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We present photometry and spectropolarimetry of the pre-main sequence star HD 106506. A photometric rotational period of ~1.416 +/- 0.133 days has been derived using observations at Mount Kent Observatory (MKO). Spectropolarimetric data taken at 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, $\Omega_{eq}$, was found to be 4.54 +/- 0.01 rad/d, with a photospheric shear $\delta\Omega$ of $0.21_{-0.03}^{+0.02}$ rad/d. This equates to an equatorial rotation period of ~1.39 +/- 0.01 days, with the equatorial region lapping the poles every ~$30_{-3}^{+5}$ days. Using the magnetic features, the equatorial rotation rate, $\Omega_{eq}$, was found to be 4.51 +/- 0.01 rad/d, with a photospheric shear $\delta\Omega$ of 0.24 +/- 0.03 rad/d. This differential rotation is approximately 4 times that observed on the Sun., Comment: 13 pages, 14 figures, accepted by MNRAS

Spectropolarimetric observations of the pre-main sequence early-G star HD 141943 were obtained at three observing epochs (2007, 2009 and 2010). The observations were obtained using the 3.9-m Anglo-Australian telescope with the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor instrument. The brightness and surface magnetic field topologies (given in Paper I) were used to determine the star's surface differential rotation and reconstruct the coronal magnetic field of the star. The coronal magnetic field at the 3 epochs shows on the largest scales that the field structure is dominated by the dipole component with possible evidence for the tilt of the dipole axis shifting between observations. We find very high levels of differential rotation on HD 141943 (~8 times the solar value for the magnetic features and ~5 times solar for the brightness features) similar to that evidenced by another young early-G star, HD 171488. These results indicate that a significant increase in the level of differential rotation occurs for young stars around a spectral type of early-G. Also we find for the 2010 observations that there is a large difference in the differential rotation measured from the brightness and magnetic features, similar to that seen on early-K stars, but with the difference being much larger. We find only tentative evidence for temporal evolution in the differential rotation of HD 141943., Comment: 11 pages, 7 figures, accepted by MNRAS

Spectroscopic and spectropolarimetric observations of the pre-main sequence early-G star HD 141943 were obtained at four observing epochs (in 2006, 2007, 2009 and 2010). The observations were undertaken at the 3.9-m Anglo-Australian Telescope using the UCLES echelle spectrograph and the SEMPOL spectropolarimeter visitor instrument. Brightness and surface magnetic field topologies were reconstructed for the star using the technique of least-squares deconvolution to increase the signal-to-noise of the data. The reconstructed brightness maps show that HD 141943 had a weak polar spot and a significant amount of low latitude features, with little change in the latitude distribution of the spots over the 4 years of observations. The surface magnetic field was reconstructed at three of the epochs from a high order (l <= 30) spherical harmonic expansion of the spectropolarimetric observations. The reconstructed magnetic topologies show that in 2007 and 2010 the surface magnetic field was reasonably balanced between poloidal and toroidal components. However we find tentative evidence of a change in the poloidal/toroidal ratio in 2009 with the poloidal component becoming more dominant. At all epochs the radial magnetic field is predominantly non-axisymmetric while the azimuthal field is predominantly axisymmetric with a ring of positive azimuthal field around the pole similar to that seen on other active stars., Comment: 18 pages, 17 figures, accepted by MNRAS

In this paper we study the feasibility of inferring the magnetic field from polarized multi-line spectra using two methods: The pseudo line approach and The PCA-ZDI approach. We use multi-line techniques, meaning that all the lines of a stellar spectrum contribute to obtain a polarization signature. The use of multiple lines dramatically increases the signal to noise ratio of these polarizations signatures. Using one technique, the pseudo-line approach, we construct the pseudo-line as the mean profile of all the individual lines. The other technique, the PCA-ZDI approach proposed recently by Semel et al. (2006) for the detection of polarized signals, combines Principle Components Analysis (PCA) and the Zeeman Do ppler Imaging technique (ZDI). This new method has a main advantage: the polarized signature is extracted using cross correlations between the stellar spectra nd functions containing the polarization properties of each line. These functions are the principal components of a database of synthetic spectra. The synthesis of the spectra of the database are obtained using the radiative transfer equations in LTE. The profiles built with the PCA-ZDI technique are denominated Multi-Zeeman-Signatures. The construction of the pseudo line as well as the Multi-Zeeman-Signatures is a powerful tool in the study of stellar and solar magnetic fields. The information of the physical parameters that governs the line formation is contained in the final polarized profiles. In particular, using inversion codes, we have shown that the magnetic field vector can be properly inferred with both approaches despite the magnetic field regime., Comment: Accepted for publication in Astronomy and Astrophysics

We have produced brightness and magnetic field maps of the surfaces of CV Cha and CR Cha: two actively accreting G and K-type T Tauri stars in the Chamaeleon I star-forming cloud with ages of 3-5 Myr. Our magnetic field maps show evidence for strong, complex multi-polar fields similar to those obtained for young rapidly rotating main sequence stars. Brightness maps indicate the presence of dark polar caps and low latitude spots -- these brightness maps are very similar to those obtained for other pre-main sequence and rapidly rotating main sequence stars. Only two other classical T Tauri stars have been studied using similar techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field patterns that are significantly more complex than those recovered for BP Tau, a fully convective T Tauri star. We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in T Tauri stars with radiative cores (V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct magnetic field studies of T Tauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of T Tauri star systems as they approach the main sequence., Comment: Accepted for publication by MNRAS: 15 pages, 11 figures

