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Fast-moving features in the debris disk around AU Microscopii

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Abstract

In the 1980s, excess infrared emission was discovered around main-sequence stars; subsequent direct-imaging observations revealed orbiting disks of cold dust to be the source1. These ‘debris disks’ were thought to be by-products of planet formation because they often exhibited morphological and brightness asymmetries that may result from gravitational perturbation by planets. This was proved to be true for the β Pictoris system, in which the known planet generates an observable warp in the disk2,3,4,5. The nearby, young, unusually active late-type star AU Microscopii hosts a well-studied edge-on debris disk; earlier observations in the visible and near-infrared found asymmetric localized structures in the form of intensity variations along the midplane of the disk beyond a distance of 20 astronomical units6,7,8,9. Here we report high-contrast imaging that reveals a series of five large-scale features in the southeast side of the disk, at projected separations of 10–60 astronomical units, persisting over intervals of 1–4 years. All these features appear to move away from the star at projected speeds of 4–10 kilometres per second, suggesting highly eccentric or unbound trajectories if they are associated with physical entities. The origin, localization, morphology and rapid evolution of these features are difficult to reconcile with current theories.

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Figure 1: High-contrast images of the AU Mic debris disk.
Figure 2: Extraction of disk substructure from the southeastern side.
Figure 3: Disk features across three epochs.
Figure 4: Projected speeds of the disk features.

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Acknowledgements

SPHERE was built by a European consortium led by IPAG (France). SPHERE was funded by the ESO, with additional contributions from CNRS, MPIA, INAF, FINES and NOVA. SPHERE also received funding from the European Commission FP6 and FP7 programmes as part of OPTICON under grant numbers RII3-Ct-2004-001566 (FP6), 226604 (FP7) and 312430 (FP7). French co-authors are supported by ANR-14-CE33-0018. Part of this work has been carried out within the framework of the National Centre for Competence in Research PlanetS supported by the Swiss National Science Foundation. C.T. and M.R.M. acknowledge the financial support of the SNSF.This study is based on observations from program 60.A-9249(C) at ESO Very Large Telescope and from program number 12228 made with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by AURA Inc. under NASA contract NAS 5-26555. We are also grateful to the ESO for releasing the commissioning data for publication. Finally, we thank P. Zarka, N. Meyer-Vernet, B. Stelzer and Q. Kral for discussions. J.S. is a NASA Postdoctoral Program Fellow.

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Authors

Contributions

A.B. reduced and analysed IRDIS data and wrote the paper. C.T. reduced the IRDIS and ZIMPOL data and contributed to the manuscript writing. A.B., C.T., A.-M.L., M.J., J.-C.A., G.S., C.G., J.D., D.M., T.H., C.D., C.G., J.O. and J.S. worked on the interpretation of the results. G.S., C.G., J.D., D.H., T.R. and J.W. re-reduced HST data. J.M., M.L., D. Mouillet., D. Mawet, J.H.G. and Z.W. operated the instrument at the telescope. A.-L.M. worked on the astrometric calibration. A.B., A.-M.L, M.L., D. Mouillet, T.H., J.-L.B., K.D., M.F., T.F., M.K., F.M., M.M., C.M., J.-F.S, H.M.S, M.T., S.U, F.V. and A.V. contributed to the instrument conception. All authors commented on the manuscript.

Corresponding author

Correspondence to Anthony Boccaletti.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Limit of detection to point sources.

The contrast is measured at 5σ using fake planets introduced to the data at discrete positions (circles) along the disk midplane to account for the self-subtraction of the ADI/KLIP algorithm. The dashed line defines the edge of the coronagraphic mask at 0.09″.

Extended Data Figure 2 Comparison of IRDIS and ZIMPOL images.

a and b show zoomed-in regions of the KLIP and LOCI reductions of the IRDIS infrared data, whereas c is taken from the conservative LOCI reduction of the ZIMPOL optical data. Features A and B are reproduced accurately in the ZIMPOL data. An additional substructure between feature B and the midplane is also detected, as indicated by arrows. The yellow star symbol indicates the position of the star.

Extended Data Figure 3 Spine of the disk measured in SPHERE IRDIS data.

The spine is measured using several reductions (noADI, ADI, KLIP) of the SPHERE IRDIS 2014 data. Average values and dispersions (error bars) are plotted as a blue line. For each region where a local maxima is identified, a Gaussian + first-order polynomial model is fitted in order to register precisely the five features.

Extended Data Figure 4 Central part of the SPHERE IRDIS image.

a shows a 12″ field of view of the SPHERE IRDIS image processed with the KLIP algorithm and b is a magnified version to indicate the bow-like deviation of the disk to the southeast in the central area (for separations shorter than ~0.7″). The horizontal dotted lines indicate the disk midplane.

Extended Data Figure 5 Positions of the disk features over time.

The positions of the features measured in the SPHERE and HST images are plotted as circles together with peak-to-valley error bars (in some cases, the errors are smaller than the symbol size). Linear fits on these three epochs illustrate the possible track of each feature. The black symbols show the location at which various inhomogeneities were reported in the literature, on the basis of older data6,7,8,9. The colour coding is the same as in Fig. 4.

Extended Data Table 1 Registration of features

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Boccaletti, A., Thalmann, C., Lagrange, AM. et al. Fast-moving features in the debris disk around AU Microscopii. Nature 526, 230–232 (2015). https://doi.org/10.1038/nature15705

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