Max Planck Institute for Astronomy - Annual Report 2005

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66 III. Scientific Work line observations have an important advantage since they also carry information about kinematics of the gas in these objects, which is however well hidden in the line profiles. The use of molecular line observations as a tool of investigations is faced with two severe difficulties. First, the chemical structure of the prestellar objects is not uniform, so different species trace different regions of the source. Second, molecular emission lines are often excited in nonsteady conditions that vary through the object. Both this facts make it challenging to extract a wealth of information about the source structure directly from the line data. This necessitates the use of sophisticated, coupled chemodynamical and radiative transfer models. By iteratively comparing the modeling results with observed quantities, the best fit and thus the basic parameters of the object can be found. At the MPIA, we are involved in the development of all necessary tools for the interpretation and modeling of molecular emission lines from the disks and clouds, including multi-dimensional chemo-dynamical models and numerical codes for radiative transfer simulations. Here we present an application of these tools to millimeter observations of AB Aur. This system is one of the best studied circumstellar disks around young Herbig Ae stars. It was intensively investigated within the entire spectral range from UV to millimeter wavelengths, including molecular lines. The main aim of our study was to determine the orientation and properties of the AB Aur system using a combination of line observations and advanced theoretical modeling. The sketch of the system is shown in Fig III.2.2 (left), together with density and temperature structure of the adopted disk model (right). Fig. III.2.3: Left – Environment around the star VV Serpentis imaged with the spItzEr Space Telescope. Blue represents emission at a wavelength of 4.5 µm, green represents 8.0 µm and red represents 24.0 µm emission. The bright star in the 20000 AU The AB Aur system consists of a flaring disk surrounded by an extended envelope. The disk shades off a toruslike region in the envelope from stellar UV flux, allowing many complex molecules to form there. We used a coherent step-by-step modeling of the AB Aur disk, its envelope physical structure, and its chemical evolution, using radiative transfer in several molecular lines. With the best-fit model we could explain most of the features in the molecular line profiles on the observed map of the disk and derive its basic parameters, like mass, size, chemical structure, and orientation. However, the lack of spatial resolution in our observations did not allow us to reveal all details of the AB Aur disk structure. Future radio-interferometers, like alMa, will provide us with more detailed information regarding chemical composition and structure of protoplanetary disks and prestellar cores. A Small Disk Acting Big The circumstellar dust- and gas disks in which planets form are often too small and too distant to be spatially resolved. This is particularly true for the spItzEr Space Telescope: it is the most sensitive infrared telescope now available, but has only limited spatial resolution. Many stars with disks, however, reside in giant molecular clouds. Some of these disks happen to lie nearly edge-on (i.e. we look at the outer edge of the disk). Pictured in the left panel of Fig. III.2.3 is the environment around the bright star VV Serpentis imaged with the spItzEr Space Telescope in false color: blue represents emission at a center is VV Ser. Right: Model image for VV Ser. The disk is so small that it resides entirely within the central bright dot. The dark wedge is the shadow cast by the small disk.
wavelength of 4.5 µm, green represents 8.0 µm and red represents 24.0 µm. The bright red star to the left is probably unrelated to VV Ser. The nebulosity surrounding VV Ser is glow from polycyclic aromatic hydrocarbons (PAHs), i.e. very small dust grains in the vicinity of the star which radiate due to quantum-excitation by ultraviolet (UV) photons originating from VV Ser. The dark wedge is not the circumstellar disk, as one may suspect, because such disks are ten to hundred times smaller than that. Instead, using multi-dimensional modeling with radiative transfer codes, K. Pontoppidan and C.P. Dullemond show that a much smaller disk casts a shadow into the environment. PAH grains residing in this shadow cannot see the UV photons by the star since their line of sight is blocked by the circumstellar disk, while PAH grains elsewhere get excited by the stellar UV photons. A model (Fig. III.2.3, right panel) shows that this explanation works. In this figure the disk is so small that it resides entirely within the central bright dot (the size of the dot is the resolution of the telescope). The dark wedge is the shadow cast by the small disk. In this way the disk is magnified about a hundred times. The opening angle of the dark wedge is equal to the opening angle (i.e. thickness) of the unresolved disk in the center of the image. The authors compared the same model also to spItzEr Space Telescope infrared spectra, and showed that the model can also explain those. Moreover, the disk structure inferred from the above image and the fitting of the spectra was used to explain a peculiar kind of variability in the optical light from the star (called UX Orionis-type variability), and they could explain nearinfrared interferometric observations of this object. In this way, the disk around VV Ser has become one of the most comprehensively modeled disks around a young star. The model has given insight into the structure of the disk and in the size-distribution of the dust particles residing in this disk. Both are very important for theories of planet formation. Signatures of Planets in Spatially Unresolved Debris Disks Main-sequence stars are commonly surrounded by debris disks, composed of cold dust continuously replenished by a reservoir of undetected dust-producing planetesimals. In a planetary system with a belt of planetesimals (like the solar system's Kuiper Belt) and one or more interior giant planets, the trapping of dust particles in the mean motion resonances with the planets can create structure in the dust disk, as the particles accumulate at certain semimajor axes. Sufficiently massive planets may also scatter and eject dust particles out of a planetary system, creating a dust-depleted region inside the orbit of the planet. In anticipation of future observations of spatially unresolved debris disks with the spItzEr Space Telescope, we are studying how the structure carved by III.2 Radiative Transfer – Link between Simulation and Observation 67 planets affects the shape of the disk‘s spectral energy distribution (SED) and consequently whether the SED can be used to infer the presence of planets. We numerically calculate the equilibrium spatial density distributions and SEDs of dust disks originated by a belt of planetesimals in the presence of interior giant planets in different planetary configurations and for a representative sample of chemical compositions. The dynamical