On the AU Microscopii debris disk

Abstract

Context. AU Mic is a young M-type star surrounded by an edge-on optically thin debris disk that shares many common observational properties with the disk around β Pictoris. In particular, the scattered light surface brightness profile falls off as ∼r −5 outside 120 AU for β Pictoris and 35 AU for AU Mic. In both cases, the disk color rises as the distance increases beyond these reference radii. Aims. In this paper, we present the first comprehensive analysis of the AU Mic disk properties since the system was resolved by Kalas et al. (2004, Science, 303, 1990. We explore whether the dynamical model, which successfully reproduces the β Pictoris brightness profile (e.g., Augereau et al. 2001, A&A, 370, 447), could apply to AU Mic. Methods. 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 (2004, Science, 305, 1442 and Krist et al. (2005, AJ, 129, 1008, 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. Finally, we evaluate the radiation and wind forces on the grains. The impact of the recurrent X-ray and UV-flares on the dust dynamics is also discussed. Results. 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 number of large grains, resulting in sub-millimeter emissions that are too low compared to the observations. From our inversion procedure, we find that the Vto H-band scattering cross sections ratio increases outside 40 AU, in line with the observed color gradient of the disk. This behavior is expected if the grains have not been produced locally, but placed in orbits of high eccentricity by a size-dependent pressure force, resulting in a paucity of large grains beyond the outer edge of the parent bodies' disk. Because of the low luminosity of AU Mic, radiation pressure is inefficient to diffuse the smallest grains in the outer disk, even when the flares are taken into account. Conversely, 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 assumedṀ 3 × 10 2Ṁ , the wind pressure overcomes the radiation pressure, and this effect is enhanced by the stellar flares. This greatly contributes to populating the extended AU Mic debris disk and explains the similarity between the β Pictoris and AU Mic brightness profiles. In both cases, the color gradient beyond 120 AU for β Pictoris and 35 AU for AU Mic, is believed to be a direct consequence of the dust dynamics.

A DEBRIS disk around the planet hosting M-star GJ 581 spatially resolved withHerschel

“…Currently, among the nearby M-stars, the only spatially resolved debris disk is around the very young M1 star AU Mic Liu et al 2004;Krist et al 2005;Wilner et al 2012) which has been modeled by Augereau & Beust (2006) and Strubbe & Chiang (2006). In addition, there are a few candidate disks with excesses above photospheric level (e.g.…”

Vertical structure of debris discs

“…To push our analysis a step further, we performed one additional run specifically designed to AU Mic (β Pictoris has a far more complex radial and vertical structure, especially its warp, and is thus not an ideal candidate for such a simple fit). For this task, we take input parameters adapted to this specific system, i.e., a "birth ring" (where all non-radiation-pressure-affected larger particles are located) centered on ∼40 AU and of width ∼5 AU (Augereau & Beust 2006;Strubbe & Chiang 2006). As in our previous runs, we consider a limiting dynamically cold case with e dyn = i initial = 0, the remaining parameters being those for our nominal set-up (Table 1).…”

“…Of course, this result should not be regarded as proof that the AU Mic disc is completely dynamically cold. It is just a 8 AU Mic is an M star where, in addition to radiation pressure, a stellar wind is also acting to place small grains on eccentric or unbound orbits (Wood et al 2002;Augereau & Beust 2006;Strubbe & Chiang 2006;Fitzgerald et al 2007). To first order, this additional force has the same dependency on particle size and radial distance as radiation pressure.…”

Detecting circumstellar disks around gravitational microlenses

“…A thin disk with such low optical depths produces relative deviations of the light curve on the order of 0.1%. Current ground-based microlensing observations require an order of magnitude higher deviation and so could detect an average optical depth >5 × 10 −3 ; according to Augereau & Beust (2006). This would be the maximal optical depth in the visible for the AU Mic debris disk, transversed in the vertical direction, but for an edge-on orientation, optical depths of 4×10 −2 could be reached.…”