Multiwavelength Vertical Structure in the AU Mic Debris Disk: Characterizing the Collisional Cascade

Abstract

Debris disks are scaled-up analogs of the Kuiper Belt in which dust is generated by collisions between planetesimals. In the collisional cascade model of debris disks, the dust lost to radiation pressure and winds is constantly replenished by grinding collisions between planetesimals. The model assumes that collisions are destructive and involve large velocities; this assumption has not been tested beyond our solar system. We present 0.″25 (≈2.4 au) resolution observations of the λ = 450 μm dust continuum emission from the debris disk around the nearby M dwarf AU Microscopii with the Atacama Large Millimeter/submillimeter Array. We use parametric models to describe the disk structure, and a Monte Carlo Markov Chain (MCMC) algorithm to explore the posterior distributions of the model parameters; we fit the structure of the disk to both our data and archival λ = 1.3 mm data (Daley et al. 2019), from which we obtain two aspect ratio measurements at 1.3 mm (h 1300 = 0.025 − 0.002 + 0.008 ) and at 450 μm (h 450 = 0.019 − 0.001 + 0.006 ), as well as the grain-size distribution index q = 3.03 ± 0.02. Contextualizing our aspect ratio measurements within the modeling framework laid out in Pan & Schlichting (2012), we derive a power-law index of velocity dispersion as a function of grain size p = 0.28 ± 0.06 for the AU Mic debris disk. This result implies that smaller bodies are more easily disrupted than larger bodies by collisions, which is inconsistent with the strength regime usually assumed for such small bodies. Possible explanations for this discrepancy are discussed.

The Geometry of the G29-38 White Dwarf Dust Disk from Radiative Transfer Modeling

“…The AU Mic debris disk was imaged with ALMA to have a half-opening angle of Θ = 1°.1-1°. 8, depending on the wavelength and specific analysis (Daley et al 2019;Vizgan et al 2022). Similar analyses found Θ = 2°.3 for the disk around HR 4796 (Kennedy et al 2018) and Θ = 2°.…”

JWST/NIRCam Coronagraphy of the Young Planet-hosting Debris Disk AU Microscopii

“…Based on modeling of the AU Mic disk in ALMA dust continuum emission, Pearce et al (2022) and Vizgan et al (2022) each provide estimates for a yet-unseen companion that might be responsible for shaping the inner edge of the disk. Pearce et al (2022) find a companion mass and semimajor axis of ∼0.44 M J and ∼21.9 au (from an inner disk radius of 28.7 au), while Vizgan et al (2022) find a companion mass and semimajor axis of ∼0.34 M J and ∼17 au (from an inner disk radius of 22.1 au). Permitting the aforementioned assumptions, our analysis indicates that we would have overall detection probabilities of ∼91% and ∼87% (respectively) for these planets in F444W.…”

“…While the methodology of Carter et al (2023) uses an 11 × 11 pixel coronagraph-centered box for relative centering, our data span a much larger temporal baseline-making small-separation changes to the diffraction pattern more likely. Within the aforementioned annular region considered for relative centering, centering for the AU Mic images explicitly excludes a rectangular region approximately aligned with the disk's major axis (adjusted for the parallactic angle of the exposure)-assuming a disk position angle of 128°.48 (Vizgan et al 2022) and a width of 12 pixels. Once these offsets are computed, we shift all of the integrations such that the stars are aligned with the reference pixel for the NIRCam long wavelength (LW) target acquisition (TA) filter, F335M.…”

“…27 The region of interest includes pixels with either r < 35 pixels or falling within a stellocentric rectangular region rotated to the approximate position angle of the disk (PA =128°. 48; Vizgan et al 2022) with a width of 12 pixels and a length of 94 pixels (where the detection in the LOCI RDI images begins to wane)-but excluding the region within three pixels of the star (where small differences in coronagraph alignment can produce significant stellar residuals that might otherwise impact disk model fitting). This region of interest is intended to include the majority of the disk flux while also including the inner background region where oversubtraction is most evident in the unconstrained LOCI result.…”

The Geometry of the G29-38 White Dwarf Dust Disk from Radiative Transfer Modeling

“…The edge-on Kuiper belt-like debris disk around β Pictoris has dust in two overlapping vertical distributions: a dynamically cold component with a half-opening angle of Θ = 0.8 • and a dynamically hot component with Θ = 6.3 • , as measured by ALMA (Matrà et al 2019). The AU Mic debris disk was imaged with ALMA to have a half-opening angle of Θ = 1.1-1.8 • , depending on the wavelength and specific analysis (Daley et al 2019;Vizgan et al 2022). Similar analyses found Θ = 2.3 • for the disk around HR 4796 (Kennedy et al 2018) and Θ = 2.9 • for the disk around q 1 Eri (Lovell et al 2021).…”