We present the first measurements of surface differential rotation on a pre-main sequence binary system. Using intensity (Stokes I) and circularly polarised (Stokes V) timeseries spectra, taken over eleven nights at the Anglo-Australian Telescope (AAT), we incorporate a solar-like differential rotation law into the surface imaging process. We find that both components of the young, 18 Myr, HD 155555 (V824 Ara, G5IV + K0IV) binary system show significant differential rotation. The equator-pole laptimes as determined from the intensity spectra are 80 days for the primary star and 163 days for the secondary. Similarly for the magnetic spectra we obtain equator-pole laptimes of 44 and 71 days respectively, showing that the shearing timescale of magnetic regions is approximately half that found for stellar spots. Both components are therefore found to have rates of differential rotation similar to those of the same spectral type main sequence single stars. The results for HD 155555 are therefore in contrast to those found in other, more evolved, binary systems where negligible or weak differential rotation has been discovered. We discuss two possible explanations for this; firstly that at the age of HD 155555 binary tidal forces have not yet had time to suppress differential rotation, secondly that the weak differential rotation previously observed on evolved binaries is a consequence of their large convection zone depths. We suggest that the latter is the more likely solution and show that both temperature and convection zone depth (from evolutionary models) are good predictors of differential rotation strength. Finally, we also examine the possible consequences of the measured differential rotation on the interaction of binary star coronae., Comment: 13 pages, 10 figures, accepted for publication in MNRAS

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 datasets 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 polarised 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 deg) 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., Comment: 17 pages, 12 figures, accepted for publication in MNRAS

In this paper we present the first results of a detailed spectroscopic and photometric analysis of the V = 11.7m eclipsing binary ASAS J052821+0338.5. With the FIES spectrograph at the Nordic Optical Telescope we have obtained a series of high-resolution spectra (R=47000) covering the entire orbit of the system. In addition we obtained simultaneous broadband photometry from three small aperture telescopes. From these spectroscopic and photometric data we have derived the system's orbital parameters and determined the fundamental stellar parameters of the two components. Our results indicate that ASAS J052821+0338.5 is a K1/K3 pre-main-sequence eclipsing binary, with component masses of 1.38 M_sun and 1.33 M_sun and a period of 3.87 days, located at a distance of 280 +/- 30 pc. The kinematics, physical location and the evolutionary status of the two stars suggest that ASAS J052821+0338.5 is a member of the approximately 11 Myr old Orion OB1a subassociation. The systems also exhibits smooth 0.15m out-of-eclipse variations that are similar to those found in RS CVn binaries. Furthermore the parameters we derived are consistent with the 10-13 Myr isochrones of the popular Baraffe stellar evolutionary models., Comment: 9 pages, 8 figures, to appear in A&A

We obtain contemporaneous observations of the surface and corona of AB Dor using ground-based circularly polarised spectra from the Anglo-Australian Telescope and X-ray data 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. The high coronal density and complex multi-polar magnetic field indicate a compact X-ray corona, which is concentrated close to the surface, with a height, H~0.3-0.4R*. There is also significant correlation between the surface and coronal active region locations. At this epoch AB Dor appears to possess one very large active longitude region; displaying enhanced activity in the form of large dark spots, strong magnetic fields and chromospheric emission. Finally, the level of rotational modulation and shape of the X-ray lightcurve depend on the distribution of magnetic field in the obscured hemisphere. The models that best reproduce the rotational modulation observed in the contemporaneous Chandra X-ray lightcurve 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., Comment: 17 pages, 14 figures, accepted by MNRAS. This version has cropped figures. For a preprint with the original figures please go to http://star-www.st-and.ac.uk/~gajh/papers07

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 datasets of the K3 dwarf Speedy Mic using the Anglo-Australian Telescope (AAT) and the 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 timescale. We identify prominences as transient absorption features in all lines of the Hydrogen Balmer series down to H_10. We use the ratios of the prominence 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 (\tau \approx 20) in the H_alpha line. The total hydrogen column density and thus the prominence masses are determined via observations of the CaII H&K lines. We find typical masses for four of the largest prominences to be in the range 0.5 - 2.3 x10^{14} kg, slightly larger than giant solar prominence masses. Rotationally modulated emission is seen outside of the H_alpha 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., Comment: 13 pages, 7 figures, accepted for publication in MNRAS, (Higher resolution greyscale images can be found at http://star-www.st-and.ac.uk/~njd2/speedy/index.html)

We present new observations of the prominence system on the K3 dwarf Speedy Mic (BO Mic, HD 197890). Using an improved technique to track the absorption features in H_alpha we find a very active prominence system with approximately ten prominences on the observable hemisphere per rotation. From a total of 25 prominences we find an average axial distance of 2.85+/-0.54 R_* which is twice the co-rotation radius above the stellar surface. We discuss the consequences of these observations on the nature of the supporting magnetic structures. Two consecutive nights, with complete phase coverage, combined with a further night after a three night gap allow us to study the evolution of the prominence system on two different timescales. Several of the prominences have counterparts at similar phases on consecutive nights. During this interval many prominences show evidence for evolution in their heights and phases of observation. Five nights (13 rotation cycles) later we recover many prominences at approximately the same phases. Whilst individual prominences change axial distances or appear/re-appear from night-to-night the underlying prominence supporting structures appear to be stable over as many as 13 stellar rotations., Comment: 10 pages, 6 figures, accepted for publication in MNRAS, (Higher resolution greyscale images can be found at http://star-www.st-and.ac.uk/~njd2/speedy/index.html)