Dwarf AB Doradus Research Articles

THE CORONAL STRUCTURE OF AB DORADUS

We perform a numerical simulation of the corona of the young, rapidly rotating K0 dwarf AB Doradus using a global MHD model. The model is driven by a surface map of the radial magnetic field constructed using Zeeman-Doppler Imaging. We find that the global structure of the stellar corona is dominated by strong azimuthal tangling of the magnetic field due to the rapid rotation. The MHD solution enables us to calculate realistic Alfv\'en surfaces and we can therefore estimate the stellar mass loss rate and angular momentum loss rate without making undue theoretical simplifications. We consider three cases, parametrized by the base density of the corona, that span the range of possible solutions for the system. We find that overall, the mass and angular-momentum loss rates are higher than in the solar case; the mass loss rates are 10 to 500 times higher, and the angular momentum loss rate can be up to $3\times{10}^4$ higher than present day solar values. Our simulations show that this model can be use to constrain the wide parameter space of stellar systems. It also shows that an MHD approach can provide more information about the physical system over the commonly used potential field extrapolation.

Magnetic activity on AB Doradus: temporal evolution of star-spots and differential rotation from 1988 to 1994

Surface brightness maps for the young K0 dwarf AB Doradus are reconstructed from archival data sets for epochs spanning from 1988 to 1994. By using the signal-to-noise ratio enhancement technique of least-squares deconvolution, our results show a greatly increased resolution of spot features than obtained in previously published surface brightness reconstructions. These images show that for the exception of epoch 1988.96, the star-spot distributions are dominated by a long-lived polar cap, and short-lived low to high-latitude features. The fragmented polar cap at epoch 1988.96 could indicate a change in the nature of the dynamo in the star. For the first time we measure differential rotation for epochs with sufficient phase coverage (1992.05, 1993.89, 1994.87). These measurements show variations on a time-scale of at least 1 year, with the strongest surface differential rotation ever measured for AB Dor occurring in 1994.86. In conjunction with previous investigations, our results represent the first long-term analysis of the temporal evolution of differential rotation on active stars.

The Coronal Topology of the Rapidly Rotating K0 Dwarf AB Doradus. I. Using Surface Magnetic Field Maps to Model the Structure of the Stellar Corona

We reanalyze spectropolarimetric data of AB Dor taken in 1996 December using a surface imaging code that can model the magnetic field of the star as a nonpotential current-carrying magnetic field. We find that a nonpotential field needs to be introduced in order to fit the data set at this epoch. This nonpotential component takes the form of a strong unidirectional azimuthal field of a similar strength to the radial field. This azimuthal field is concentrated around the boundary of the dark polar spot recovered at the surface of the star using Doppler imaging. As polarization signatures from the center of starspots are suppressed, it is unclear whether or not this nonpotential component genuinely represents electric current at the unspotted surface or whether it results from the preferred detection of horizontal field in starspot penumbrae. This model contains 20% more energy than the corresponding potential field model at the surface. This amount of free energy drops to under 1% about 1R* above the photosphere. We use these surface maps to model the coronal structure of the star. The mixed radial polarities at the pole in the surface maps support closed coronal loops in the high-latitude regions, indicating that a component of the X-ray emission may originate in this area. Assuming that the field remains closed out to 5R*, we find stable surfaces where prominences may form out to the observed distances using this coronal model.

Modelling surface magnetic field evolution on AB Doradûs due to diffusion and surface differential rotation

From Zeeman Doppler images of the young, rapidly-rotating K0 dwarf AB Doradus, we have created a potential approximation to the observed radial magnetic field and have evolved it over 30 days due to the observed surface differential rotation, meridional flow and various diffusion rates. Assuming that the dark polar cap seen in Doppler images of this star is caused by the presence of a unipolar field, we have shown that the observed differential rotation will shear this field to produce the observed high-latitude band of unidirectional azimuthal field. By cross-correlating the evolved fields each day with the initial field we have followed the decay with time of the cross-correlation function. Over 30 days it decays by only 10 percent. This contrasts with the results of Barnes et al (1998), who show that on this timescale the spot distribution of He699 is uncorrelated. We propose that this is due to the effects of flux emergence changing the spot distributions.

