Chemodynamics of circumstellar non-solar-metallicity disks

Very young stars form in collapsing interstellar clouds, initially forming massive gas disks from which gas flows down onto the central star. Inside these disks, planets also begin to form and grow. It is therefore important to better understand the dynamics and composition of such disks. Although disks can now be observed with advanced telescopes, their slow evolutionary processes are followed in sophisticated computer simulations that will also be the subject of our project. In such simulations, one usually assumes that the gas has the same composition as the Sun or other neighboring stars in the Milky Way (mostly hydrogen and helium with admixtures of heavier elements), but planet formation which depends on the admixtures of heavy elements may proceed very differently in disks with different composition. This is in particular also true in distant galaxies or was the case for older population stars in the Milky Way. New generations of telescopes like JWST and the E-ELT will soon be able to observe such non-solar circumstellar disks in detail. In our project, we will numerically simulate the disk evolution assuming very non-solar composition, computing the dynamic motion of gas and dust in the disks together with chemical reactions taking place in the gas. We will proceed along two lines: First, we will perform detailed studies of chemical reactions taking place in various disks using advanced models that also compute temperatures and the gas ionization at every point in the disk. These models are static and do not evolve in time, but give a precise snapshot description of a disk. And second, we will perform hydrodynamic simulations including important chemical reactions that change the properties of the gas in time. In all calculations, we will also consider the radiation from the star and its propagation through the disk itself, which induces chemical reactions. Our most important goals are to understand how the composition of a disk evolves for different initial compositions, how it can form clumps that may be important for planet formation, and how gas moves down onto the star, perhaps in episodic bursts, again for different gas compositions. We will emphasize the link to future observing opportunities with large telescopes. The proposed procedures are novel and have not been attempted before. First, we will combine detailed chemical calculations with the dynamical simulations following the disk evolution reliably over millions of years. Second, we will explore new territory with disk compositions completely different from the standard solar mixture. Because the composition is important for chemical processing, we will address the crucial issue of the formation of organic molecules as precursors and conditions for the formation of life in various planetary systems over cosmic time.

Stars form in collapsing interstellar clouds, initially forming massive gas disks from which gas flows down onto the central star. Inside these disks, planets also begin to form and grow. This process, however, crucially depends on the inventory of chemical elements heavier than hydrogen and helium, which are referred to as metals or metallicity. The metallicity is lower in distant galaxies than in the Milky Way. It is therefore important to better understand the dynamics and chemical composition of such low-metallicity disks, since new generations of telescopes like the ESO ELT will soon be able to observe them in detail. We studied the evolutionary processes in circumstellar disks using sophisticated computer simulation models, which were specifically developed to probe the low-metallicity environments. We found that some physical effects that are pertinent to the young forming stars in the Solar neighborhood, such as sharp brightening known as the FU Orionis-type objects, can also be present in low-metallicity environments, proving the universal character of the star formation process. However, the chemical evolution of circumstellar disks at low metallicities differs from that of the solar-metallicity counterparts and cannot be understood or reproduced by scaling down the respective chemical species abundances of the solar-metallicity disk. Our work also demonstrated that disk gravitational fragmentation can be a promising pathway to form wide-orbit giant planets in low-metallicity environments, which can be of fundamental importance for constraining the chronology of the origins of life in the Universe. The evolution of the star and its circumstellar disk is often interweaved with both positive and negative feedback loops. We found that stars in low metallicity environments rotate faster than their solar-metallicity cousins, their disks are heated differently and have shorter lifetimes compared to the solar-metallicity counterparts, which can help us understand disk evolution and dispersal not only in distant low-metallicity galaxies, but also in low-metallicity worlds in the Milky Way halo.