Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

the central star. They regulate the distribution of angular momentum, provide the material and the energy to
launch jets, provide the raw material to form planets, etc... Not surprisingly, a large fraction of our activities
has been historically devoted to the search and study of these disks around young solar-like stars. With time
however, our spectrum of activities has broadened. Today, it includes the study of both to the inner parts of
these disks and the study of the mass-loss phenomenon, itself strongly coupled to the accretion process, and
possibly to the star-disk interaction zone. Disks around brown dwarfs and A stars are also considered now,
further broadening our spectrum.
To carry this research, LAOG proved to be a unique place. For example, to study the mass-loss process,
strong ties with SHERPAS gave us access to coherent, cutting edge physical models of MHD disk winds able to
predict jet rotation rates and emission-line ratios (Pesenti et al. 2004) (both now observed) while the presence of
GRIL and the technical group gave us prime access, very early on, to the best available high angular resolution
data obtained with adaptive optics and interferometry. In the sections below, we describe some of these results.
6.3.1 Observation and models of young accretion disks
The Outer dust disk
Studies of the properties of T Tauri disks were initially based on the analysis of the spectral energy distribution
(SED) only, for lack of available images (e.g., Beckwith et al. 1990, AJ, 99, 924). The first images of disks
seen in scattered light became available in 1996 with HST in the optical and ground-based adaptive optics in
the near-infrared (Burrows et al. 1996, ApJ, 473, 437 for HH 30 with HST; Roddier et al. 1996, ApJ, 463, 326
for GG Tau with AO). Maps of disks at longer wavelengths became available roughly at the same time with
millimeter interferometers (e.g., Dutrey et al. 1996, A&A, 309, 493).
Today, SED fitting still provides useful constraint on the disk structure, (e.g., inner radius, flaring surface,
vertical inner rim, ec) and to some extent on the dust properties (size distribution, opacity law) but the more
difficult image fitting, coupled with SED fitting, is proving much more powerful.
After being very active in searching for new disks in the previous 4-year period, but see Chauvin et al.
(2002) for the recent discovery of the edge-on disk Lk Hα 263 C, our efforts are now going into improving the
wavelength coverage of known disks, from scattered light images in the optical and near- to mid-infrared, e.g.,
Stapelfeldt et al. (2003); Duchêne et al. (2004) to thermal emission maps in the millimeter range, e.g., (Duchêne
et al. 2003). Our image fitting models, largely improved by Christophe Pinte (ongoing PhD thesis), is based
on a powerful 3D Monte Carlo radiative transfer scheme. The analysis and publication of several datasets is
currently underway.
In the visible and near-infrared (shortwards of 2µm), we have used VLT and HST to gather additional
images of several known disks. This effort involves long standing collaborations with Karl Stapelfeldt (JPL)
and Andrea Ghez (UCLA).
Simultaneous multi-wavelength image fitting is important because the scattering properties (i.e., albedo,
phase function, polarisation) of dust grains depend on wavelength and these model offer a better probe of the
grain size distribution and disk parameters than fitting a single image does.
Comparing our previous HST image of the edge-on disk around HK Tau B (Stapelfeldt et al. 1998) to a
2.2µm image of the system that we obtained with VLT’s NAOS (see Fig.6.1) shows that the dust grains in that
disk are at least as forward-throwing in the near-infrared than they are in the visible (Ménard et al. 2006),
clearly showing the presence of larger grains. This result is further supported by the detection of scattered light
at 12 microns (McCabe et al. 2003, see Fig.6.1).
This result is extremely interesting as grains several microns in size are needed to produce significant scattering
at 10µm. This encouraging results prompted us to obtain more observations in the mid-infrared (3–20 µm)
on other disks, in particular during the first public observing campaign offered with the laser-guided adaptive
optics systems on the Keck telescope and during commissioning of VISIR at the VLT. In all disks observed so
far, we have revealed the presence of much larger dust grains than those found in the interstellar medium.
In addition, in the GG Tau circumbinary ring, Duchêne et al. (2004) demonstrated that the scattered
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