stars?

This paper (2005, A&A 442, L31) by Gregg Wade and collaborators presents evidence for kG fields in young pre-main sequence Herbig Ae/Be stars. The case of HD 72106, showing both obvious Zeeman signatures in circular polarisation line profiles (Stokes V, top curves) and abundance spot signatures in unpolarised profiles (Stokes I, bottom curves) at two nearby epochs, is illustrated on the right. These stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. These detections represent additional contradictions to the hypothesis of Hubrig et al, who claim that magnetic fields appear in intermediate-mass stars only after 30% of their main sequence evolution is complete. More on these results can be obtained from ADS.


Discovery of a strong magnetic field on the O star HD 191612

This paper (2006, MNRAS 365, L6) by J-F Donati and collaborators presents evidence for a strong magnetic field in the O star HD 191612, making it the second O star with a detected field. This detection (see Zeeman signature detected in the Stokes V profile, upper curve) strongly suggests that the period of spectroscopic variations recently evidenced for this star by Walborn et al (equal to 538 days) is the rotation period, making HD 191612 the slowest known rotator among massive stars (despite a spectral line broadening of about 100 km/s). In this context, HD 191612 appears as an evolved version of the very young magnetic O star theta1 Ori C (the brightest star of the Orion trapezium); angular momentum dissipation through a magnetically confined wind is presumably at the origin of the very slow rotation of HD 191612. More on these results can be obtained from ADS.

A search for magnetic fields in the variable HgMn star α Andromedae

With this work, the authors (G Wade and collaborators) conducted an extensive search for magnetic fields in the photosphere of α And. They acquired in particular new circular polarisation spectra with the MuSiCoS and ESPaDOnS spectropolarimeters. The polarimetric data provide no convincing evidence for photospheric magnetic fields. The highest-S/N phase- and velocity-resolved Stokes V profiles, obtained with ESPaDOnS, allow to place a 3σ upper limit of about 100 G on the possible presence of any undetected pure dipolar, quadrupolar or octupolar surface magnetic fields, and just 50 G for fields with significant obliquity.
Read the full paper on astro-ph/0601616.



© Jean-François Donati, last update Sep 20 2007.