ESPaDOnS
first scientific results
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.
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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).
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The simple magnetic field structure of V374 Pegasi. © Jardine & Donati |
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 Jun 06 2006.