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

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C2D Spitzer/IRAC and MIPS data of star forming regions, coupled with our optical surveys of the same clouds, are used to study the presence and properties of disks around brown dwarfs and weak-line T Tauri stars. 6.4 Selected topics on the IMF and the properties of low-mass Populations 6.4.1 The low-mass population of nearby Star Forming Regions The brown dwarf population of the Taurus Molecular cloud In 2000, we initiated a deep large scale optical survey of the Taurus molecular cloud at CFHT. Today, the coverage of the cloud is of order of 30% and we have unveiled 17 new brown dwarfs in this low density star forming region (Guieu et al. 2005, and PhD Thesis). Taking into account the 4 brown dwarfs also discovered by our group earlier, Martin et al. 2001, we have ∼doubled the number of known substellar objects in this star forming region. We have studied the spatial distribution of the brown dwarfs in Taurus, and contrarily to previous claims, we find that there is no brown dwarf deficit in Taurus. The ratio of brown dwarfs to stars in Taurus is now consistent with other estimations elsewhere, in Orion for example. We also find that a significant number of brown dwarfs show Hα emission and other signatures of accretion, indicative of the presence of circumstellar disks. Using our collaboration with the Spitzer C2D legacy team, we are currently investigating the mid-infrared excess of numerous brown dwarfs in Taurus. These results show that half of the brown dwarfs in Taurus harbor a significant infrared excess, hence possess a circumstellar disk. Figure 6.6: Spatial distribution of newly found BD in Taurus (open triangles) and previously known BD (open circles). Black dots locate Taurus young stars. Image credit: S.Guieu. 90
Multiplicity of protostars Following our investigation of multiple systems among low-mass PMS and ZAMS stars aimed at constraining the star formation process from the statistics of multiplicity (e.g., Duchêne, 1999, A&A, 341, 547), we have recently focussed on the earliest stages of star formation by investigating the multiplicity of near-infrared embedded sources in star forming regions. These embedded sources are the observable objects closest to the protostellar stage. Duchêne & Bouvier, with Co-I André & Motte (Saclay), and Bontemps (Bordeaux) derived the fraction of wide binaries among these sources from direct near-IR imaging at CFHT and ESO. An excess of multiple systems compared to older low-mass stars in the field is found. This new result, for the youngest stellar objects known, tends to confirm the ubiquity of fragmentation during molecular core collapse, thus leading to multiple protostars which later decay through dynamical relaxation or are disrupted by gravitational encounters in dense clusters (Duchêne et al. 2004). Taking advantage of the unique capabilities of the near-infrared wavefront sensor of the NACO Adaptative Optics system on VLT, we are now in the process of investigating the tightest of these protostellar systems, down to a resolution of 10 AU. Preliminary results reveal a number of close systems with separations in this range, see Fig. 6.7. A complete census of multiple protostellar systems over the range 10-1500 AU awaits the completion of the VLT NACO runs. Meanwhile, we have started follow up spectroscopy of the faintest and most embedded companions to protostars, with the aim to search for proto-brown dwarfs. In 2005,we will also obtain the first VLTI-MIDI mid-infrared interferometric observations of these objects, in an attempt to detect the tightest embedded multiple systems, thereby setting constraints on the fragmentation scenario for systems just a few AU appart, and to determine the structure of the dusty envelopes surrounding these protostars. L1689 IRS 5 (ophiuchus) SVS 20 (Serpens) WL 20 (Ophiuchus) 0.5" Figure 6.7: Three examples of multiple protostellar systems observed with VLT/NACO. Note the strange physionomy of the third companion of the WL 20 system, which is not an artefact but most likely an edge-on disk. Image credit G. Duchêne and J.Bouvier While our current results indicate a high frequency of multiplicity among protostars indeed, thus supporting fragmentation during core collapse, we find no evidence for the existence of small-N unstable aggregates (N=5- 10) in the protostellar stage, contrary to what several numerical models of core collapse predict through multiple fragmentation. This disagreement probably points to the simplified physics included so far in the numerical models which, for instance, neglect magnetic field, use a poorly constrained equation of state, and do not take into account feedback effects resulting from the strong wind of young protostars onto the collapsing cloud. Our results regarding the end products of core collapse in several star forming regions thus provide some guidance to improve numerical models. On the nature of Infrared companions to T Tauri stars Taking advantage of the high-angular resolution imaging and spectroscopic devices available on the VLT and Keck telescopes, we have been studying for the last few years the nature of the so-called ”infrared companions” to T Tauri stars. These objects are best exemplified by T Tau Sa, the optically undetectable companion to the prototype young stellar object T Tau. The spectral energy distribution of these objects would classify them as embedded, Class I protostars, but their close vicinity (a few tens to hundreds of AU) to more evolved T Tauri stars suggests otherwise. Using the Keck telescope in collaboration with Andrea Ghez (UCLA), we have obtained medium resolution near-infrared spectra of two such objects (T Tau Sa and V773 Tau D (Duchêne 91 E N
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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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Name Grade Comm. Project Berger Jea
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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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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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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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Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

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