Doin’ the twist: secular changes in the surface differential rotation on AB Doradus

We present measurements of the rotation rates of individual starspots on the rapidly rotating young K0 dwarf AB Doradus, at six epochs between 1988 December and 1996 December. The equatorial rotation period of the star decreased from 0.5137 to 0.5129 days between 1988 December and 1992 January. It then increased steadily, attaining a value of 0.5133 days by 1996 December. The latitude dependence of the rotation rate mirrored the changes in the equatorial rotation rate. The beat period between the equatorial and polar rotation periods dropped from 140 days to 70 days initially, then rose steadily. The most rigid rotation, in 1988 December, occurred when the starspot coverage was at a maximum. The time-dependent part of the differential rotation is found to have Delta Omega / Omega ~ 0.004, which should alter the oblateness of the star enough to explain the period changes observed in several close binaries via the mechanism of Applegate (1992).

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 topology and prominence patterns on AB Doradus

We report new Zeeman--Doppler imaging observations of the rapidly rotating young K0 dwarf AB Doradus, obtained with the Anglo-Australian Telescope in 1996 December. From such observations, simultaneous brightness and magnetic images of the stellar photosphere of AB Dor were reconstructed at three different epochs. The magnetic topology of AB Dor is found to be very complex, with at least 12 different radial field regions of opposite polarities located all around the star. Significant azimuthal field fluxes are also detected in the form of one negative polarity region close to the equator, a series of positive polarity patches at intermediate latitudes and an almost complete ring of negative polarity encircling the rotational pole at high latitudes. In particular, the azimuthal polarities we reconstruct are in very good agreement with those obtained by Donati & Cameron, confirming that this field component is directly related to the dynamo-generated large-scale toroidal magnetic structure. The triple polarity latitudinal pattern observed for this structure in the upper hemisphere of AB Dor indicates that the degree of the underlying large-scale poloidal structure in an axisymmetric spherical harmonics expansion is equal to or greater than five. It also strengthens the idea that the dynamo processes operating in AB Dor feature a non-solar component distributed throughout the convective zone. From the subtle distortion of successive brightness images, we can also confirm the surface differential rotation first measured on this star by Donati & Cameron in both sense and magnitude, with a pole rotating more slowly than the equator by about one part in 220. Finally, the rotation periods we measure for four prominences (from the recurrence rate of their spectral signatures in Balmer lines) confirm the presumption that such clouds are anchored at intermediate to high latitudes. The intrinsic variability of these prominences is not associated with any abrupt changes in the photospheric brightness or magnetic distributions, implying that they essentially result from the reorganization of coronal field lines.

Doppler Imaging of Stars with Surface Inhomogeneities

AbstractI review recent progress in the field of stellar surface imaging, with particular reference to advanced methods for mapping surface-brightness distributions on magnetically active late-type stars. New signal enhancement techniques, utilising profile information from hundreds or thousands of photospheric lines simultaneously, allow images to be derived for stars several magnitudes fainter than was previously possible. For brighter stars, the same techniques make it possible to map features as small as two or three degrees in extent on the stellar surface. While this opens up whole new areas of research, such as the ability to use starspot tracking to study surface differential rotation patterns on single and binary stars, caution must be exercised in the treatment of “nuisance” parameters such as the stellar rotation rate, surface abundances and radial velocity. At the very high S:N levels we now use, the effects of systematic errors in these parameters are easier to identify, isolate and eliminate. This leads to the possibility of making precise radial velocity variations (at the few hundred m s−2 level or better) in late-type stars even with equatorial rotation speeds as high as 100 km s−1. This is particularly topical given the recent discovery that one of our prime imaging targets, the young southern K0 dwarf AB Doradus, has an astrometric companion in a highly eccentric orbit with an inferred mass close to the H-burning limit.

