Zeeman Doppler Imaging Research Articles

Mapping of Stellar Surfaces with Doppler and Zeeman Doppler Imaging

AbstractDoppler imaging has been used to image dark spots on the surfaces of rapidly rotating, magnetically active stars for the last 20 years. More recently, methods have been developed for combining line-profile information from large numbers of spectral lines simultaneously. This has allowed starspot distributions to be mapped in sufficient detail to allow tracking of individual spots over several stellar rotations, delineating surface differential rotation patterns for a number of stars. Zeeman-Doppler imaging allows the creation of stellar magnetograms, which are providing the first insights into the 3D topology of stellar coronal magnetic fields. The advent of cryogenic infrared echelle spectrographs opens up exciting new possibilities for Doppler imaging in molecular lines of species such as TiO, OH, and FeH.

High-Resolution Spectroscopy of some Active Southern Stars

High-resolution échelle spectra of 42 nearby southern solar-type stars have been obtained, in a search for young, single, active, and rapidly rotating sun-like stars suitable for Doppler imaging and Zeeman Doppler imaging studies. As a result of this survey, 13 stars were determined to be youthful with ages less than 600 Myr (Hyades age) and eight of these were found to have projected rotational velocities in excess of 15 km s−1. In addition, five spectroscopic binary systems were identified. Of those stars observed for this survey, HD 106506 is the most outstanding target for mapping active regions. It is an apparently young and single star with rapid rotation (v sin i ∼ 80 km s−1), strong Hα chromospheric activity (log R′Hα ∼ −4.2), and deformation of the spectral line profiles indicating the presence of large starspots.

Polar caps on active stars: magnetic flux emergence and transport

In recent years, Zeeman Doppler imaging of rapidly rotating solar-like stars has shown high-latitude magnetic flux patterns of intermingled magnetic polarities. Such high-latitude intermingling of positive and negative flux is inconsistent with our present understanding of how magnetic flux emerges on the Sun and is transported poleward. To determine how these patterns may arise, magnetic flux transport simulations are carried out. These simulations follow the evolution of the radial magnetic field at the surface of the star as new magnetic bipoles emerge and are advected poleward by the surface effects of differential rotation, meridional flow and supergranular diffusion. To produce intermingling of flux at high latitudes, key parameters such as the emergence profiles and transport coefficients are varied from presently used solar values. In doing so, it is found that, in order to explain the high-latitude intermingling, at least two of these parameters must be changed. First, the emergence profile must be extended to higher latitudes (λ = 50°-70° ), and secondly the value of the meridional flow must be increased by around a factor of 10 (∼100 m s -1 ). The results show that the observed intermingling of high-latitude flux can only occur through a flux emergence and transport process that is significantly different from that which occurs on the Sun. Observable features produced by both of these changes are considered, and the significance of the simulations to future observing programmes discussed. Finally the emergence profile and transport coefficients that best fit the observations of the young active star AB Dor (period = 0.514 d) are put forward.

Photospheric magnetic field and surface differential rotation of the FK Com star HD 199178

We present spectropolarimetric observations of the FK Com star HD 199178 obtained between 1998 December and 2003 August at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We report the detection of a photospheric magnetic field and reconstruct its distribution by means of Zeeman–Doppler imaging. We observe large regions where the magnetic field is mainly azimuthal, suggesting that the dynamo processes generating the magnetic activity of HD 199178 may be active very close to the stellar surface. We investigate the rapid evolution of surface brightness and magnetic structures from a continuous monitoring of the star over several weeks in 2002 and 2003. We report that significant changes occur in the distribution of cool-spots and magnetic regions on typical time-scales of the order of two weeks. Our spectropolarimetric observations also suggest that the surface of HD 199178 is sheared by differential rotation, with a difference in rotation rate between equatorial and polar regions of the order of 1.5 times that of the Sun.

