3.3 Polarimetry

The most direct way to detect and study stellar magnetic fields is to use polarimetric techniques. Polarimetry of stars other than the Sun is, however, a rather challenging task and represents a relatively small field of stellar astrophysics. This, on one hand, is due to limited instrumental capabilities and, on the other hand, due to disk-integrated observations of the Stokes parameters. The latter results in significant cancellation of the signal from regions of mixed polarity fields and, thus, only large-scale magnetic fields can be detected from disk-integrated polarimetric measurements, which still requires the accuracy of the order $10-3 -10- 4$ . Such low polarimetric signals from starspots require large collecting areas of telescopes. An overview of the existing instruments and future projects was made by Semel and López Ariste (2001 ). A list of the available instruments is given in Table 2.


Instrument	Telescope/Observatory	Resolving power	Wavelengths, nm


SEMPOL $*$ /UCLES	Siding Spring( $3.9 m$ )/AAT	70 000	370-870
MuSiCoS $**$	TBL( $2 m$ )/Pic-du-Midi	35 000	450-660
SARG SP	TNG( $3.6 m$ )/Tenerife	46 000-164 000	370-1020
ESPaDONS	CFHT( $4 m$ )/Hawaii	70 000	370-1000

Table 2:

High-resolution spectropolarimeters for studying starspots. Remarks: $*$ SEMPOL is a visitor instrument, $**$ MuSiCoS will soon be replaced by NARVAL (R $~$ 50 000).

The new generation large telescopes are not yet equipped with high-resolution spectropolarimetric capabilities. The only existing project is PEPSI at the double $8 m$ LBT (Strassmeier et al., 2004 ). Low-resolution spectropolarimetry is possible with FORS1 at VLT (Appenzeller et al., 1998 ). A promising approach to high-precision polarimetry is provided by a ZIMPOL-type spectropolarimeter, which is a transportable system being combined with a spectrograph (Stenflo, 2001 ).

Magnetic fields in G, K, and M main-sequence stars have been deduced from the broadening of line profiles (e.g., Valenti et al., 1995 ) or even Zeeman-split spectral lines in M-type dwarf flare stars (Johns-Krull and Valenti, 1996 ). Using SEMPOL at AAT, Donati et al. (1997 ) reported detection of Zeeman polarisation in several kinds of active stars, including RS CVn systems, a dwarf flare star of BY Dra type, a FK Com star, and a T Tau star. Several new detections have been made with MuSiCoS (Wade, 2003 ).