ASCA X-ray Spectra of Quiescent and Flaring Emission from AB Doradus

The Japanese satellite ASCA carries X-ray detectors which combine excellent sensitivity and high spectral resolution. We observed the young, rapidly-rotating K dwarf AB Doradus with ASCA in November 1993. The star’s X-ray flux was essentially steady for the first half of the observation, and then a series of flares occurred during the second half. The flares showed rise times of 30 minutes and decay times of several hours. The quiescent X-ray luminosity was 2 × 1030 ergs s−1 (0.5-10 keV). At the flare peaks, LX(> 0.5 keV)/Lbol = .002.AB Dor has a relatively high count rate in the ASCA data and we can obtain high-quality spectra for different periods of the light curve. In Fig. 1 we present spectra corresponding to the period of quiescence, the rising phase and peak of each of the three flares combined, and the decay periods of each of the flares combined. The rise/peak curve (upper) has been multiplied by 2 for purposes of display. We have subtracted the quiescent spectrum from the flare spectra (i.e., used it as a background spectrum) for this display and subsequent analysis.The detailed differences between these spectra display the importance of the high spectral resolution provided by ASCA. One important difference, not readily evident in this figure, is the Fe K line complex at 6.4-6.7 keV. This line, a diagnostic of hot plasma, is strong in the rise/peak spectrum, moderate in the decay spectrum and weak in the quiescent spectrum. A time profile of the counts in this energy range shows little significant emission during the quiescent periods but strong emission at the flare peaks.

X-ray Spectral Diagnostics for Coronal Loops in the Active K Dwarf AB Doradus

AbstractWe discuss theoretical X-ray spectra for coronal loop models in the rapidly rotating young K dwarf AB Doradus (HD 36705), as a typical representative of active X-ray bright stars. The loop models are based on EXOSAT and IUE observations, and further motivated by a possible connection between the observed X-ray flares and co-rotating clouds of neutral hydrogen (a few/day). The resulting synthetic spectra between 0.5 – 7 keV can be approximated by a linear combination of three distinct temperature components. Two components are sufficient between 0.5 – 2.5 keV. Below 0.1 keV the loop spectra deviate significantly from the few component fits. To test some basic assumptions (dynamic vs. static, constant vs. variable cross-sectional area), useful constraints on the DEM(T)-distribution could be obtained with the grating-spectrometer of XMM with 103 – 104sec exposure times. The ratio of the He-type (O VII) forbidden and intercombination lines at 0.56 keV will provide sufficient density diagnostics, to distinguish e.g.between compact and large loops. The crystal-spectrometers of XMM and XSPECT could achieve the same but with longer (105 sec) observing time. The strong Ly α line of O VIII at 0.65 keV can be observed with the crystals in 104 sec, and used even for rotational modulation and flare studies, and giving additional information about flows in flaring loops. At the Iron 6.7 keV lines, where the gratings do not work, the crystals should be used together with low resolution devices, to set constraints on the hottest gas at loop summits. Our discussion applies also to several brighter cool stars but with shorter observing time (like Capella and HR 1099, which are over 3 times X-ray brighter than AB Dor).

X-ray spectral diagnostics for coronal loop sizes and heating rates in the active K dwarf AB Doradus

We present a set of theoretical X-ray spectra derived from models of coronal loops in thermal and hydrostatic equilibrium on the rapidly-rotating K0 dwarf AB Doradus. This star is a typical representative of active X-ray bright stars. The coronal loop models are based on EXOSAT and IUE observations of AB Dor. The resulting synthetic spectra are folded through the expected responses of the XMM and XSPECT missions. Useful constraints on the DEM(T) distribution could be obtained for a star as bright as AB Dor with the grating spectrometer of XMM in 10 3 to 10 4 s exposure times. The ratio of the He-type (O VII) forbidden and intercombination lines at 0.56 keV will provide density diagnostics sufficient to determine whether, for example, the X-rays originate mainly in compact, dense flare loops or in more extended loops of lower density. The strong O VIII line at 0.65 keV can be observed with the Bragg crystal spectrometers of both missions, in 10 4 s, and may be used for studies of rotational modulation and of flow velocities in flaring loops. At the Iron 6.7 keV lines, the crystal spectrometers and the lower-resolution CCD and GSPC instruments can be used to set constraints on the hottest gas at the loop summits.