Stellar surface imaging: mapping brightness and magnetic fields

AbstractRapidly rotating late‐type stars typically display signs of magnetic activity that exceed those seen on the Sun by over two orders of magnitude. The techniques of Doppler imaging and Zeeman Doppler imaging have been instrumental in unveiling magnetic activity patterns at the photospheres of these active stars. Essentially, these techniques work by inverting time‐series of high resolution spectra to produce temperature, brightness and/or magnetic field maps at the surfaces of stars. I will describe how these techniques work and review what they have taught us about the nature of magnetic activity in rapid rotators over the last 20 years. Finally, I will conclude by outlining the capabilities of these techniques in light of new instrumentation that is now becoming available. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic Loop Models: from Sun to Stars

I review recent progress in determining the nature of the loop structures that form the coronae of solar-like stars. This progress has been driven by observational advances, in particular the new results from X-ray satellites (Chandra and XMM-Newton) and the availability of surface magnetograms from Zeeman-Doppler imaging. It is now clear that stars that are similar to the Sun in mass, but which rotate more rapidly, have a very different magnetic field structure. Their surfaces are more heavily spotted, with spots appearing at all latitudes, extending all the way up to the rotation pole. Their coronae are correspondingly much brighter in X-rays, containing plasma that is hotter and denser than on the Sun. In addition, stellar coronae can support massive co-rotating prominences out to many stellar radii. Recent efforts in modelling these magnetic structures are now bringing together both the surface magnetograms and also the coronal X-ray emission. The resulting coronal loop models show complex loop structures on all scales, with much of the X-ray emission coming from high latitudes where is does not suffer rotational self-eclipse. The observed high densities and X-ray emission measures are a natural consequence of the high magnetic flux density at the surface. The stripping of the corona due to centrifugal effects at high rotation rates can also explain the saturation and supersaturation of X-ray emission with increasing rotation rates, and the recent observation of a high rotational modulation in a supersaturated star.

Surface Differential Rotation of Evolved Fast Rotators

Zeeman-Doppler Imaging enables one to estimate the short term temporal evolution of surface brightness and magnetic structures, under the effect of differential rotation. We present here spectropolarimetric observations secured between 1998 and 2002 for two evolved active stars: the K1 subgiant of the RS CVn system HR 1099 and the single FK Com giant HD 199178. Differential rotation is detected both from brightness and magnetic images, indicating that the rotational shear, roughly solar in magnitude for the single star, is significantly weaker in the binary system. This result suggests that tidal forces, rather than stellar evolution, could be responsible for the lower rotational shear and thus the longer spot lifetime reported for binary systems.

Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099

We present here spectropolarimetric observations of the RS CVn system HR 1099 (V711 Tau) secured from 1998 February to 2002 January with the spectropolarimeter MuSiCoS at the Telescope Bernard Lyot (Observatoire du Pic du Midi, France). We apply Zeeman–Doppler imaging and reconstruct surface brightness and magnetic topologies of the K1 primary subgiant of the system, at five different epochs. We confirm the presence of large, axisymmetric regions where the magnetic field is mainly azimuthal, providing further support to the hypothesis that dynamo processes may be distributed throughout the whole convective zone in this star. We study the short-term evolution of surface structures from a comparison of our images with observations secured at close-by epochs by Donati et al. at the Anglo-Australian Telescope. We conclude that the small-scale brightness and magnetic patterns undergo major changes within a time-scale of 4–6 weeks, while the largest structures remain stable over several years. We report the detection of a weak surface differential rotation (both from brightness and magnetic tracers) indicating that the equator rotates faster than the pole with a difference in rotation rate between the pole and the equator about four times smaller than that of the Sun. This result suggests that tidal forces also affect the global dynamic equilibrium of convective zones in cool active stars.

Dynamo processes and activity cycles of the active stars AB Doradus, LQ Hydrae and HR 1099

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.

