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

systems: HD 141569, HD 32297, HD 181327. Other disks like HR 4796 and AU Mic for example were also
studied by our group. (Augereau et al. 1999, A&A, 348, 557; Boccaletti et al. 2003).
Since debris disks are optically thin, their dust density profiles can be directly extracted from spatially
resolved scattered light images. When available, the colors of the disk (in scattered light) can be related directly
to the size distribution of the smallest dust grains (Boccaletti et al. 2003; Augereau et al. 2004). The situation
is slightly more complex for edge-on disks (Beta Pic, AU Mic, HD32297 for instance) and a specific inversion
procedure was developed in order to reconstruct the dust density from brightness profiles (e.g., Augereau &
Beust, A&A, submitted for AU Mic). These profiles, once included into the debris disk radiative tranfert model
we developped to adjust the SED, give insight on the dust properties (grain size distribution, composition).
The model results serve as inputs for our dynamical modeling of the colliding planetesimals disks that release
the observed dust grains (e.g., Augereau et al. 2001). They are also incorporated into the chemistry model of
the remnant gas disk around HD 141569 that we have detected with the IRAM Interferometer (Augereau et al.
in prep.).
Although the inner regions of most debris disks appear to be largely dust-depleted, the actual amount of
dust is poorly known. Our group is involved in the detection and modeling of a very small amount of dust
in the Vega inner system (within about 1AU) based on CHARA/FLUOR infrared interferometric observations
(Absil et al., submitted). This exo-zodiacal dust population around Vega may be related to collisions among
asteroid-like objects and/or due to evaporating comets.
Beta Pictoris itself has been studied now for years by our group. In the past 4 years, a collaboration has
been initiated with members of the Laboratoire de Planétologie de Grenoble (LPG). The goal was to better
understand the physics of planetesimal evaporation in the vicinity of the star. Indeed, repeated transient changes
in the absorption spectrum of Beta Pictoris had been attributed for years to the sublimation of star-grazing
planetesimals. The goal of the collaboration was to translate models of solar system comets to the case of Beta
Pictoris to better understand the physics of their sublimation and better constrain the model. This constituted
the thesis of C.Karmann under the supervision of H.Beust. The main outcome of the study was that the
planetesimals in the Beta Pic system contain probably very little ice, and that their sublimation rate depends
on their distance to the star but also on their history, with possible seasonal effects (Karmann et al., 2001;
2003).
6.3.5 Dynamical evolution of young circumstellar environments
Our team has also developped a long-term collaboration with the group of John Papaloizou (QMWC and Cambridge,
UK) to model the complex structures of spatially resolved debris disks. The models involve pertubing
planets like in the Beta Pic and Vega disks (Augereau et al. 2001; Reche et al., in prep.) but also the effect
of radiation and wind drag forces on the grains (e.g., AU Mic, Augereau & Beust, submitted). In the case of
Beta Pic, this type of approach has allowed to place an upper limit on the gas mass in the system (Thébault &
Augereau, 2005). Some systems like HR 4796 are not isolated and the effect of stellar companions must be investigated.
In the case of HD 141569 for instance, we show that the large-scaled spiral structure observed likely
results from the secular perturbation of the disk by the two bound M-star companions (Augereau & Papaloizou
2004, see Fig. 6.5). As part of the PhD thesis of R.Reche, we are now exploring the effects of an additional
perturber, a Jupiter-mass planet, on the inner disk structure that remains unexplained.
The grains observed in debris disks arise from mutual collisions between larger particles (up to km-sized
bodies). We have thus started to numerically investigate the outcome of collisions in debris disks to predict the
size distribution as a function of time and distance from the star to see if it could affect the interpretation of
the observations, especially the mid-infrared excesses measured with ISO and Spitzer.
6.3.6 Other Topics in Star Formation
X-ray emission from Young Stellar Objects
Many members FOST are involved in several long exposure observations of star forming region called ‘large
projects’, a highly competitive category of Chandra and XMM-Newton observations. This effort is lead by
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