Stellar spectropolarimetry


Spectroscopy aims at measuring the flux distribution of a light source (a star for instance) as a function of the radiation wavelength (i.e. colour in the particular case of visible light). These flux distributions are called spectra. Here is for instance what the spectrum of the Sun looks like at optical wavelength, both globally and locally (around a green wavelength of 500 nm):

The local sharp dips we see all around the place are called spectral lines and are due to the absorption of the solar radiation from the solar inner by the atoms of the outer layers (the photosphere). The three deeper spectral lines seen above are from neutral iron. Their shape tells us physical information on the photospheric plasma (e.g. temperature, turbulence). Nowadays, stellar spectroscopy often involves a a high-resolution echelle spectrograph comprising at least a diffraction grating, a cross-dispersing prism, a collimator, a camera and a digital bidimensional CCD detector. Such an instrument can record a very wide spectral domain (as much as the whole optical domain from 370 to 900 nm, i.e. from the near ultraviolet to the near infrared) in a single exposure, like for instance the Fiber-fed Extended Range Optical Spectrograph (or FEROS) built by ESO for their 1.5m telescope in Chile.


Radiation from a light source can be polarised. When the electric vector of the electromagnetic radiation vibrates in a fixed plane, the radiation is called linearly polarised. When the electric vector describes an helix about the direction of propagation, the radiation is said to be circularly polarised. A combination of both usually yields elliptical polarisation. Polarimetry aims at measuring the degree to which a radiation from a light source is polarised, as well the polarisation state of the corresponding light. A polarimeter usually involves retardation components (cristalline plates or Fresnel rhombs, retarding one component of the electric vibration with respect to the other by a fixed amount) in association with a birefringent cristal (which splits the two orthogonal states of linear polarisation of the incoming beam into two separate beams). To perform a circular polarisation light analysis for instance, one normally uses at least a quarter-wave plate (changing the incoming circular polarisation into linear polarisation) and a birefringent cristal (to search for potential linear polarisation emerging from the quarter-wave plate).
Polarimetric observations of stellar sources can inform us on the geometry and chemical composition of circumstellar environments (winds, discs, dust envelopes) through the continuum polarisation scattering processes produce. It can also tell us about stellar surface magnetic fields through the line profile polarisation that the Zeeman effect generates in photospheric spectral lines.

Stellar spectropolarimetry

Although spectroscopy of unpolarised light and broadband photopolarimetry are quite common tools for modern astronomers, the combination of both, called spectropolarimetry, is much more unusual. The coupling of both instruments is indeed not totally trivial. While the polarisation analysis is usually performed at the primary or Cassegrain focus of a telescope (to minimise instrumental polarisation produced by oblique reflections in the telescope), high-resolution spectroscopy is often done at Coudé focus (for better spectrograph stability). A double-fibre feed (one fibre for each orthogonal polarisation state) must therefore be used to convey the light from the Cassegrain focus down to the Coudé room and couple both foci with no compromise either on the accuracy of the polarisation analysis or on the spectrograph stability.
Only very few such instruments exist worldwide. Those we currently use are the polarimeter of the MuSiCoS Echelle spectrograph (on the 2m TBL telescope of Pic du Midi), or with Semel's visitor polarimeter on the UCL Echelle Spectrograph of the 3.9m Anglo-Australian Telescope. I am presently involved in the development of a new generation instrument for the Canada-France-Hawaii Telescope, named ESPaDOnS.

Related publications

Donati J.-F. , ``ESPaDOnS: An Echelle SpectroPolarimetric Device for the Observation of Stars at CFHT'', in: Trujillo Bueno J., Sanchez Almeida J. (eds.), proceedings of the Solar Polarisation Workshop #3 (2003). ASP Conf. Series (in press).

Donati J.-F. , Catala C., Mathys G., et al., ``High resolution spectropolarimetry on the VLT'', in: Monnet G., Bergeron J. (eds.), ``Scientific Drivers for ESO Future VLT/VLTI Instrumentation'' (2001). Springer, Berlin (in press)

Donati J.-F. , ``Spectropolarimetry on giant telescopes'', in: Mathys G., Solanki S.K., Wickramasinghe D.T. (eds.), ``Magnetic fields across the HR diagram'' (2001). ASP Conf. Series 248, 563

Donati J.-F., Catala C., Wade G.A., Gallou G., Delaigue G., Rabou P., ``A dedicated polarimeter for the MuSiCoS échelle spectrograph'' (1999) A&AS 134, 149

Donati J.-F. , Catala C., Landstreet J.D., ``ESPaDOnS: An Echelle SpectroPolarimetric Device for the Observation of Stars at CFHT'', in: Martin P., Rucinski S. (eds.), `proceedings of the ``fifth CFHT users' meeting'' (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

Semel M., Donati J.-F., Rees D.E., ``Zeeman-Doppler Imaging of active stars III. Instrumental and technical considerations'' (1993) A&A 278, 231

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