Polar fields for AB Doradus

Polar spots are often observed on rapidly-rotating cool stars, but the nature of the magnetic field in these spots is as yet unknown. While Zeeman–Doppler imaging can provide surface magnetic field maps over much of the observed stellar surface, the Zeeman signature is suppressed in the dark polar regions. We have determined the effect on the global coronal structure of three current models for this polar field: that it is composed (a) of unipolar field, (b) of bipolar regions or (c) of nested rings of opposite polarity. We take as an example the young, rapid rotator AB Doradus (Prot= 0.514 d). By adding these model polar fields into the surface field maps determined from Zeeman–Doppler imaging, we have compared the resulting coronal structure with the observable properties of the corona – the magnitude and rotational modulation of the X-ray emission measure and the presence of slingshot prominences trapped in the corona around the Keplerian corotation radius. We find that only the presence of a unipolar spot has any significant effect on the overall coronal structure, forcing much of the polar field to be open.

Magnetic Fields in B, (peculiar) A, and F stars - recent results from MuSiCoS Data

Ultra-high precision measurements of Stokes V Zeeman signatures have been obtained using the MuSiCoS spectropolarimeter (Donati et al. 1999) at the Pic du Midi obseravtory, aimed at studying the distribution of weak and tangled magnetic fields in F, A, and late-B type stars. We will discuss recent results obtained from these data, including the complete absence of magnetic fields (with an upper limit of around 25 G) in the non-Ap/Bp sample, and the relatively easy detection of magnetic fields in nearly all of the Ap/Bp stars. In addition polarimetric data in all four Stokes parameters ( I , V , Q , and U ) has been obtained for a sample of roAp stars, a pulsating subgroup within the magnetic Cp stars, with the aim to study vertical (stratification) and horizontal ( via Zeeman Doppler Imaging) abundance characteristics and their relation to the magnetic field geometry. We will present results of our data analysis, using the Least Squares Deconvolution (LSD) technique (Donati et al. 1997) and with the goal of determining the surface abundance distribution of rare earth elements and the magnetic field geometry of these stars.

Stellar prominences and coronal magnetic fields

Rapidly rotating solar type stars display varying amounts of chromospheric emission in their Balmer lines. The profiles of these lines often display rapid variability. Since the mid-1980s, high-cadence echelle spectroscopy of numerous such stars has revealed that some of the variability is caused by concentrations of neutral hydrogen, trapped in the stellar coronal magnetic field at distances up to a few stellar radii from the rotation axis. In this paper we review the observations and the tomographic techniques that can be applied to mapping these slingshot prominence systems. We discuss the possible mechanisms for formation and mechanical support of these condensations, in the light of recent efforts to map 3-dimensional stellar coronal magnetic fields and X-ray emission measure distributions, with the help of Zeeman-Doppler imaging.

Inferring X-ray coronal structures from Zeeman-Doppler images

We have modelled the X-ray emission from the young rapid rotator AB Doradus (P = 0.514 days) using as a basis Zeeman-Doppler maps of the surface magnetic field. This allows us to reconcile the apparently conflicting observations of a high X-ray emission measure and coronal density with a low rotational modulation in X-rays. The technique is to extrapolate the coronal field from the surface maps by assuming the field to be potential. We then determine the coronal density for an isothermal corona by solving hydrostatic equilibrium along each field line and scaling the plasma pressure at the loop footpoints with the magnetic pressure. We set the density to zero along those field lines that are open and those where at any point along their length the plasma pressure exceeds the magnetic pressure. We then calculate the optically thin X-ray emission measure and rotational modulation for models with a range of coronal densities. Although the corona can be very extended, much of the emission comes from high-latitude regions close to the stellar surface. Since these are always in view as the star rotates, there is little rotational modulation. We find that emission measures in the observed range of 10^52.8 - 10^53.3 per cubic centimetre can be reproduced with densities in the range 10^9 - 10^10.7 per cubic centimetre for coronae at temperatures of 10^6-10^7 K.

