[Paper Review] On the AU Mic debris disk: density profiles, grain properties and dust dynamics

J.‐C. Augereau, H. Beust|ArXiv.org|Apr 13, 2006

Stellar, planetary, and galactic studies1 references52 citations

TL;DR

This study investigates the AU Mic debris disk using scattered light profiles and spectral energy distribution, showing that stellar wind pressure—enhanced by recurrent flares—explains the disk's extended structure and similarity to β Pictoris. Unlike radiation pressure, the wind efficiently redistributes small grains, producing a r⁻⁵ brightness profile and color gradient beyond 35 AU via size-dependent dynamics.

ABSTRACT

We present the first comprehensive analysis of the AU Mic debris disk properties since the system was discovered by Kalas et al. (2004), and we explore whether the dynamical model, successful to reproduce the Beta Pic brightness profile could apply to AU Mic. We calculate the surface density profile of the AU Mic disk by performing the inversion of the near-IR and visible scattered light brightness profiles measured by Liu (2004a) and Krist et al. (2005), respectively. We discuss the grain properties by analysing the blue color of the disk in the visible (Krist et al. 2005) and by fitting the disk spectral energy distribution. We show that irrespective of the mean scattering asymmetry factor of the grains, most of the emission arises from an asymmetric, collisionally-dominated region that peaks close to the surface brightness break around 35 AU. The elementary scatterers at visible wavelengths are found to be sub-micronic, but the inferred size distribution underestimates the amount of large grains, resulting in too low sub-millimeter emissions compared to the observations. From our inversion procedure, we find that the V- to H-band scattering cross sections ratio increases outside 40 AU, in line with the observed color gradient of the disk. We show that a standard, solar-like stellar wind generates a pressure force onto the dust particles that behaves much like a radiation pressure force. With an assumed Mdot ~ 300 Mdot_sun, the wind pressure overcomes the radiation pressure and this effect is enhanced by the stellar flares. This explains the similarity between the Beta Pic and AU Mic brightness profiles. In both cases, the color gradient beyond 120 AU for Beta Pic and 35 AU for AU Mic, is believed to be a direct consequence of the dust dynamics.

Motivation & Objective

• Understand the origin of the AU Mic debris disk's extended scattered light profile and color gradient.
• Assess whether dynamical mechanisms successful for β Pictoris apply to AU Mic.
• Determine the role of stellar flares and wind forces in shaping dust distribution.
• Reconcile observed sub-millimeter emission with inferred grain size distributions.
• Evaluate the impact of time-variable radiation pressure due to flares on dust particle orbits.

Proposed method

• Invert near-IR and visible scattered light brightness profiles (from Liu 2004a and Krist et al. 2005) to derive the surface density profile.
• Analyze disk color and spectral energy distribution to infer grain properties and size distribution.
• Model radiation and wind forces on dust grains, including time-variable radiation pressure from flares.
• Use orbital mechanics to compute effective β_pr ratios under periodic flares, treating flares as transient perturbations.
• Integrate energy changes from flare episodes to derive an effective orbital evolution with time-averaged β_pr.
• Compare model predictions (brightness, color, sub-mm emission) with observations to test consistency.

Experimental results

Research questions

• RQ1 Why does the AU Mic disk exhibit a surface brightness profile that falls as r⁻⁵ beyond ~35 AU, similar to β Pictoris?
• RQ2 What causes the observed blue color gradient in the visible and increasing V-to-H band cross-section ratio beyond 40 AU?
• RQ3 How do recurrent X-ray and UV flares affect dust dynamics in the AU Mic system?
• RQ4 Why is the observed sub-millimeter emission lower than predicted by grain size distributions inferred from scattered light?
• RQ5 Can stellar wind pressure, enhanced by flares, explain the extended dust distribution in AU Mic as in β Pictoris?

Key findings

• The scattered light emission in AU Mic primarily arises from an asymmetric, collisionally dominated region peaking near 35 AU, the surface brightness break.
• Elementary scatterers in visible light are sub-micron grains, but the inferred size distribution underestimates large grains, leading to too low predicted sub-millimeter emission.
• The V-to-H band scattering cross-section ratio increases beyond 40 AU, consistent with a color gradient driven by size-dependent dust dynamics.
• Grains beyond the parent body disk outer edge are depleted of large particles due to pressure forces placing them on high-eccentricity orbits.
• Radiation pressure is inefficient at dispersing small grains in AU Mic due to its low luminosity, even with flares.
• A standard solar-like stellar wind with Ṁ ≈ 3×10² M⊙/yr generates a pressure force comparable to radiation pressure, enhanced by flares, which efficiently populates the outer disk and explains the r⁻⁵ profile and color gradient.