Interferometric observations of pre-main sequence disks - Caltech ...

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16 Scientific overview the gas and sediment on the disk mid plane, increasing the collisional efficiency and probably reaching a dynamical equilibrium in which large particles form smaller ones through destructive collisions and vice versa. Given the extreme complexity of the problem, different models gives often contradictory results and the formation of plan- etesimals remain at the moment an open question. As consequence of the grain growth and settling, circumstellar disks are predicted to be vertically stratified with small grains in the uppermost layers of the disk and large grains in the disk mid plane. Furthermore, grain growth should be much slower in the outer regions of the disk than in the regions closer to the star, which should be completely depleted of small grains. The situation becomes even more complicated if we consider that inward radial circulations, due for example to the disk viscosity and/or to the dust drag due to the gas, bring material close to the star where dust sublimates. At the same time, opposite radial circulations, due, for example, to the stellar wind or to the magnetorotational instability, move the inner material outward permitting the ricondensation of grains and the formation of crystalline structures (Gail, 2004). Despite all the theoretical difficulties in understanding the dust evolution, grain properties in disks can be explored with a large number of techniques and over large range of wavelengths. It is worth to stress that since the disks around pre-main sequence stars have high optical depth, observations at optical and infrared wavelengths allow to ob- tain information only on grains located in the very superficial layer of the disk where τ≃1. Such observations are thus sensitive only to a very small fraction of the dust mass which is concentrated on the disk mid plane (see Eq. 1.2). An important exception to this general rule is represented by near-infrared interferometric observations which are able to spatially resolve the regions close to the star where the dusty disk is truncated by the dust evaporation. We will show in Chapter 4 that in this case, the measure of the location of the dust evaporation radius gives information on dust grains present in the disk mid plane. Particularly important to the study of the chemical composition, size and structure of the dust grains is the mid-infrared spectral region (between∼ 3µm and∼ 100µm),
1.3 Grain growth and planetary formation 17 rich in vibrational resonances of many dust species. In disks around HAe and TTS, this dust features are generally seen in emission and are thought to originate in the optically thin disk surface layer, directly illuminated by the stellar radiation (Calvet et al., 1991; Chiang and Goldreich, 1997). The more common features observed around TTS and HAe stars are those of amorphous and crystalline silicates, located around 10µm and 20µm. Recent Spitzer data (e.g. Furlan et al., 2006) show a large variation in the strength and shape of this features, confirming the correlation firstly identified by van Boekel et al. (2005): weaker is the feature flatter is its shape. This is interpreted in terms of the growth of silicate grains from sub-micron to micron sizes; if grains grow further, the silicate emission disappears. The presence of crystalline grains around HAe and TTS stars implies that the pris- tine material underwent a drastic change in its structure due to some, not yet known, energetic process. Using spectrally resolved mid-infrared interferometry, van Boekel et al. (2004) showed for the first time that the disk surface inside 1-2 AU around three HAe stars is much more crystalline than the outer disk surface, supporting the idea that crys- tallization occurs in the inner disk (Gail, 2004). However crystalline silicates are also present at larger distances (∼ 20 AU), and in the comets in our Solar System, suggesting that the radial drift can have an effective role in the dust evolution within circumstellar disks. At millimetric and centimetric wavelengths disks become optically thin to their own thermal radiation and the observations can probe the bulk of the dust mass concentrated on the disk mid plane. The SED of the continuum emission at these wavelengths can be directly related to the grain emissivity which, in turn, depends on the grain properties. The millimetric dust opacity can be approximated by a power law, k∼λ −β , where the exponentβdepends strongly on the grain size distribution and on its upper limit. In a size distribution composed only by grains larger than the observing wavelength, i.e. centimeter size grains, the dust opacity is grey andβ=0, whileβis close or larger than unity only for sub-millimeter grains (see Fig. 1.3). If the disk emission is optically thin (τν≪1) and the Reyleigh-Jeans approximation is verified (Bν(T)∝λ −2 ), the emitted
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Page 5 and 6: CONTENTS I Introduction 1 1 Scienti
Page 7 and 8: CONTENTS v 4.5 Comparison with prev
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Page 12 and 13: 4 Scientific overview the critical
Page 14 and 15: 6 Scientific overview Log νFν Log
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Page 28 and 29: 20 Scientific overview We propose a
Page 31 and 32: CHAPTER 2 2.1 A bit of history Intr
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Page 51 and 52: 3.1 Introduction 43 Tauri stars (Ei
Page 53 and 54: 3.2 Model description 45 adapt the
Page 55 and 56: 3.2 Model description 47 log(δR/R
Page 57 and 58: 3.2 Model description 49 the pressu
Page 59 and 60: 3.3 Results 51 Photospherical heigh
Page 61 and 62: 3.3 Results 53 ever, is sufficientl
Page 63 and 64: 3.3 Results 55 Sec. 3.3.1 (see Fig.
Page 65 and 66: 3.4 Discussion 57 the value ofǫ, t
Page 67 and 68: 3.4 Discussion 59 sample of Fig. 3.
Page 69 and 70: 3.5 Summary and conclusions 61 R in
Page 71 and 72: 3.6 Appendix 63 However, the bright
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68 Large dust grains in the inner r
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Face-on disk image Baseline (m) V 2
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V 2 1 0.8 0.6 0.4 0.2 0 0 50 100 15
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V 2 1 0.8 0.6 0.4 0.2 0 0 50 100 15
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CHAPTER 5 More on the “puffed-up
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5.1 Fuzzy and sharp rims 97 opacity
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5.2 New shapes for the rim 99 grain
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5.3 New observational results 101 (
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5.3 New observational results 103 w
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CHAPTER 6 Constraining the wind lau
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6.1 Introduction 109 λ Fλ (erg cm
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6.2 Observations and data reduction
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6.3 Results and discussions 113 V V
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6.4 Conclusions 115 note however th
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Part IV The structure of the outer
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120 The keplerian disk of HD 163296
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Part V Conclusions and future prosp
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152 Summary and Conclusions ∼1 an
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154 Summary and Conclusions disk in
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CHAPTER 9 Future developments As my
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9.1 Gas in the inner disk of Herbig
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9.2 Probing planet formation throug
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Published papers • Isella, A.; Te
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172 BIBLIOGRAPHY [2000] Bodenheimer
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174 BIBLIOGRAPHY [2004] Gail H.-P.,
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176 BIBLIOGRAPHY Arizona Press, Tuc
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178 BIBLIOGRAPHY [2001] Thompson A.
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