The magnetic structure of sunspots and starspots

The single most important quantity determining the properties of sunspots and presumably starspots is their magnetic field. First an overview of the magnetic structure of sunspots is given, some of the progress made in recent years is described and some of the unsolved questions are pointed out. Both observational and theoretical aspects are dealt with. Finally, the magnetic structure of starspots is discussed. After presenting the evidence for (and against) their magnetic nature the signature of starspots in Zeeman Doppler images is described. It is pointed out that if the properties of sunspots are extrapolated to giant spots on rapidly rotating stars then the observed signature can be at least qualitatively explained.

Mapping potential fields on the surfaces of rapidly rotating stars

ABSTRA C T We present a technique that combines Zeeman Doppler imaging (ZDI) principles with a potential field mapping prescription in order to gain more information about the surface field topology of rapid rotators. This technique is an improvement on standard ZDI, which can sometimes suffer from the suppression of one vector component because of the effects of stellar inclination, poor phase coverage or lack of flux from dark areas on the surface. Defining a relationship between the different vector components allows information from one component to compensate for reduced information in another. We present simulations demonstrating the capability of this technique and discuss its prospects.

Comparisons of images derived from independent Zeeman Doppler imaging codes

We compare surface brightness and magnetic field surface maps for the K0 dwarf AB Doradus, obtained using two independently developed Zeeman Doppler imaging codes. We find that even the fine structure in the two sets of images shows excellent agreement. Magnetic field maps appear to be less sensitive to line-modelling effects than brightness maps. We argue that these results point to the consistency and reliability of Doppler imaging techniques even when using poorly sampled data sets, provided that the spectra used are of sufficiently high signal-to-noise ratio.

Magnetic cycle of LQ Hydrae: observational indications and dynamo model

We present a model for the differential rotation and dynamo activity of the young rapidly rotating K0 dwarf LQ Hya (Prot=1.6 d). As might be expected from observations of the similar rapid rotator AB Dor, the predicted differential rotation is small (≃0.8 per cent) but extremely efficient in generating magnetic fields. The dynamo, which is of a distributed type, produces a globally axisymmetric field with radial and azimuthal components that are of the same magnitude and display a phase-lag in their evolution of about π/2. This is consistent with the long-term Zeeman—Doppler imaging study by Donati. The latitudinal distribution of flux is, however, a little different from that observed and the cycle period of 3.2 yr is somewhat shorter than suggested by the observations.

The potential magnetic field of AB Doradus: comparison with Zeeman-Doppler images

The potential magnetic field of AB Doradus: comparison with Zeeman-Doppler images

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 Tue, 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, El 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 (1.3 kms -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.

Zeeman Doppler Imaging of Stars with the AAT

AbstractZeeman Doppler Imaging (ZDI) is a recent technique for measuring magnetic fields on rapidly rotating, active stars. ZDI employs spectropolarimetry taken at different rotational phases to derive information on the magnetic field distribution over the stellar surface. The Zeeman effect is used to identify the presence of a magnetic field, and variations in Doppler wavelength shifts across the rapidly rotating star allow fields to be resolved on different parts of the visible disk. Analysis of the spectra can be used to produce both thermal and surface magnetic images. ZDI requires very high S/N spectra to be acquired within a time interval short compared to the stellar rotation period. As a result, a large-aperture telescope is needed. Since an initial successful test in 1989, the 3·9 m Anglo-Australian Telescope has been used to obtain ZDI spectra of active stars of different evolutionary stages. The observations have concentrated on the K subgiant in the RSCVn system HR 1099 to monitor changes on this bright and active star. With the advent in 1991 of ZDI spectropolarimetry with the AAT échelle spectrograph, it has become possible to co-add the polarisation signature from the many magnetically sensitive lines recorded simultaneously. As a result, stellar magnetic field detections of unprecedented quality have been obtained. The aims of this paper are to briefly outline the principles of ZDI, describe the instrumental setup at the AAT and present some preliminary results from recent observations.