models are necessary to estimate the enhancement of particles near the mean motion resonances with the planets and to determine how many particles drift inside the planet’s orbit. On the basis of the SEDs and predicted spItzEr colors we discuss what types of planetary systems can be distinguished and the main parameter degeneracies in the model SEDs. For the simulation of the debris disk SEDs we use a radiative transfer tool which is optimized for optically thin dust configurations with an embedded radiative source. The density distribution, radiative source, and dust parameters can be selected either from an internal database or defined by the user. Furthermore, this tool is optimized for studying circumstellar debris disks where large grains (with radii � 1 µm) are expected to determine the far-infrared through millimeter dust reemission spectral energy distribution. The tool is available at http://aida28.mpia-hd.mpg.de/~swolf/dds. In Figure III.2.4 we compare the emission arising from the different compositions (keeping the particle size approximately constant). These simulations demonstrates the dependence of a debris disk SED on the dust grain properties (chemical composition, density, and size distribution) and the mass and location of the perturbing planet. The SED of a debris disk with interior giant planets is fundamentally different from that of a disk without planets; the former shows a significant decrease of the NIr and MIr flux due to the clearing of dust inside the planet's orbit. The SED is particularly sensitive to the location of the planet, i.e., to the area interior to the planet‘s orbit that is depleted in dust. However, some degeneracies can complicate the interpretation of the spectral energy distribution in terms of planet location. For example, the SED of a dust disk dominated by Fe-poor silicate grains has its minimum at wavelengths longer than those of a disk dominated by carbonaceous and Fe-rich silicate grains. Because the SED minimum also shifts to longer wavelengths when the gap radius increases (because of a decrease in the mean temperature of the disk), we note that there might be a degeneracy between the dust grain chemical composition and the semimajor axis of the planet clearing the gap. For example, note the similarities in the shape of the SEDs arising from a dust disk with a 3 M J planet at 1AU that is dominated by MgSiO 3 grains and that arising from a disk with a 3 M J planet at 30 AU that is dominated by MgFeSiO 4 grains (see Fig. III.2.5). This illustrates the importance of obtaining spectroscopic observations able to constrain the grain chemical composition, and/or highresolution images, able to spatially resolve the disk.
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Contents Preface ..................
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6 I. General I.1 Scientific Goals U
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8 I. General massive protostars and
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10 I. General achieve higher angula
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12 I. General The institute is co-l
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14 I. General Point Source sensitiv
Page 18 and 19: 16 I. General Edinburgh Groningen D
Page 20 and 21: 18 II. Highlights II.1 Light Echoes
Page 22 and 23: 20 II. Highlights 10 arcsec Fig. II
Page 24 and 25: 22 II. Highlights AB Dor C AB Dor A
Page 26 and 27: 24 II. Highlights II.3 The First He
Page 28 and 29: 26 II. Highlights II.4 Planetesimal
Page 30 and 31: 28 II. Highlights azimuthal directi
Page 32 and 33: 30 II. Highlights ticles fall faste
Page 34 and 35: 32 II. Highlights ANTU Seyfert 2 ga
Page 36 and 37: 34 II. Highlights cretion disk, one
Page 38 and 39: 36 II. Highlights 8.5 �m 2.3 ly 0
Page 40 and 41: 38 II. Highlights II.6 Massive Star
Page 42 and 43: 40 II. Highlights log M� /pc2 �
Page 44 and 45: 42 II. Highlights II.7 Observations
Page 46 and 47: 44 II. Highlights log IR luminosity
Page 48 and 49: 46 II.8 Dynamics, Dust, and Young S
Page 50 and 51: 48 II. Highlights and we see a temp
Page 52 and 53: 50 III. Selected Research Areas III
Page 54 and 55: 52 III. Scientific Work Fig. III.1.
Page 56 and 57: 54 III. Scientific Work the Data Ar
Page 58 and 59: 56 III. Scientific Work The Field o
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Page 66 and 67: 64 III. Scientific Work z [AU] 1 0
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Page 72 and 73: 70 III. Scientific Work Column dens
Page 74 and 75: 72 III.3 The Galaxy-Dark Matter Con
Page 76 and 77: 74 III. Scientific Work redshift su
Page 78 and 79: 76 III. Scientific Work Mocking the
Page 80 and 81: 78 III. Scientific Work tion rate o
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Page 84 and 85: 82 III. Scientific Work DDO 53 HO I
Page 88 and 89: 86 III. Scientific Work tribution o
Page 90 and 91: 88 IV. Instrumental Development All
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Page 112 and 113: 110 V. People and Events 1� 10�
Page 114 and 115: 112 V. People and Events V.2 Open H
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116 V. People and Events presented.
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118 V. People and Events The IMPRS
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120 V. People and Events Fig. V.5.2
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124 V. People and Events These oppo
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130 V. People and Events V.10 The P
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132 V. People and Events Abb. V.11.
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134 V. People and Events V.13 Four
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136 V. People and Events Lemke: I w
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138 V. People and Events Fig. V.13.
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140 V. People and Events V.14 Where
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142 V. People and Events against sp
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144 Staff Directors: Henning (Actin
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146 Working Groups Rainer Lenzen, H
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148 Cooperation with industrial Fir
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150 Conferences Conferences and Mee
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152 Conferences D. Lemke: JWST-miri
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154 Conferences/Service in Comitees
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156 Publications Szalay, I. Szapudi
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158 Publications Hamilton, C. M., W
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160 Publications Brinkmann, D. G. Y
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162 Publications Zheng, X. Z., F. H
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164 Publications of the wheel mecha
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166 Publications ASP Conf. Ser. 331
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The Max Planck Society The Max Plan
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