Springer Online Journal Archives 1860-2000
Abstract In the task of studying stellar magnetic fields, polarimetric methods have been intensively used in Ap stars. But the observational material classically used to reconstruct stellar magnetic structures (average longitudinal magnetic field as a function of rotational phase) is not rich enough in spatial information to derive geometries more complex than centered or decentered dipoles. In solar-type stars, all evidences of activity recently detected on their surfaces (starspots, flares, ...) indicate they are most likely magnetic stars. But polarimetric methods have always failed in these stars, probably due to the complex magnetic topologies encountered which even prevented until now a simple detection (Borra, Edwards, and Mayor, 1984). With the Zeeman broadening measurement technique proposed by Robinson (1980), no reliable results can be derived for rapid rotators, which are otherwise presumed to be the best candidates for magnetic detections. Once more, if temperature inhomogeneity charts are already available for solar-type stars (Vogt, 1987), spatial information on their magnetic distributions has conversely not yet been obtained. The new option, recently proposed by Semel (1989) and qualified by Donati, Semel, and Praderie (1989), is based on the rotational modulation study of a rapid rotator Stokes parameter V(λ), obtained with both high spectral resolution R, and high signal-to-noise ratio S/N. Since the magnetic information used refers to localized strips on the stellar disc (as a consequence of the star rotation), multipolar structures can thus be resolved. A new instrumentation and observing procedure have been defined for ZDI, in order to obtain very high S/N data. The method has been successfully tested on two bright magnetic Ap stars: a magnetic detection was obtained on ɛ UMa and a 15-point phase coverage of α 2 CVn is available for the reconstruction of complete 2D abundance and magnetic mappings of its photosphere. Concerning solar-type stars, a numerical simulation was carried out in order to determine the observational constraints required for the detection of ‘typical’ magnetic field similar to those reported in slow rotators with the Robinson method (Saar, 1988). The specifications needed are S/N ≥ 400 per 40 mÅ pixel and R ∼- 6 × 104.
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