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

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15.4 Transport phenomena in accretion-ejection flows The existence of accretion-ejection structures raises a number of prominent issues, which, for most of them, have been around from the very beginning of this field of research. 15.4.1 Jet stability The exceptional propagation length of jets, as compared to their radii, raises the question of the physical mechanisms responsible for their stability. This question has at least two different aspects: (1) Jet global stability properties. Purely hydrodynamical jets are quickly destroyed due to the development of the Kelvin-Helmholtz instability. MHD jets seem to be more stable with respect to Kelvin-Helmholtz modes. However, such MHD jet are prone to be unstable with respect to purely MHD (current- and pressure-driven) instabilities, the outcome of which is still unknown on theoretical grounds. These instabilities are well-known to be quite disruptive in the fusion context, so that the very small level of research activity in the astrophysics community on these issues is all the more surprising. (2) Particle acceleration. However, one certainly does not want to quench every possible mode of instability in MHD jets as some turbulence is required to accelerate the non-thermal particle populations which are responsible for the high energy emission of these objects. In particular, in the framework of the two-flow model developed by the SHERPA team, it is essential to understand and characterize the processes responsible for the turbulent stirring of the pair plasma, which tap the energy reservoir of the large scale MHD jet. In order to make progress on these issues, the effect of pressure- and current-driven instabilities in jets are examined ab initio. Pressure-driven instabilities are expected to be most disruptive in jets confined by the hoop-stress, and a linear analysis of the problem has been undertaken (Kersale, Longaretti & Pelletier, 2000, A&A, 363,1166; Longaretti 2003; Longaretti & Baty in preparation). A complete review of the question of jet structure and stability, based on both the astrophysical and fusion literature, has also been written (Longaretti 2005). 15.4.2 Transport in accretion disks The question of mass and angular momentum transport in accretion disks is one of the oldest issues in this branch of astrophysics, and has not yet been resolved in a satisfying way. In spite of the remarkable progress undergone in the last fifteen years, with the (re)discovery of the “Magneto-Rotational Instability” (MRI, Balbus & Hawley 1991, ApJ, 376, 214), and the (essentially numerical) characterization of the induced turbulent transport in the nonlinear regime (e.g. Stone et al. 1996, ApJ, 463, 656), many issues are still acutely open: (1) Not all disks, or disk regions, are ionized enough to sustain MHD activity (e.g. Gammie 1996, ApJ, 457, 355, Matsumura & Pudritz 2003, ApJ, 598, 645). The transport in these regions must therefore be sustained through non-MHD mechanisms. (2) The accretion-ejection structures most actively studied in our group do require a fairly high level of turbulent resistivity to be self-consistently maintained (Ferreira & Pelletier 1995, A&A, 295, 807). It is unclear whether the MRI can provide it. To settle these questions, we have first addressed and completely reinvestigated the old issue of the existence of subcritical turbulence in keplerian flows. The underlying idea is that all linearly stable flows accessible to laboratory experiments are observed to undergo a transition to turbulence (called subcritical). The initial proposal by Shakura & Sunyaev (1973, A&A, 24, 337) was that such a mechanism was at work in keplerian disks (which are hydrodynamically stable). This picture has given rise to controversial points of view in the astrophysics literature (Balbus, Hawley & Stone 1996, ApJ, 467, 76; Richard & Zahn 1999, A&A, 347, 734). Due to the formidable complexity of the problem, little progress had been accomplished on this issue at a fundamental level, until the last decade, where an interesting breakthrough has been operated in the fluid 150
mechanics community, for non-rotating shear flows. Based on this new understanding, the question has been reinvestigated in rotating flows (such as the keplerian flows), through i/ a phenomenological analysis (Longaretti 2002); ii/ a reinvestigation of all the relevant laboratory experimental data (Longaretti and Dauchot 2005; Longaretti and Dauchot, in preparation; Dubrulle et al. 2005); and iii/ numerical simulations (Lesur and Longaretti 2005). This work has lead to the conclusion that a stabilizing rotation does not quench the transition to turbulence, but nevertheless considerably reduces the efficiency of the subcritical turbulent transport, thereby settling this old dispute. Geoffroy Lesur has started a PhD in September 2004, under the supervision of Pierre-Yves Longaretti, with the aim to investigate in a more systematic way the question of turbulent transport in disks and jets, both from analytic and numerical points of view. 15.5 Physics of high energy sources 15.5.1 Seyfert galaxies Seyfert galaxies are strong X-ray emitters with a characteristic X-ray spectra. It is roughly power-law like above 2 keV, with the presence of a high energy cut-off near 100 keV (e.g. Petrucci et al. 2001, ApJ, 556, 716; Zdziarski et al. 2000, ApJ, 542, 703). Secondary components, like a fluorescent iron line near 6.4 keV and a bump in the 10-50 keV range are also generally present. The X-ray emission is generally supposed to be produced by a hot and optically thin thermal plasma (a corona) localized above the accretion disk and comptonizing the disk photons. The secondary components are thought to result from the Compton reflection of the X-rays on the disk surface. Due to the lack of sensitivity in the hard X-ray/soft gamma ray energy range (0.1-1 MeV) of the detectors, the real nature of the