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We present the photometry and spectropolarimetry of the pre-main-sequence star HD 106506. A photometric rotational period of 1.416 0.133 d has been derived using observations at Mount Kent Observatory (MKO). Spectropolarimetric data obtained with the 3.9-m Anglo-Australian Telescope (AAT) were used to derive spot occupancy and magnetic maps of the star through the technique of Zeeman Doppler imaging (ZDI). The resulting brightness maps indicate that HD 106506 displays photospheric spots at all latitudes including a predominant polar spot. Azimuthal and radial magnetic images of this star have been derived, and a significant azimuthal magnetic field is indicated, in line with other active young stars. A solar-like differential rotation law was incorporated into the imaging process. Using Stokes I information the equatorial rotation rate, , was found to be 4.54 0.01 , with a photospheric shear of . This equates to an equatorial rotation period of 1.39 0.01 d, with the equatorial region lapping the poles every d. Using the magnetic features, the equatorial rotation rate, , was found to be 4.51 0.01 , with a photospheric shear of 0.24 0.03 . This differential rotation is approximately four times that observed on the Sun. [ABSTRACT FROM AUTHOR]

We present the first maps of the surface magnetic fields of a pre-main-sequence binary system. Spectropolarimetric observations of the young, 18 Myr, HD 155555 (V824 Ara, G5IV+K0IV) system were obtained at the Anglo-Australian Telescope in 2004 and 2007. Both data sets are analysed using a new binary Zeeman–Doppler imaging (ZDI) code. This allows us to simultaneously model the contribution of each component to the observed circularly polarized spectra. Stellar brightness maps are also produced for HD 155555 and compared to previous Doppler images. Our radial magnetic maps reveal a complex surface magnetic topology with mixed polarities at all latitudes. We find rings of azimuthal field on both stars, most of which are found to be non-axisymmetric with the stellar rotational axis. We also examine the field strength and the relative fraction of magnetic energy stored in the radial and azimuthal field components at both epochs. A marked weakening of the field strength of the secondary star is observed between the 2004 and 2007 epochs. This is accompanied by an apparent shift in the location of magnetic energy from the azimuthal to radial field. We suggest that this could be indicative of a magnetic activity cycle. We use the radial magnetic maps to extrapolate the coronal field (by assuming a potential field) for each star individually – at present ignoring any possible interaction. The secondary star is found to exhibit an extreme tilt (≈75°) of its large-scale magnetic field to that of its rotation axis for both epochs. The field complexity that is apparent in the surface maps persists out to a significant fraction of the binary separation. Any interaction between the fields of the two stars is therefore likely to be complex also. Modelling this would require a full binary field extrapolation. [ABSTRACT FROM AUTHOR]

We obtain contemporaneous observations of the surface and corona of AB Doradus (AB Dor), a young single cool star, using ground-based circularly polarized spectra from the Anglo-Australian Telescope and X-ray light curves and spectra from the Chandra satellite. The ground-based data are used to construct surface magnetic field maps, which are extrapolated to produce detailed models of the quiescent corona. The X-ray data serve as a new test for the validity of these coronal models. We find that AB Dor's X-ray corona must be concentrated close to its surface, with a height, ; this height is determined by the high coronal density and complex multipolar magnetic field from the surface maps. There is also significant correlation between the positions of surface and coronal active longitudes as determined from the surface spot and magnetic field maps and the X-ray light curve. At this epoch (2002 December) AB Dor appears to possess one very large active longitude region, covering almost half the star; displaying enhanced activity in the form of large dark spots, strong magnetic fields and chromospheric emission. This is unusual as previous surface maps of AB Dor typically display more active regions that span a wider range of longitudes. Finally, the level of rotational modulation and shape of the X-ray light curve depend on the distribution of magnetic field in the obscured hemisphere (AB Dor is inclined by 60°). The models that best reproduce the rotational modulation observed in the contemporaneous Chandra X-ray light curve and spectra require the magnetic field in the obscured hemisphere to be of the same polarity as that in the observed hemisphere. The Sun shows different behaviour, with the leading polarity reversed in the opposite hemisphere. The X-ray observations provide a unique constraint on the magnetic structure in the obscured hemisphere. [ABSTRACT FROM AUTHOR]

Observations of stellar prominences on young rapidly rotating stars provide unique probes of their magnetic fields out to many stellar radii. We compare two independently obtained data sets of the K3 dwarf Speedy Mic (BO Mic, HD 197890) using the Anglo-Australian Telescope (AAT) and the European Southern Observatory (ESO) Very Large Telescope (VLT). Taken more than a fortnight apart, they provide the first insight into the evolution of the prominence system on such a young rapidly rotating star. The largest prominences observed transiting the stellar disc are found at very similar rotational phases between the epochs. This suggests that the magnetic structures supporting the prominences retain their identity on a two to three week time-scale. By taking advantage of the high signal-to-noise ratio and large wavelength range of the VLT observations, we identify prominences as transient absorption features in all lines of the hydrogen Balmer series down to H10. We use the ratios of the prominence equivalent widths (EWs) in these lines to determine their column densities in the first excited state of hydrogen. We determine the optical depths, finding prominences to be rather optically thick (τ≈ 20) in the Hα line. The total hydrogen column density and thus the prominence masses are determined via observations of the Ca II H&K lines. We find typical masses for four of the largest prominences to be in the range 0.5–2.3 × 1014 kg, slightly larger than giant solar prominence masses. Rotationally modulated emission is seen outside of the Hα line. These loops of emission are shown to be caused by prominences seen off the stellar disc. We find that all of the large emission loops can be associated with prominences we see transiting the stellar disc. This, combined with the fact that many prominences appear to eclipse the off-disc emission of others, strongly suggests that the prominence system is highly flattened and likely confined to low stellar latitudes. [ABSTRACT FROM AUTHOR]