We investigate how the observed large-scale surface magnetic fields of low-mass stars (∼0.1 -2 M ), reconstructed through Zeeman-Doppler imaging (ZDI), vary with age t, rotation and X-ray emission. Our sample consists of 104 magnetic maps of 73 stars, from accreting pre-main sequence to main-sequence objects (1 Myr t 10 Gyr). For non-accreting dwarfs we empirically find that the unsigned average large-scale surface field |B V | is related to age as t −0.655±0.045 . This relation has a similar dependence to that identified by Skumanich (1972), used as the basis for gyrochronology. Likewise, our relation could be used as an age-dating method ("magnetochronology"). The trends with rotation we find for the large-scale stellar magnetism are consistent with the trends found from Zeeman broadening measurements (sensitive to large-and small-scale fields). These similarities indicate that the fields recovered from both techniques are coupled to each other, suggesting that small-and large-scale fields could share the same dynamo field generation processes. For the accreting objects, fewer statistically significant relations are found, with one being a correlation between the unsigned magnetic flux Φ V and P rot . We attribute this to a signature of star-disc interaction, rather than being driven by the dynamo.

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In this paper, we present new brightness and magnetic surface images of the young K0 dwarfs AB Doradus and LQ Hydrae, and of the K1 subgiant of the RS CVn system HR 1099 (=V711 Tauri), reconstructed from Zeeman–Doppler imaging spectropolarimetric observations collected at the Anglo‐Australian Telescope during five observing campaigns (totalling 50 nights), from 1998 January to 2002 January. Along with the older images of the same stars (published in previous papers), our complete data set represents the first long‐term series on temporal fluctuations of magnetic topologies of very active stars. All of the magnetic images presented here indicate that large regions with predominantly azimuthal magnetic fields are continuously present at the surfaces of these stars. We take this as further evidence that the underlying dynamo processes that produce them are probably distributed throughout the entire convective zone (and not confined at its base, as in the Sun). We speculate that the radial and azimuthal field maps that we recover correspond, respectively, to the poloidal and toroidal components of the large‐scale dynamo field. We find, in particular, that some signatures, for instance the relative fraction of magnetic energy stored in the large‐scale poloidal and toroidal field components, and the polarity of the axisymmetric component of the field, are variable with time, and provide potentially fruitful diagnostics for investigating magnetic cycles in active stars other than the Sun. We report here the detection of partial polarity switches in some of the axisymmetric field components of two of our programme stars (AB Dor and LQ Hya), suggesting that the dynamo operating in these stars may be cyclic.

Zeeman-Doppler imaging studies have shown that the magnetic fields of T Tauri stars can be significantly more complex than a simple dipole and can vary markedly between sources. We collect and summarize the magnetic field topology information obtained to date and present Hertzsprung-Russell (HR) diagrams for the stars in the sample. Intriguingly, the large scale field topology of a given pre-main sequence (PMS) star is strongly dependent upon the stellar internal structure, with the strength of the dipole component of its multipolar magnetic field decaying rapidly with the development of a radiative core. Using the observational data as a basis, we argue that the general characteristics of the global magnetic field of a PMS star can be determined from its position in the HR diagram. Moving from hotter and more luminous to cooler and less luminous stars across the PMS of the HR diagram, we present evidence for four distinct magnetic topology regimes. Stars with large radiative cores, empirically estimated to be those with a core mass in excess of ∼40% of the stellar mass, host highly complex and dominantly non-axisymmetric magnetic fields, while those with smaller radiative cores host axisymmetric fields with field modes of higher order than the dipole dominant (typically, but not always, the octupole). Fully convective stars stars above 0.5 M appear to host dominantly axisymmetric fields with strong (kilo-Gauss) dipole components. Based on similarities between the magnetic properties of PMS stars and main sequence M-dwarfs with similar internal structures, we speculate that a bistable dynamo process operates for lower mass stars ( 0.5 M at an age of a few Myr) and that they will be found to host a variety of magnetic field topologies. If the magnetic topology trends across the HR diagram are confirmed they may provide a new method of constraining PMS stellar evolution models.

We present the analysis performed on spectropolarimetric data of 97 O-type targets included in the framework of the MiMeS (Magnetism in Massive Stars) Survey. Mean Least-Squares Deconvolved Stokes I and V line profiles were extracted for each observation, from which we measured the radial velocity, rotational and nonrotational broadening velocities, and longitudinal magnetic field B ℓ . The investigation of the Stokes I profiles led to the discovery of 2 new multi-line spectroscopic systems (HD 46106, HD 204827) and confirmed the presence of a suspected companion in HD 37041. We present a modified strategy of the Least-Squares Deconvolution technique aimed at optimising the detection of magnetic signatures while minimising the detection of spurious signatures in Stokes V . Using this analysis, we confirm the detection of a magnetic field in 6 targets previously reported as magnetic by the MiMeS collaboration (HD 108, HD 47129A2, HD 57682, HD 148937, CPD-28 2561, and NGC 1624-2), as well as report the presence of signal in Stokes V in 3 new magnetic candidates (HD 36486, HD 162978, HD 199579). Overall, we find a magnetic incidence rate of 7 ± 3%, for 108 individual O stars (including all O-type components part of multi-line systems), with a median uncertainty of the B ℓ measurements of about 50 G. An inspection of the data reveals no obvious biases affecting the incidence rate or the preference for detecting magnetic signatures in the magnetic stars. Similar to A-and B-type stars, we find no link between the stars' physical properties (e.g. T eff , mass, age) and the presence of a magnetic field. However, the Of?p stars represent a distinct class of magnetic O-type stars.