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

6.3.3 The physics and origins of mass-loss in T Tauri stars
The physical mechanism by which mass is ejected from accreting systems and collimated into jets is another
fundamental problem in star formation that FOST is interested in. MHD accretion-driven wind models appear
required to explain the efficient collimation and the large ejection to accretion ratio of 0.01-0.1. Two broad
classes of models have been proposed: ejection from the stellar surface, or magnetocentrifugal ejection from
the associated accretion disk. Distinguishing between these scenarii is vital not only for understanding the jet
phenomenon in itself, but also for modelling the formation of exoplanets, as they have distinct implications on
the density structure and the migration processes in the inner regions of protoplanetary disks.
At the T Tauri phase (at ages > 10 6 yrs), most of the residual infalling envelope is evacuated, allowing direct
observation of the inner 100 AUs where ejection models predict that most of the collimation and acceleration
processes occur. Since 1996, C.Dougados and co-workers have been conducting sub-arcsecond observations of
the central regions (a few 100 AUs) of the jets associated with T Tauri stars. These observations are confronted
to detailed predictions from MHD wind models to constrain the jet launching mechanism in these sources.
This work is done in close collaboration with S. Cabrit (LERMA, Obs. de Paris), J. Ferreira (LAOG) and
P. Garcia (University of Porto). A thesis work on this subject was conducted by N.Pesenti (LAOG). These
observational constraints have so far favored a disk wind origin: the increase of jet widths with distance, the
onion-like structure of the DG Tau jet with faster gas nested inside slower material, and the detection of
rotation signatures are all successfully reproduced by MHD wind models with launch radii of 0.1-3 AU. Our
main contributions in the past 4 years are detailed below.
One of the major breakthroughs, in recent years, has been the report of tentative rotation signatures in 4
T Tauri microjets (Bacciotti et al. 2000, ApJ, 537, L49; Woitas et al. 2002, ApJ, 580, 336; Coffey et al. 2004,
Ap&SS, 292, 553). It has been soon recognized that the measure of azimuthal velocities is a powerful tool to
constrain the streamline launching radius (Bacciotti et al. 2002, ApJ, 576, 222). Azimuthal velocities directly
computed from observed velocity shifts indicate launching radii in the range 0.1-3 AU, the strongest indication
to date of a disc wind origin. In his thesis work, Nicolas Pesenti, has shown however that projection and dilution
effects significantly alter the derivation of azimuthal velocities from observed velocity shifts and that a careful
and detailed comparison with model predictions is required (Pesenti et al. 2003). Extending the analytical
work of Anderson et al. (2003), we have also computed predicted azimuthal velocities in the context of X- and
stellar-winds (Ferreira et al. submitted).
In order to fully test the reported rotation detections in T Tauri jets, we have also launched an observational
campaign with the IRAM Plateau de Bure interferometer to map rotation in the associated disk. So far disk
rotation has been checked against jet rotation for only 2 sources and the results are perplexing. While the
DG Tau circumstellar disk appears to rotate in the same direction as reported for the jet (Testi et al. 2002,
A&A, 394, L31), the RW Aur disk does not! (Cabrit et al. A&A submitted). These observations further suggest
that care should be taken in the interpretation of reported T Tauri jet rotation signatures. Further jet sources
will be observed at IRAM/PdBI in the forthcoming years within the JETSET/RTN collaboration.
Exploring jet tracers in the near-infrared domain is critical to analyse current AO observations obtained on
8m-class telescopes and efficiently prepare future observations with AMBER/VLTI (The LAOG is co-I on 2
GTO AMBER/VLTI observations on T Tauri jet sources). Another aspect of Pesenti’s thesis work was therefore
to develop a tool for the prediction of the near-infrared [Fe ii] line emission of wind models. This tool was first
applied to the self-similar disk wind models developped by Ferreira and collaborators from SHERPAS (Pesenti
et al. 2003).
6.3.4 Observations and models of the more evolved “Debris disks”
Debris disks are circumstellar dust disks imaged in scattered light around young main sequence stars (typically
10 Myr or older). Contrary to genuine protoplanetary disks, these environments contain almost no gas and are
optically thin. Their dynamics is therefore mostly gravitational. These disks are interesting because they are
probably young planetary systems that are not fully dynamically relaxed. Their study is then of great interest
for our understanding of the processes that give birth and make evolve planetary systems in general.
The prototype of these disks is the dust disk orbiting the star Beta Pictoris which remained, for many years,
the only one known. In the past 5 years, our group has been involved in the discovery of new debris disk
87