This paper reports the results of 5 yr (5 runs, 23 nights) of
spectropolarimetric observations of active stars with the UCL Echelle Spectrograph of
the Anglo-Australian Telescope. Two hundred and twenty five
circularly (and four linearly) polarised spectra were recorded on 28
objects (21 active stars and 7 calibration standards) using the new
technique of Zeeman-Doppler imaging.
To extract polarisation echelle spectra from raw frames, we
developed a new dedicated automatic software package (called ESpRIT
and utilizing optimal extraction techniques) whose detailed
description is given in the paper. For each recorded spectrum, we
extract ``mean'' polarised and unpolarised 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 to the older
technique of Donati et al. (1992a) 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, witnessing 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 red
parts of the spectral domain indicates that the magnetic regions we
detect are mostly 500 to 1,000 K cooler than, 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 (1.3 km/s 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.