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A measure of the size of the magnetospheric accretion region in TW Hydrae

Abstract

Stars form by accreting material from their surrounding disks. There is a consensus that matter flowing through the disk is channelled onto the stellar surface by the stellar magnetic field. This is thought to be strong enough to truncate the disk close to the corotation radius, at which the disk rotates at the same rate as the star. Spectro-interferometric studies in young stellar objects show that hydrogen emission (a well known tracer of accretion activity) mostly comes from a region a few milliarcseconds across, usually located within the dust sublimation radius1,2,3. The origin of the hydrogen emission could be the stellar magnetosphere, a rotating wind or a disk. In the case of intermediate-mass Herbig AeBe stars, the fact that Brackett γ (Brγ) emission is spatially resolved rules out the possibility that most of the emission comes from the magnetosphere4,5,6 because the weak magnetic fields (some tenths of a gauss) detected in these sources7,8 result in very compact magnetospheres. In the case of T Tauri sources, their larger magnetospheres should make them easier to resolve. The small angular size of the magnetosphere (a few tenths of a milliarcsecond), however, along with the presence of winds9,10 make the interpretation of the observations challenging. Here we report optical long-baseline interferometric observations that spatially resolve the inner disk of the T Tauri star TW Hydrae. We find that the near-infrared hydrogen emission comes from a region approximately 3.5 stellar radii across. This region is within the continuum dusty disk emitting region (7 stellar radii across) and also within the corotation radius, which is twice as big. This indicates that the hydrogen emission originates in the accretion columns (funnel flows of matter accreting onto the star), as expected in magnetospheric accretion models, rather than in a wind emitted at much larger distance (more than one astronomical unit).

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Fig. 1: VLTI-GRAVITY observations of TW Hya.
Fig. 2: Visibility plot of TW Hya.

Data availability

This work is based on observations collected at the European Southern Observatory (ESO) under ESO programme 0102.C-0408(C). The raw data are publicly available in the ESO Science Archive Facility. The reduced data that support the findings of this study are available from the corresponding author under reasonable request.

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Acknowledgements

We thank C. Manara for providing the XSHOOTER spectrum of TW Hya and the template of the stellar photosphere. This material is based upon works supported by Science Foundation Ireland under grant number 18/SIRG/5597. M.K. was funded by the Irish Research Council (IRC), grant GOIPG/2016/769. R.F. acknowledges support from the Chalmers Initiative on Cosmic Origins (CICO) postdoctoral fellowship. A.C.G. and T.P.R. have received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement number 743029). A.N. acknowledges the hospitality of DIAS. A.A., M.F. and P.J.V.G. acknowledge funding by the Fundação para a Ciência e a Tecnologia, with grant references UID/FIS/00099/2013 and SFRH/BSAB/142940/2018. Th.H. acknowledges support from the European Research Council under the Horizon 2020 Framework Program via the ERC Advanced Grant Origins 832428.

Author information

Authors and Affiliations

Consortia

Contributions

GRAVITY is developed as a collaboration by the Max Planck Institute for Extraterrestrial Physics, LESIA of Paris Observatory and the IPAG of Université Grenoble Alpes/CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Centro Multidisciplinar de Astrofisica Lisbon and Porto, and the European Southern Observatory. Authors from these institutes contributed to the concept, design, assembly, instrumental tests, science cases, verification and implementation of GRAVITY and its subsystems, and the data reduction pipeline. P.J.V.G. conducted the observations. R.G.L., K.P. and M.K. reduced the data. R.G.L. and M.K. analysed the data. A.N. estimated the location of the disk rim, and the corotation and truncation radii. R.F. performed the model fitting of the continuum-subtracted visibilities. R.G.L. wrote the manuscript. A.N., T.P. and A.C.G. edited the manuscript. R.G.L., A.N., A.C.G., T.P.R., R.F., M.K., L.K., K.P., J.S.-B., M.B., C.D., L.L., W.B., P.J.V.G., Th.H., P.C., G.D., T.Z. and R.G. discussed the results and their implications, and commented on the manuscript.

Corresponding author

Correspondence to R. Garcia Lopez.

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Extended data figures and tables

Extended Data Fig. 1 Best-fit model to the continuum-subtracted Brγ line visibilities.

Continuum subtracted visibilities are represented in colour in the u–v plane. The symbol size indicate the error of each single data point. For comparison, the average visibility error is represented by the dark full circle at the bottom right of the figure. Contours represent the visibility values of the best two-dimensional Gaussian model.

Extended Data Fig. 2 Sketch of the inner-disk region of TW Hya.

The main features of the inner disk are represented: the dusty disk (brown), the dust sublimation radius located at about 7.5R⁎, the inner gaseous disk (blue), truncated by the stellar magnetosphere (red) at about 3.5R⁎, along with the Brγ line emitting region, which is probably tracing the width of the accretion columns.

Extended Data Table 1 Observation log of the VLTI GRAVITY+UT high-resolution observations of TW Hya

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GRAVITY Collaboration. A measure of the size of the magnetospheric accretion region in TW Hydrae. Nature 584, 547–550 (2020). https://doi.org/10.1038/s41586-020-2613-1

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