Like the Sun and its
sunspots,
cool stars or chemically peculiar stars are know to exhibit abundance,
brightness and/or magnetic surface inhomogeneities. If we cannot yet resolve these stars
with direct stellar surface imaging techniques (i.e. interferometry) yet,
indirect methods have been developed to help us examine the detailed distribution
of such surface inhomogeneities, as we do for instance on our Sun, and get some information
of the underlying physical processes, namely the dynamo and
microscopic diffusion phenomena.
For rapidly rotating objects, spectroscopic observations recorded with both high spectral
and temporal resolution can be converted into spatially
resolved maps of stellar surfaces. We first use the fact that stellar rotation (through
the
Doppler effect) associates different radial velocities to different regions of the visible
stellar hemisphere, for instance negative and positive radial velocities for the approaching
and receding limbs respectively. Therefore, as the star rotates and surface inhomogeneities
get carried accross the visible hemisphere, the associated spectral line profile distortions
will successively appear at different radial velocities. Following such spot-induced profile
distortions in either unpolarised or polarised light in dynamic spectra can tell us (with the
help of tomographic techniques) where the parent spots are located at the surface of the star,
what is the brightness, abundance or magnetic field strength inside the spots, but also
how field lines are oriented within these spots. These techniques are known as
Doppler imaging (DI) when the aim is to image brightness/abundance
inhomogeneities imaging of stellar surfaces, and Zeeman-Doppler imaging
(ZDI) in the case of magnetic field imaging. With ZDI, we obtained the
first magnetic image (i.e. with no a priori constraint on
the large-scale field topology) of a star other than the Sun.
Below is an example of unpolarised (Stokes I, left panel) and
circularly polarised (Stokes V, right panel) dynamic spectra
one can obtain, in the particular case of AB Dor, a very young star that arrived very recently
on the main sequence. One can distinctly see on these graphs line profile distoritions
moving from the left side of the line profile (depicted with
a vertical line at velocity -60 km/s) to the right side of the
line profile (depicted with a vertical line at velocity 120 km/s). In such a graph,
spots at different latitudes cross the line profile at different rates: a spot near the
equator crosses the whole line profile from the blue wing (at
-60 km/s) to the red wing (at 120 km/s), while a spot near the
rotation pole only affects the central part of the line profile (around a radial
velocity of 30 km/s). Similarly, the way the amplitude
of the Stokes V signatures are modulated throughout their transit across the spectral line
tells us (to a certain extent) how the magnetic field is oriented
within the corresponding magnetic region.
© MNRAS
The brightness (upper panel) and magnetic
(lower panels) maps reconstructed from the above data set are presented below.
One can see that fairly detailed maps of the stellar surface (typical longitude resolution
of about 3 degrees close to the equator in this particular example) can be obtained with
such a technique.
© MNRAS
Monitoring the long-term evolution of such magnetic topologies can tell us which type of dynamo process is operating in these objects, and gives us a unique opportunity of studying magnetic activity cycles in stars other than the Sun. In the particular maps shown above, up to ten different brightness features and twelve magnetic regions of opposite polarities can be identified. Note in particular that the azimuthal component of the vector field is significant, and is probably generated by a non-solar type dynamo, distributed throughout the whole convective zone.
Comparing different brightness/magnetic maps obtained several rotation cycles apart also enables us to measure the amount of surface differential rotation on a star. In the particular case of AB Dor, we proved that the differential rotation of this object is very similar to that of the Sun both in sense (with a pole rotating slower than the equator) and magnitude (about 120 d are required for the equator to lap the pole by one complete rotational cycle) even though this star rotates more than 50 times faster than the Sun.
Yet another potential application is to extrapolate the reconstructed photospheric field up to the corona (see above in the particular case of one of our AB Dor images). In a second step, we compare the resulting 3D magnetic structure (and its temporal evolution) with successive maps of the magnetically confined prominences (the stellar analogue of solar prominences, see below) that orbit the star (detected through the signatures they imprint in strong spectral lines such as Halpha for instance). It gives us the opportunity of studying the dynamics of stellar coronae in a very broad context by extending the solar studies to rapid rotators where the centrifugal force plays a major role. This work is done in collaboration with M. Jardine and, A. Collier Cameron, from the School of Physics and Astronomy at the University of St. Andrews in Scotland: many other similar images and movies can be found on their own web sites.
