Surface magnetic fields of non degenerate stars

Why studying stellar magnetic fields?

Magnetic field is an important parameter in many astrophysical problems and at virtually all spatial scales, from interstellar dust grains to planets and stars, and to galaxies and clusters of galaxies. In stars in particular, they play a crucial role on their interior, their atmospheres and their circumstellar environment, by influencing (in sometimes a rather drastic way) the transport processes of chemical elements and angular momentum. Studying the topology of magnetic fields, and in particular their large- and small-scale structures, can first help us understand their physical origins, whether they are produced whithin stellar plasma through hydrodynamical processes (dynamo fields) like that of the Sun, or represent a fossil remnant from a previous evolutionary stage (fossil fields) like those of chemically peculiar stars. It also helps understanding the potential impact of these magnetic fields on long-term stellar evolution.

Zeeman-Doppler imaging

Using high resolution echelle spectropolarimeters (like that of the MuSiCoS spectrograph) and cross-correlation techniques such as Least-Squares Deconvolution, one can detect stellar magnetic fields through the Zeeman signatures they generate in the shape and polarisation state of spectral line profiles (the Zeeman effect). Zeeman-Doppler imaging (ZDI) works best for moderate to ultra-fast rotators, for which circular polarisation signatures of individual unipolar magnetic regions are associated to different Doppler velocities and thus no longer mutually cancel as in conventional polarimetric methods. This method was successfully used to obtain the first direct detection (i.e. from spectropolarimetric data) of magnetic fields in cool stars other than the Sun.

Dynamo fields of active stars

Although small (with a typical relative size of 0.1% of the unpolarised continuum), Zeeman signatures from surface magnetic fields of cool active stars have now been detected with ZDI in about 20 objects, from pre-main sequence (2 wTTS and 1 cTTS) to main-sequence (3 ZAMS objects and 1 dMe star), to red-giant branch stage (12 RS CVn system, 1 FK Com star). Below is an example of such circular polarisation (i.e. Stokes V) signatures (upper curves) for the very young weak-line T Tauri star V410 Tauri (left panel) and for the evolved FK Com giant YY Men (right panel). Note that these Stokes V signatures show several sign reversals indicating that the underlying field structure is probably quite complex.


Monitoring such Zeeman signatures throughout a full stellar rotation cycle allows one to map the detailed structure of the vector field topology and study the way dynamo processes operate in convective zones of cool stars.

Fossil fields of hot stars

Magnetic chemically peculiar (Ap) stars also feature large-scale field structures often modeled as offcentred dipoles. ZDI can also be used to detect line profile circular (Stokes V) and linear (Stokes Q and U) polarisation Zeeman signatures generated by these large-scale field structures. Below is an example of Stokes V (left panel, upper curve) and Stokes U (left panel, upper curve) line profiles in the particular case of the Ap star 49 Cam (note the different amplification factors for the circular and linear Zeeman signatures).

Monitoring such Zeeman signatures throughout a full stellar rotational cycle should allow one to study how the underlying large-scale magnetic topologies differ from simple dipolar structures, and thus test in detail the fossil origin currently invoked for these field structures.

Other forms of stellar magnetism?

There is very likely other classes of non-degenerate magnetic stars than the only two we know of at the moment (solar-type and chemically peculiar stars), and probably other forms of stellar magnetism as well. We indeed have ample indirect evidence that magnetic fields are likely present at the surface of stars of other types. Magnetic fields in accretion discs for instance (produced through some kind of shear-flow dynamo) are now invoked by many astronomers to explain the anomalously high viscosity observed in such objects, but have not been detected yet. Hot stars (O stars, Wolf-Rayet stars) and warmer ones (Ae/Be Herbig stars, Be stars, A/B supergiants) are also very good candidates for instance, for which magnetic fields are very often invoked to explain the azimuthal structuration observed in their strong winds. The recent field detection obtained in the Herbig Ae star HD 104237 (see below) is a direct confirmation of this point.

