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Post Info TOPIC: AB Doradus
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RE: AB Doradus
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Title: The size of ABDorA from VLTI/AMBER interferometry
Authors: J.C. Guirado, J.M. Marcaide, I. Marti-Vidal, J.-B. Le Bouquin, L.M. Close, W.D. Cotton, J. Montalban

The pre-main sequence (PMS) star ABDorA is the main component of the quadruple system ABDoradus. The precise determination of the mass and photometry of the close companion to ABDorA, ABDorC, has provided an important benchmark for calibration of theoretical evolutionary models of low-mass stars. The limiting factor to the precision of this calibration is the age of the system, as both the mass and luminosity of ABDorA and C are well monitored by other ongoing programs. In this paper we present VLTI/AMBER observations of ABDorA which provide a direct measurement of the size of this star, 0.96±0.06 Rsun. The latter estimate, combined with other fundamental parameters also measured for this star, allows a precise test of PMS evolutionary models using both H-R diagrams and mass-radius relationships. We have found that our radius measurement is larger than that predicted by the models, which we interpret as an evidence of the oversizing produced by the strong magnetic activity of ABDorA. Considering, at least partially, this magnetic effect, theoretical isochrones have been used to derive constraints to the age of ABDorA, favouring an age about 40-50 Myr for this system. Older ages are not completely excluded by our data.

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Blobrana


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Title: The Coronal Structure of AB Doradus
Authors: O. Cohen, J.J. Drake, V.L. Kashyap, G.A.J. Hussain, T.I. Gombosi

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 Alfven 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, parametrised 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 x 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.

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Blobrana


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Posts: 131433
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AB Doradus C
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Title: Coronagraphic near-IR photometry of AB Dor C
Authors: A. Boccaletti, G. Chauvin, P. Baudoz, J.-L. Beuzit

Observations of low-mass companions for which the dynamical masses are well constrained help to improve the calibration of evolutionary models. Such observations thereby provide more confidence in the estimation of the mass of a companion using the photometric methods expected for the next generation of planet finder instruments. The commissioning of a new coronagraph at the Very Large Telescope (VLT) was the occasion to test the performance of this technique on the well-known object AB Dor A and its 0.09 solar masses companion AB Dor C. The purpose of this paper is to refine the photometric analysis on this object and to provide an accurate photometric error budget. In addition to coronagraphy, we calibrated the residual stellar halo with a reference star. We used standard techniques for photometric extraction. The companion AB Dor C is easily detected at 0.185" from the primary star, and its magnitudes in H and Ks are in agreement with an M5.5 object, as already known from spectroscopic observations. However, these new measurements make the earlier J-band photometry less reliable. Finally, the comparison with evolutionary models supports an age of (75 ± 25) Myr, contrary to previous analyses. These observations demonstrate that coronagraphic observations can be more efficient than direct imaging, not only to improve contrast, but also to provide a better photometric estimation as long as a good calibration of the stellar halo is achieved.

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Blobrana


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Posts: 131433
Date:
AB Doradus
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Title: Modelling the time variation of the surface differential rotation in AB Doradus and LQ Hydrae
Authors: A. F. Lanza

Sequences of Doppler images of the young, rapidly rotating late-type stars AB Dor and LQ Hya show that their equatorial angular velocity and the amplitude of their surface differential rotation vary versus time. Such variations can be modelled to obtain information on the intensity of the azimuthal magnetic stresses within stellar convection zones. We introduce a simple model in the framework of the mean-field theory and discuss briefly the results of its application to those solar-like stars.

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