Observations of fast-moving features in the debris disk of AU Mic on a three-year timescale: Confirmation and new discoveries^★,★★

¹ LESIA, Observatoire de Paris, PSL Research Univ., CNRS, Univ. Paris Diderot, Sorbonne Paris Cité(c), UPMC Paris 6, Sorbonne Univ., 5 place Jules Janssen, 92195 Meudon, France
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ CRAL, UMR 5574, CNRS/ENS-L/Université Lyon 1, 9 av. Ch. André, 69561 Saint-Genis-Laval, France
⁵ Aix Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁶ Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
⁹ Steward Observatory, 933 North Cherry Avenue, The University of Arizona Tucson, Tucson, AZ 85719, USA
¹⁰ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001 Vitacura, Santiago 19, Chile
¹¹ Eureka Scientific, 2452 Delmer, Suite 100, Oakland, CA 96002, USA
¹² Space Telescope Science Institute, 3700 San Martin Dr. Baltimore, MD 21218, USA
¹³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁴ Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
¹⁵ Núcleo Milenio Formación Planetaria - NPF, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
¹⁶ Department of Physics & Astronomy, College of Charleston, 66 George Street, Charleston SC 29424, USA
¹⁷ Department of Physics and Astronomy, The University of Oklahoma, 440 W. Brooks St., Norman, OK 73019, USA
¹⁸ Exoplanets & Stellar Astrophysics Laboratory, Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
²⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
²¹ Université Cote d’Azur, OCA, CNRS, 66 George Street, Charleston, SC 29424, USA
²² Institute for Astronomy, University of Edinburgh, Blackford Hill View, Edinburgh EH9 3HJ, UK
²³ Département d’Astronomie, Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
²⁴ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
²⁵ Núcleo de Astronoma, Facultad de Ingeniera, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
²⁶ NOVA Optical Infrared Instrumentation Group, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

Received: 14 December 2017
Accepted: 8 March 2018

Abstract

Context. The nearby and young M star AU Mic is surrounded by a debris disk in which we previously identified a series of large-scale arch-like structures that have never been seen before in any other debris disk and that move outward at high velocities.

Aims. We initiated a monitoring program with the following objectives: (1) track the location of the structures and better constrain their projected speeds, (2) search for new features emerging closer in, and ultimately (3) understand the mechanism responsible for the motion and production of the disk features.

Methods. AU Mic was observed at 11 different epochs between August 2014 and October 2017 with the IR camera and spectrograph of SPHERE. These high-contrast imaging data were processed with a variety of angular, spectral, and polarimetric differential imaging techniques to reveal the faintest structures in the disk. We measured the projected separations of the features in a systematic way for all epochs. We also applied the very same measurements to older observations from the Hubble Space Telescope (HST) with the visible cameras STIS and ACS.

Results. The main outcomes of this work are (1) the recovery of the five southeastern broad arch-like structures we identified in our first study, and confirmation of their fast motion (projected speed in the range 4–12 km s⁻¹); (2) the confirmation that the very first structures observed in 2004 with ACS are indeed connected to those observed later with STIS and now SPHERE; (3) the discovery of two new very compact structures at the northwest side of the disk (at 0.40′′ and 0.55′′ in May 2015) that move to the southeast at low speed; and (4) the identification of a new arch-like structure that might be emerging at the southeast side at about 0.4′′ from the star (as of May 2016).

Conclusions. Although the exquisite sensitivity of SPHERE allows one to follow the evolution not only of the projected separation, but also of the specific morphology of each individual feature, it remains difficult to distinguish between possible dynamical scenarios that may explain the observations. Understanding the exact origin of these features, the way they are generated, and their evolution over time is certainly a significant challenge in the context of planetary system formation around M stars.