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Refereed Papers

2009

Surface magnetic fields on two accreting T Tauri stars: CV Cha and CR Cha

G. A. J. Hussain, A. Collier Cameron, M. M. Jardine, N. Dunstone, J. Ramirez Velez, H.C. Stempels, J.-F. Donati, M. Semel, G. Aulanier, T. Harries, J. Bouvier, C. Dougados, J. Ferreira, B.D. Carter, W.A. Lawson

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 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. These T Tauri stars are more massive and thus have larger radiative cores than the other classical T Tauri stars that have been studied using similar techniques: V2129 Oph and BP Tau. Both CV Cha and CR Cha show magnetic field patterns that are more complex than those recovered for BP Tau. 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. By comparing our maps with previously published maps of classical T Tauri stars, we infer that magnetic field patterns on T Tauris, and their underlying magnetic field generation mechanisms evolve quickly as they develop radiative cores. This may have implications for the efficiency with which T Tauri stars can effectively lock onto their surrounding disks under the magnetospheric accretion model scenario. Further observations of T Tauri stars are needed to confirm this hypothesis.

Revised draft submitted to MNRAS; this can be accessed here

Testing the magnetically confined wind model for Cep using Chandra and XMM phase-resolved X-ray observations

Favata, F., Neiner, C., Testa, P., Hussain, G.A.J., Sanz-Forcada, J.

We have performed a set of phase-resolved X-ray observations of the magnetic B star Beta Cep, for which theoretical models predict the presence of a confined wind emitting X-rays from stationary shocks. We obtained four observations spaced in rotational phase with XMM-Newton and with Chandra. A detailed analysis of the data was performed to derive both photometric and spectral parameters from the EPIC data, searching for rotational modulation, and to derive the location of the X-ray plasma from the line ratios in the He-like triplets of N, O and Ne from the RGS data. The LETG data were used to constrain the presence of bulk motions in the plasma. The strong rotational modulation predicted by the early, static magnetically confined wind model for the X-ray emission is not observed in Beta Cep. The small modulation present goes in the opposite direction, pointing to the absence of any optically thick disk of neutral material, and showing a modulation consistent with the later, dynamic models of magnetically confined wind models in B stars. The lack of observed bulk motion points to the plasma being confined by a magnetic field, but the low plasma temperature and lack of any flaring show that the plasma is not heated by magnetic reconnection. Therefore, the observations point to X-ray emission from shocks in a magnetically confined wind, with no evidence of an optically thick, dense disk at the magnetic equator.

A&A, in press (arXiv:0806.2275)

2008

Differential rotation on both components of the pre-main-sequence binary system HD 155555

Dunstone, N., Hussain, G.A.J., Collier Cameron, A., Marsden, S., Jardine, M., Barnes, J., Ramirez Velez, J., Donati, J.-F.

We present the first measurements of surface differential rotation on a pre-main-sequence binary system. Using intensity (Stokes I) and circularly polarized (Stokes V) time-series spectra, taken over 11 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 lap times as determined from the intensity spectra are 80 d for the primary star and 163 d for the secondary. Similarly, for the magnetic spectra we obtain equator-pole lap times of 44 and 71 d, respectively, showing that the shearing time-scale of magnetic regions is approximately half of 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: first that at the age of HD 155555 binary tidal forces have not yet had time to suppress differential rotation and 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.

2008, MNRAS, 387, 1525

The first magnetic maps of a pre-main-sequence binary star system - HD155555

Dunstone, N., Hussain, G.A.J., Cameron, A., Marsden, S., Jardine, M., Barnes, J., Ramirez Velez, J., Donati, J.

We present the first maps of the surface magnetic fields of a pre-main-sequence binary system. Spectropolarimetric observations of the young, 18Myr, HD155555 (V824Ara, 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 HD155555 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.

2008, MNRAS, 387, 481

Magnetospheric accretion on the T Tauri star BP Tauri

Donati, J., Jardine, M., Gregory, S., Petit, P., Paletou, P., Menard, J., Cameron, A., Harries, T., Hussain, G.A.J. et al.

From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star (cTTS) BP Tau. Circular polarization signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2 per cent of the stellar surface. The strong mainly axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully convective dwarfs. It suggests that magnetic fields of fully convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their convective zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau.

2008, MNRAS, 386, 1234

2007

Spectropolarimetric observations of the transiting planetary system of the K dwarf HD 189733

Moutou, C., Donati, J.-F., Savalle, R., Hussain, G.A.J, Alecian, E., Bouchy, F., Catala, C., Collier Cameron, A., Udry, S., Vidal-Madjar, A.

Context: With a Jupiter-mass planet orbiting at a distance of only 0.031 AU, the active K2 dwarf HD 189733 is a potential candidate in which to study the magnetospheric interactions of a cool star with its recently-discovered close-orbiting giant planet. Aims: We decided to explore the strength and topology of the large-scale magnetosphere of HD 189733, as a future benchmark for quantitative studies for models of the star/planet magnetic interactions. Methods: To this end, we used ESPaDOnS, the new generation spectropolarimeter at the Canada-France-Hawaii 3.6 m telescope, to look for Zeeman circular polarisation signatures in the line profiles of HD 189733 in 2006 June and August. Results: Zeeman signatures in the line profiles of HD 189733 are clearly detected in all spectra, demonstrating that a field is indeed present at the surface of the star. The Zeeman signatures are not modulated with the planet's orbital period but apparently vary with the stellar rotation cycle. The reconstructed large-scale magnetic field, whose strength reaches a few tens of G, is significantly more complex than that of the Sun; it involves in particular a significant toroidal component and contributions from magnetic multipoles of order up to 5. The Ca ii H & K lines clearly feature core emission, whose intensity is apparently varying mostly with rotation phase. Our data suggest that the photosphere and magnetic field of HD 189733 are sheared by a significant amount of differential rotation. Conclusions: Our initial study confirms that HD 189733 is an optimal target for investigating activity enhancements induced by closely orbiting planets. More data are needed, densely covering both the orbital and rotation cycles, to investigate whether and how much the planet contributes to the overall activity level of HD 189733.

2007, A&A, 473, 651

The coronal structure of AB Dor determined from contemporaneous Doppler imaging and X-ray spectroscopy

G. A. J. Hussain, M. Jardine, J.-F. Donati, N. S. Brickhouse, N. J. Dunstone, K. Wood , A. K. Dupree, A. Collier Cameron, F. Favata

Abstract: A We obtain contemporaneous observations of the surface and corona of AB Dor, a young single cool star, using ground-based circularly polarised spectra from the Anglo- Australian Telescope and X-ray lightcurves 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 Dors X-ray corona must be concentrated close to its surface, with a height, H 0.30.4R; this height is determined by the high coronal density and complex multi-polar magnetic field from the surface maps. There is also signif- icant correlation between the positions of surface and coronal active longitudes as determined from the surface spot and magnetic field maps and the X-ray lightcurve. At this epoch (2002 December) AB Dor appears to possess one very large active longi- tude 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 lightcurve de- pend on the distribution of magnetic field in the obscured hemisphere (AB Dor is inclined by 60 degrees). 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.

2007, MNRAS, 377, 1488


 
 
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last updated: 2009.01.21