Abstract
Abstract We present new high-fidelity optical coronagraphic imagery of the inner ∼50 au of AU Mic’s edge-on debris disk using the BAR5 occulter of the Hubble Space Telescope Imaging Spectrograph (HST/STIS) obtained on 2018 July 26–27. This new imagery reveals that “feature A,” residing at a projected stellocentric separation of 14.2 au on the southeast side of the disk, exhibits an apparent “loop-like” morphology at the time of our observations. The loop has a projected width of 1.5 au and rises 2.3 au above the disk midplane. We also explored Transiting Exoplanet Survey Satellite photometric observations of AU Mic that are consistent with evidence of two starspot complexes in the system. The likely co-alignment of the stellar and disk rotational axes breaks degeneracies in detailed spot modeling, indicating that AU Mic’s projected magnetic field axis is offset from its rotational axis. We speculate that small grains in AU Mic’s disk could be sculpted by a time-dependent wind that is influenced by this offset magnetic field axis, analogous to co-rotating solar interaction regions that sculpt and influence the inner and outer regions of our own Heliosphere. Alternatively, if the observed spot modulation is indicative of a significant misalignment of the stellar and disk rotational axes, we suggest that the disk could still be sculpted by the differential equatorial versus polar wind that it sees with every stellar rotation.
Highlights
AU Mic is a nearby (9.8 pc; Gaia Collaboration et al 2018), M1Ve star that is a member of the 23±3 Myr old β Pic moving group (Mamajek & Bell 2014)
We found that while chromatic residuals were fully mitigated with our disk-obscurred BAR5 observation of AU Mic on 22 Sept 2018, using this orbit as a point spread function (PSF) template led to substantial PSF subtraction residuals caused by differential wavefront errors induced by the non-contemporaneous (e.g. ∼2 month separation) of this observation from our 3 sequential orbits that resolved AU Mic’s disk
It is not possible to determine whether the “loop-like” morphology of feature A remains coherent as the feature moves within the disk, due to the single-epoch nature of these high resolution data
Summary
AU Mic is a nearby (9.8 pc; Gaia Collaboration et al 2018), M1Ve star that is a member of the 23±3 Myr old β Pic moving group (Mamajek & Bell 2014). Chiang & Fung (2017) proposed that the moving features in AU Mic’s disk are caused by the interaction between the star’s wind and repeated “dust avalanche” events These avalanches are triggered in a zone marked by the intersection of AU Mic’s primary debris ring and a proposed secondary ring of dust left behind by the catastrophic disruption of an object up to the size of the Kuiper Belt Object Varuna (radius 450km; Lellouch et al 2013).
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