high energy continuum of Seyfert galaxies remains unclear. We have recently tested two different geometries of the disk-corona configuration using the most well adapted data for the study of the Seyfert high energy continuum i.e. the BeppoSAX 1 brightest Seyfert sample (Petrucci & Dadina 2005 in preparation). This subsample contains 28 objects. It appears that both geometries agree with the data but more importantly, we show that the behaviors of the physical parameters of the models (mainly the temperature and optical depth of the X-ray corona) are strongly geometry dependent, resulting in completely different physical interpretations. This work is an extension of previous works done on a smaller sample (Petrucci et al, 2000, ApJ, 540, 131; Petrucci et al. 2001, ApJ, 556, 716) and underlines the weakness of the actual constraints. Stronger and less ambiguous constraints on the nature of the coronal plasma can be obtained from variability studies. For example, thermal models, where hot and cold phases are in radiative equilibrium, predict that the X-ray spectrum of the sources should harden when the corona temperature increases (Haardt et al. 1997, ApJ, 476, 620) while non-thermal models predict the reverse (Petrucci et al. 2001, A&A, 374, 719). The analysis of the ∼1 month simultaneous IUE/RXTE monitoring campaign on NGC 7469, performed in 1996, appears to be in agreement with thermal comptonization emission (Nandra et al. 2000, ApJ, 544, 734). We have performed a more recent reanalysis of these data with realistic comptonization codes that gives another support to this interpretation (Petrucci et al. 2004). Missions with X-ray/γ-ray broad band capabilities and higher sensitivity, like the future ASTROE-2 mission, are required to progress in this field. On the other hand, the highly sensitive instruments of the XMM-Newton satellite enable very new and exciting results concerning the fluorescent iron line. Broad iron lines are clearly present in some Seyferts and in objects like MCG-6-30-15, their broad profiles suggest the presence of spinning black hole, close to the extreme Kerr value (Fabian et al., 2000, PASP, 112, 1145) and require very steep disk emissivity laws (Wilms et al. 2001, MNRAS, 328, L27; Fabian et al. 2002, MNRAS, 335, L1), steeper than standard accretion disk one. This is in agreement with the model we proposed some years ago where the irradiating X-ray source was concentrated on the disk axis, above the black hole (Henri & Petrucci 1997, A&A, 326, 87; Petrucci & Henri 1997, 326, 99 but see also Martocchia & Matt 1996, A&A, 282, L53; Martocchia et al. 2002, A&A, 383, L23). More recently, redshifted narrow iron lines have been also observed in an increase number of objects (e.g. Yaqoob et al. 2003, ApJ, 596, 85; Longinotti et al. 2004; Porquet et al. 2004, A&A, 427, 101; Turner et al. 2004, ApJ, 603, 62; Iwasawa et al. 2004, MNRAS, 335, 1073). These lines can be interpreted as signatures of small magnetic flares illuminating small part of the accretion disk. We are investigating the presence of such a line in the Seyfert 1 1 BeppoSAX was an Italian-Dutch satellite covering the 0.1-200 keV energy range. It stopped observing in 2003. 151
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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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and accretion disks with heavy nume
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emphasis on the chemistry of planet
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Part III TEAM ASTROMOL Sub-arcsecon
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• we develop quantum and classica
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Table shows not only the volume of
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• Herschel-HIFI: Astromol is invo
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particular, we calculated a 9D H2O-
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• The hot corinos. More spectacul
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the massive deeply embedded protost
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tens of visual magnitudes, hence pr
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quantum VCC-IOS approximation may f
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Rate constant (cm 3 s -1 ) 1e-09 1e
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• One of us (PV) is strongly invo
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sublimation of water ice trapped ei
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If the chemistry in the first phase
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4.2.2 Molecular Physics The theoret
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Source Herschel Time Astromol Class
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several important studies that anti
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Chapter 5 Selected Publications •
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Part IV TEAM FOST The first image o
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393254 (5MJup at 55 AU) & AB Pic (1
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2.2 µ m 0.5" 11.7 µ m Figure 6.1:
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Figure 6.3: Contribution of heating
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such mid-term (several weeks) magne
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systems: HD 141569, HD 32297, HD 18
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C2D Spitzer/IRAC and MIPS data of s
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et al. 2002; 2003). We have found t
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Figure 6.8: Distribution of separat
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with only three known to date. Most
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300 objects, a size sufficient for
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