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

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such mid-term (several weeks) magnetospheric cycles from the observed modulation of both accretion and wind diagnostics in long time series of observations (Bouvier et al. 2003). These results first confirm the validity of the magnetospheric accretion paradigm but also prompt new developments in the analytical and numerical models. As observations suggest, the magnetically-mediated accretion process is highly non-axisymmetric and quite time-dependent as a result of the feedback of accretion flow onto the magnetospheric structure. These refinements are now just being included into the latest numerical simulations of magnetospheric accretion in young stars (e.g., Romanova et al. 2004, ApJ, 610, 920). These results have been acknowledged in the community and prompted invitations to give reviews at several international conferences, the most recent one being a full review chapter awarded on this subject (with Bouvier PI) at the Protostar & Planets V conference (Hawaii, Oct.2005). This work, led by our group at LAOG (Bouvier, Dougados), has been made possible through a large number of collaborations, sometimes funded by official programs (e.g. MAE 2003-2004, Econet 2005-2006), and multiple visits of collaborators (from 2 weeks up to 1 yr; CNRS, MENRT): K. Grankin and M. Ibrahimov, Uzbekistan; S. Alencar and J. Vasconcelos, Brazil; J. Muzerolle, USA; C. Clarke, UK. With a number of stringent observational constraints now at hand, we have embarked into the development of more realistic MHD models to describe the magnetospheric accretion/ejection process in young stars. Within a collaborative framework involving both FOST (Bouvier) and SHERPAS (Ferrreira) at LAOG, as well as an expert MHD group at University Utrecht (Keppens) a joint thesis has been started (N. Bessolaz) in September 2004 on the development of 2D numerical simulations of magnetically-channelled flows. A broader perspective is also offered by the start of the FP6 RTN JETSET (2005-2008) of which LAOG is a node (Dougados PI) and which includes the investigation of the magnetospheric accretion process as a possible source of jet outflows. Finally, we will now attempt the first direct detection of the magnetospheric cavity in young stars through interferometric measurements with VLTI/AMBER in GTO programs (Dougados, Bouvier, Malbet). X-rays to probe the star-disk interaction during accretion outbursts The typical accretion rates in T Tauri stars (10 −7,−8 M⊙/yr) is too weak for accretion pressure to rule over magnetospheric pressure, forcing the infalling material to proceed along the field lines, i.e., magnetospheric accretion (see previous section). For higher accretion rates however, the inner rim of the disk moves closer to the photosphere, possibly reaching the photosphere for extreme accretion rates. A boundary layer results. Young solar-like stars are thought to experience these phases of extreme accretion (10 −4,−5 M⊙/yr) several times during their early life. During these accretion bursts, known as FU Orionis and EX Lupi phases, the luminosity of the object is dominated by the emission of the heated inner disk. In that case, direct study of the star-disk interaction zone is next to impossible by “classical” methods. In November 2003, the “anonymous” star V1647 Ori erupted dramatically and produced the rise of the McNeil’s nebula, discovered serendipitously by amateur astronomer McNeil in Jan. 2004. This outburst is explained by an increase of the disk accretion rate from 6 × 10 −7 to 10 −5 M⊙ yr −1 . N.Grosso from FOST, in collaboration with J.Kastner (Rochester Institute of Technology) used Chandra (an american X-ray satellite observatory) to reveal a factor ∼100 increase in the X-ray flux of this source compared to pre-outburst values. The coincidence of this surge in X-ray brightness with the optical/infrared outburst demonstrates that strongly enhanced high energy emission occurs as a consequence of high accretion rates in V1647 Ori (Kastner et al. 2004). Follow-up observations with XMM-Newton confirmed the high levels of X-ray emission observed with Chandra, and showed enhanced X-ray variability from V1647 Ori in outburst (Grosso et al. 2005). The observed variability of the X-ray flux does not appear typical of X-ray flares from other young stellar objects, and could be produced by the Keplerian rotation of a warped accretion disk. A follow-up project at IRAM has been started by Grosso in collaboration with team ASTROMOL to characterise the accretion disk of V1647 Ori. 86
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
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LABORATOIRE D’ASTROPHYSIQUE DE GR
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III Team ASTROMOL 41 2 Presentation
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8.1.3 Graduate Students . . . . . .
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VI Team SHERPA 145 15 Results 147 1
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Part I Foreword: Presentation of th
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Part II Executive Summary A 18th ce
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of CNRS for technicians and enginee
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Figure 1.3: New LAOG organigram (20
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heart attack. Manuel went back to t
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Figure 1.6: PhD student repartition
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• LAOG researchers (and publishin
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and published 95 papers in four yea
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• National astronomy programs. 4
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1.5 From dreams to reality: Human r
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• ANR Alterdetect, with IMEP : si
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Mixed GRIL-other team profiles 1. A
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Name Grade Comm. Project Berger Jea
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• First fringes in the H band wit
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Table 15.1: List of the permanent S
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15.4 Transport phenomena in accreti
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Figure 15.2: Spectral energy distri
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states, combining a still powerful
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Excitation of turbulence by Cosmic
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Table 15.2: Former PhD students of
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Chapter 16 Perspectives The distinc
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Ray background. The second project
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164
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- Ceccarelli Cecilia (AT) - Delfoss
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Among the various areas of expertis
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-responsable du groupe ”logiciel
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Chef de l’ Equipe ASTROMOL du LAO
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jury de concours: 1 concours IR CNR
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Chapter 20 Appendix 4: The Jean Mar
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20.7 Production informatique ¡¢£
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20.10 Formation des utilisateurs La
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20.13 Darwin ¡¢£¤¥¦§¨©�
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Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

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