Spectropolarimetric observations of active stars

This paper reports the results of five years (five runs, 23 nights) of spectropolarimetric observations of active stars with the UCL Echelle Spectrograph of the Anglo-Australian Telescope. 225 circularly (and four linearly) polarized spectra were recorded on 28 objects (21 active stars and seven calibration standards) using the new technique of Zeeman-Doppler imaging. To extract polarization echelle spectra from raw frames, we developed a new dedicated automatic software package (called ESprit, which utilizes optimal extraction techniques) whose detailed description is given in the paper. For each recorded spectrum, we extract `mean' polarized and unpolarized profiles using `least-squares deconvolution', a technique similar to cross-correlation, which can enhance enormously the sensitivity of Zeeman-Doppler imaging, by up to 7.5 mag in flux with respect to a single average line analysis or by 4.5 mag compared with the older technique of Donati et al. in the particular case of a K1 star. Magnetic field is detected unambiguously on 14 objects, namely the weak-line T Tauri star V410 Tau, the pre-main-sequence binary HD 155555, the ZAMS stars AB Dor and LQ Hya, the dwarf flare star CC Eri, the RS CVn systems HR 1099, EI Eri, TY Pyx, CF Tuc, SZ Psc, II Peg, IM Peg and IL Hya, and the FK Com star YY Men. Marginal field detections are also obtained for the weak-line T Tauri star HD 283572 and the Herbig Ae star HD 104237. Except on HR 1099 and II Peg, our results represent the first direct field detections ever reported on these objects, and in particular the first direct field detection on as young a star as V410 Tau. Most of the magnetic signatures we detect on cool stars show several sign reversals throughout the line profile, indicating that the parent field structure is rather complex and must feature (as expected) many small-scale magnetic regions of different polarities. For all stars on which Zeeman detections are recorded with sufficient accuracy (namely LQ Hya, CC Eri, HR 1099, EI Eri, II Peg, IL Hya and YY Men), differential least-squares deconvolution from both the blue and the red parts of the spectral domain indicates that the magnetic regions we detect are mostly 500 to 1000 K cooler than, and sometimes at the same temperature as, but never warmer than the surrounding photosphere. Serendipitous results include the first detection (i) of small-amplitude radial velocity variations (1.3kms^-1 peak to peak) of the Herbig Ae star HD 104237 with small enough a period (37.5+/-1 min) that they must be due to stellar pulsations and (ii) of the solar-like secondary component of the RS CVn system IL Hya.