Donati J.-F. ,
``Rotation and magnetic fields of solar-like stars'',
in: Maeder A., Eenens P. (eds.),
IAU Symposium 215 on Stellar Rotation (2003).
ASP Conf. Series (in press)
McIvor T., Jardine M.M., Cameron A.C., Wood K., Donati J.-F.,
``Polar fields for AB Doradus'' (2003) MNRAS 345, 601
Donati J.-F., Cameron A.C., Semel M., Hussain G.A.J., Petit P.,
Carter B.D., Marsden S.C., Mengel M., Lopez Ariste A., Jeffers S.V., Rees D.E.,
``Dynamo processes and activity cycles of the active
stars AB Dor, LQ Hya and HR 1099'' (2003) MNRAS 345, 1145
Donati J.-F. ,
``Surface magnetic fields and differential rotation
of solar-like stars'',
in: Arnaud J., Meunier N. (eds.),
Proceedings of the workshop dedicated to Jean-Louis Leroy on
``Magnetism and Activity of the Sun and Stars'' (2003).
EDP Sciences, EAS 9, 169
Garcia-Alvarez D., Foing B.H., Montes D., et al.,
``Simultaneous optical and X-ray observations of
flares and rotational modulation on the RS CVn binary HR 1099 (V711 Tau)
from the MUSICOS 1998 campaign'' (2003) A&A 397, 285
Jardine M.M., Cameron A.C., Donati J.-F.,
``The global magnetic topology of AB Doradus''
(2002) MNRAS 333, 339
Pointer G.R., Jardine M.M., Cameron A.C., Donati J.-F.,
``Modelling surface magnetic field evolution
on AB Doradus due to diffusion and surface differential rotation''
(2002) MNRAS 330, 160
Donati J.-F. ,
``Imaging the magnetic topologies of cool
active stars'', in: Boffin H., Steeghs D., Cuypers J. (eds.),
AstroTomography, Indirect imaging methods in observational astronomy (2001).
Springer, Berlin, p. 207
Jardine M.M., Cameron A.C., Donati J.-F., Pointer G.R.,
``Prominence support in potential field
configurations around rotating stars'' (2001) MNRAS 324, 201
Hussain G.A.J., Donati J.-F., Cameron A.C., Barnes J.,
``Comparisons of images derived from independent
Zeeman Doppler imaging codes'' (2000) MNRAS 318, 961
Jardine M., Barnes J., Donati J.-F., Cameron A.C.,
``The potential magnetic field of
AB Doradus; comparison with Zeeman-Doppler images''
(1999) MNRAS 305, L35
Donati J.-F. ,
``Surface magnetic fields of late-type stars'',
in: Butler C.J., Doyle J.G. (eds.), Brendan Byrne memorial workshop on
``Solar and Stellar Activity: Similarities and Differences'' (1999).
ASP Conf. Series, vol. 158, p. 27
Donati J.-F.,
``Magnetic cycles of HR 1099 and LQ Hydrae''
(1999) MNRAS 302, 457
Donati J.-F., Cameron A.C., Hussain G.A.J., Semel M.,
``Magnetic topology and prominence patterns on
AB Doradus''
(1999) MNRAS 302, 437
Barnes J.R., Cameron A.C., Unruh Y.C., Donati J.-F., Hussain G.A.J.,
``Latitude distributions and lifetimes of starspots
on G dwarfs in the Alpha Persei cluster''
(1998) MNRAS 299, 904
Donati J.-F., Cameron A.C.,
``Differential rotation and magnetic polarity patterns
on AB Doradus''
(1997) MNRAS 291, 1
Donati J.-F., Brown S.F.,
``Zeeman Doppler imaging of active stars.
V: Sensitivity of maximum entropy maps to field orientation''
(1997) A&A 326, 1135
Donati J.-F., Brown S.F., Semel M., Rees D.E., et al.,
``Photospheric imaging of the RS CVn system HR 1099''
(1992) A&A 265, 682
© Jean-François Donati, last update on 2003 Nov 5