Issue |
A&A
Volume 455, Number 3, September I 2006
|
|
---|---|---|
Page(s) | 987 - 999 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20054250 | |
Published online | 16 August 2006 |
On the AU Microscopii debris disk
Density profiles, grain properties, and dust dynamics
1
Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France e-mail: augereau@obs.ujf-grenoble.fr
2
Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
Received:
23
September
2005
Accepted:
15
April
2006
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
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 V- to 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 assumed ,
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.
Key words: stars: circumstellar matter / stars: individual: AU Mic / stars: flare / planetary systems: formation / scattering
© ESO, 2006
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