Annual Report 2011 Max Planck Institute for Astronomy

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38 II. Highlights and may solve a long-standing problem in cosmology, that of the shallow central slopes of low-mass dark halos. Furthermore, many questions are raised that warrant further investigations. We will search for similar objects over a broader redshift range to find out how their frequency evolves with cosmic time, search for less extreme counterparts to sample different evolutionary stages of similar objects, and examine through highresolution spectroscopy the metal content, dynamical masses, and ionization parameters of the most extreme bursts. What is perhaps most important is that we suddenly have a way to study dwarf galaxies at large cosmological look-back times. This new avenue will prove to be a valuable complement to the detailed studies that unravel the inner workings of nearby, present-day dwarf galaxies. Arjen van der Wel, Hans Walter Rix & the Candels team
III. Selected Research Areas III.1 Planetary population synthesis Planetary population synthesis is a new method that allows us to improve our understanding of planet formation and evolution by using statistical observational constraints provided by the extrasolar planets. They are now known in a sufficiently high number to treat them no more just as single objects, but also as a population characterized by a number of statistical distributions. With planetary population synthesis, the global effects of many different physical mechanisms occurring during planet formation and evolution can be put to the observational test. Introduction The number of known extrasolar planets has increased very rapidly in the last few years. Currently, there are several hundred confirmed exoplanets known, which were mostly found by the spectroscopic radial velocity technique. Additionally, there are more than two thousand candidates from the Kepler satellite which were detected with extremely precise photometric transit measurements. These detections have revealed an exiting diversity in the properties of planetary companions, which was not expected from the structure of our own planetary system, the Solar System. The detections have however not only revealed diversity, but also a number of interesting correlations and structures in the properties of the planets. These insights were in particular possible thanks to the large number of planets now known. This allows for the first time to look at the (extrasolar) planets no more as single objects only but as a population that is characterized by a number of statistical properties. Understanding these statistical properties from the point of view of planet formation theory is one of the most important goals of planetary population synthesis. The special interest in a statistical, population wide approach also comes from the fact that the knowledge about a single planet is often limited. For the majority of the extrasolar planets, still only a few orbital elements (semimajor axis, eccentricity,…) and a minimum mass are known (or a radius, but no mass in the case of the Kepler candidates). For a limited number of planets, which are typically planets around bright stars for which both the mass and the radius are known, more observational constraints can be derived like the mean density or the atmospheric structure, so that they are investigated in details. Also the Solar System provides a large body of precise observational constraints against which planet formation models must be tested. In order to benefit also from the large number, but individually limited data sets, statistical methods are necessary. This is important since several future surveys like the Gaia space mission or the Sphere survey with the VLT will yield additional, similar statistical data sets for comparisons. The fundamental assumption behind planetary population synthesis is that the observed statistical properties (like the distribution of the planetary masses or semimajor axes) can be explained by studying the physical processes like the accretion of gas and solids occurring during the formation phase of the planets, given the initial conditions. These conditions for the planet formation process are the properties of the protoplanetary disk which are seen to surround most newly born stars. Observations of such circumstellar disks show that they come, as planets, with a variety of properties in terms of e.g. their mass or lifetime. For an individual planetary system, the properties of the protoplanetary disk from which it formed are mostly unknown, except maybe for the dust-to-gas ratio in the disk, which is likely correlated with the stellar metallicity that can be measured spectroscopically today. This means that the initial conditions are also only known in a statistical sense, which again makes a statistical approach appropriate. Observational constraints Fig. III.1.1 shows two of the most important statistical observational constraints. Explaining the structures seen in the two plots is one of the declared goals of planetary population synthesis. The left panels shows the (minimum) mass M versus the semimajor axis a of the exoplanets together with the planets of the Solar System. The extreme diversity, but also the existence of certain structures in the a – M diagram is obvious. For exoplanets, the mass-distance diagram has become a representation of similar importance as the Hertzsprung- Russell diagram for stellar astrophysics. In the plot, one can distinguish several groups of planets. There are for example massive, close-in planets without equivalent in the Solar System. Such hot Jupiters are found around approximately 1 % of solar like stars (Mayor et al. 2011). A class of extrasolar planets that has only been detected in the last few years thanks to the progress in the observational precision are low-mass planets with masses between 1 to 30 Earth masses. These super-Earths and mini-Neptunes seem to be very abundant, since roughly 48 % of FGK stars are found to have such a companion with a period of up to 100 days (Mayor et al. 2011). 39
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Annual Report 2011 Max Planck Institute for Astronomy

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