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.

Table 2: High-resolution spectropolarimeters for studying starspots. Remarks: * SEMPOL is a visitor instrument, ** MuSiCoS will soon be replaced by NARVAL (R ∼ 50 000).
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

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., 1995Jump To The Next Citation Point) or even Zeeman-split spectral lines in M-type dwarf flare stars (Johns-Krull and Valenti, 1996Jump To The Next Citation Point). Using SEMPOL at AAT, Donati et al. (1997Jump To The Next Citation Point) 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).

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