Max Planck Institute for Astronomy - Annual Report 2005

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62 III. Scientific Work Fig. III.1.10: The HubblE Space Telescope photo presented at the 2006 General Assembly of the International Astronomical Union in Prague, showing observations of the star forming association LH 95 in the Large Magellanic Cloud with the Advanced Camera for Surveys. This sharp image reveals a large number of low-mass infant stars coexisting with young massive stars, changing dramatically the picture that we had for stellar associations in the Magellanic Clouds. Credits: Nasa, Esa, and D. A. Gouliermis (MPIA). with high-resolution ground-based observations with the most advanced near-infrared detectors, will allow the characterization of the PMS stars and other stellar objects under formation, and the study of their IMF. This would open a complete new field of study for the Magellanic Clouds. (Dimitrios A. Gouliermis, Wolfgang Brandner, Thomas Henning)
III.2 Radiative Transfer – Link between Simulation and Observation Almost all information about the objects in the Universe is obtained through the analysis of the radiation we receive from them. One therefore might expect that the calculation of the transfer of radiation within an object and in the interstellar medium is an astrophysical problem that has been solved long ago. Surprisingly the reverse is true: Among the numerous processes important in astrophysics, radiative transfer is one of the most difficult problems. The MPIA is among the few institutes worldwide where programs are used that are able to trace the radiation even in complex three-dimensional structures. This is particularly important for objects surrounded by gas and dust envelopes – such as forming and young stars or debris disks around main-sequence stars where planets are possibly forming, as well as the central black holes in active galactic nuclei. The Seven Dimensions of Radiative Transfer According to current notion, star formation starts with the gravitational collapse of the cores of cold molecular clouds. Due to the conservation of angular momentum, the gas and dust distribution around the forming protostar flattens, creating a circumstellar disk in later stages. In order to follow the evolution from a molecular cloud core to a completed star sophisticated simulations are required where the motion of the gas and dust is being traced using magnetohydrodynamical calculations. Here, radiative transfer is playing an important double role: On the one hand, the radiation carries energy through the structure, contributing to its heating or cooling; on the other hand, the appearance of the structure at a certain wavelength can only be determined from the calculation of radiative transfer. Yet radiative transfer was still neglected or calculated in a much simplified way in simulations so far. The reason for this is the high dimensionality of the problem: The radiation field is not only a function of space and time but also of direction and wavelength. Compared to other physical quantities like density or magnetic field it thus has three additional dimensions. Accordingly, a simulation calculating the magnetohydrodynamics and the radiative transfer equally correctly would use almost all the computing time for the radiative transfer. In approximative radiative transfer calculations, at least the transfer of the mean energy through the system is calculated correctly. Within the »flux limited diffusion«, e.g., the radiation field is calculated correctly in the optically very thin or very thick case, which suffices for many applications. However, as soon as spectral energy distributions or spatially resolved images of the objects have to be calculated, a correct radiative transfer is required. This is due to the fact that the appearance of the object at a certain wavelength is dominated by the radiation originating from the spatial region where the optical depths equal unity. But exactly in this region, the approximations break down – which is why it is also the numerically most demanding region. Radiative Hydrodynamics: Diffusion plus Ray-tracing Thus, the numerical calculation of the hydrodynamic evolution of accretion disks under simultaneous consideration of radiative transfer is still a great challenge. Realistic thermodynamics of the disk, however, cannot be done without radiative transfer. Flux limited diffusion so far has been the method of choice in this field since correct radiative transfer without approximations would consume too much computing time to be able to follow the dynamical evolution of the optically thick disk. Concerning the incoming radiation of the central star the diffusion approximation can be used only under certain circumstances. But if the simple local diffusion approximation with an assumed local black-body radiation is replaced by a wavelength dependent diffusion approach, as has been done by colleagues in Pasadena, even with modern computers the numerical effort is so high again that the time evolution of a disk can no longer be simulated in all three spatial dimensions. Therefore, in collaboration with Willy Kley (Tübingen), we have extended the simple diffusion approach by a ray-tracer. This ray-tracer allows to calculate precisely the absorption of the stellar radiation in the accretion disk. The radiation energy absorbed is then added to the diffusion approach as a source term. The test results are encouraging, showing that the »diffusion plus ray-tracing« approach and an exact continuum radiative transfer are providing quantitatively comparable results (Fig. III.2.1). Although the errors are too large for a reliable determination of the spectral energy distribution or an intensity map of the disk they allow to determine locally a sufficiently accurate sound velocity, which is just the quantity needed for hydrodynamics. This new algorithm is being applied first to protoplanetary accretion disks around luminous young stars and young planets embedded in their parent disk. 63
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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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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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154 Conferences/Service in Comitees
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156 Publications Szalay, I. Szapudi
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162 Publications Zheng, X. Z., F. H
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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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