A detailed monitoring of how the detected Zeeman signature is rotationnally modulated is presently undertaken in order to study the topology of the associated field structure and therefore its probable physical origin. We can nevertheless say already that, given the high level of intrinsic variability we observe for this star, it is unlikely that the field is fossil. A solar-type dynamo field on the other hand would be highly surprising as this star does not trigger a vigorous enough convection. We speculate that such a field may also be produced through a shear-flow dynamo within the upper atmospheric layers that suffer strong rotational braking from the magnetised wind.

Related publications

Budding E., Carter B.D., Mengel M.W., Slee O.B., Donati J.-F., ``A Radio and Optical Study of the Active Young F Star HR 1817 (=HD 35850)'' (2002) PASA 19, 527

Shorlin S.L.S, Wade G.A., Donati J.-F., Landstreet J.D., Petit P., Sigut T.A.A., Strasser S., ``A sensitive search for magnetic fields in B, A and F stars'' (2002) A&A 392, 637

Donati J.-F., Babel J., Harries T.J., Howarth I.D., Petit P., Semel M., ``The magnetic field and wind confinement of theta 1 Orionis C'' (2002) MNRAS 333, 55

Donati J.-F., Wade G.A., Babel J., Henrichs H.F., de Jong J.A., Harries T.J., ``The magnetic field and wind confinement of beta Cephei: new clues for interpreting the Be phenomenon?'' (2001) MNRAS 326, 1265

Donati J.-F. , ``Stellar magnetic fields'', in: Catala C., Zahn J.-P. (eds.), Aussois courses on stellar physics (2001). Paris Observatory press, p. 173

Bagnulo S., Wade G.A., Donati J.-F., Landstreet J.D., Leone F., Monin D.N., Stift M.J., ``A study of polarised spectra of magnetic CP stars: predicted vs. observed Stokes IQUV profiles for beta CrB and 53 Cam'' (2001) A&A 369, 889

Wade G.A., Donati J.-F., Landstreet J.D., Shorlin S.L.S., ``Spectropolarimetric measurements of magnetic Ap and Bp stars in all four Stokes parameters'' (2000) MNRAS 313, 823

Wade G.A., Donati J.-F., Landstreet J.D., Shorlin S.L.S., ``High precision magnetic field measurements of Ap and Bp stars'' (2000) MNRAS 313, 851

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., Wade G.A., ``On the magnetic field and circumstellar environment of the young O7 star theta1 Orionis C'' (1999) A&A 341, 216

Donati J.-F., ``Exploration observationnelle des topologies magnétiques stellaires'' (1998) Habilitation à diriger des recherches, Université Toulouse III

Donati J.-F. , ``Surface magnetic fields of non-degenerate stars'', in: Kaper L., Fullerton A. (eds.), ESO workshop on ``Cyclical variability in stellar winds'' (1998), Springer, Berlin, p. 212

Donati J.-F., Semel M., Carter B.D., Rees D.E., Cameron A.C., ``Spectropolarimetric observations of active stars'' (1997) MNRAS 291, 658

Catala C., Boehm T., Donati J.-F., Semel M., ``Circular polarisation and variability in the spectra of the Herbig Ae/Be stars I. The Fe II 5018 A and the He I 5876 A lines of AB Aurigae'' (1993) A&A 278, 187

Donati J.-F., Semel M., Rees D.E., ``Circularly polarised spectroscopic observations of RS CVn systems'' (1992) A&A 265, 669

Donati J.-F., Semel M., del Toro Iniesta J.C., ``The magnetic field of the Ap star epsilon UMa'' (1990) A&A 233, L17

Donati J.-F., Semel M., Rees D.E., Taylor K., Robinson R.D., ``Detection of a magnetic region on HR 1099'' (1990) A&A 232, L1

Donati J.-F., Semel M., ``Zeeman-Doppler Imaging: a new option for magnetic field study of Ap and solar-type stars'' (1990) Solar Phys. 128, 227

Donati J.-F., Semel M., Praderie F., ``Zeeman-Doppler Imaging of active stars II. Numerical simulation and first observational results'' (1989) A&A 225, 467

© Jean-François Donati, last update on 2003 Jun. 16