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.
Highlights
M stars, the most common stars in the galaxy, are privileged targets for exoplanet research
As in Sezestre et al (2017), we studied four different scenarios: (1) the source of dust is fixed in the system, (2) the source is in Keplerian motion around the star with no constraint on the direction of the trajectories (“orbiting free” case), (3) the source is in Keplerian motion and all the structures move toward the observer (“forward” case), and 4) the source is in Keplerian motion and all the structures move away from the observer (“backward” case)
While the motion is obvious, even on a timescale of a few months, the morphology of each individual feature changes, implying that the measurement of their locations based on centroids and their associated projected speeds is complicated by this evolution such that we can no longer consider that the whole train of structures moves as a solid block
Summary
M stars, the most common stars in the galaxy, are privileged targets for exoplanet research. The gain in angular resolution and contrast with SPHERE compared to previous observations unveils several structures in the form of undulations or arches in the northeast part of the southeast side of the disk (Boccaletti et al 2015) Comparing these observations to reprocessed HST/STIS images from 2010/2011, we were able to identify five recurrent patterns across the three epochs, labeled A–E. Small dust particles would be released from this place and expelled by the stellar wind This theory involves several assumptions to qualitatively match the observations, but the dynamical behavior of the grains once released is quite similar to the case of a fixed source as proposed in Sezestre et al (2017).
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