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Young stellar binaries: The structure of circumbinary accretion disks

Subject Area Astrophysics and Astronomy
Term from 2012 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 210359296
 
Final Report Year 2018

Final Report Abstract

A substantial fraction of protoplanetary disks form around stellar binaries. The binary system generates a time-dependent non-axisymmetric gravitational potential, inducing strong tidal forces on the circumbinary disk. This leads to a change in basic physical properties of the circumbinary disk, which should in turn result in unique structures that are potentially observable with the current generation of observatories / instruments. We performed studies with the goal to identify these characteristic structures, constrain the physical conditions that cause them, and evaluate the feasibility of observing them in circumbinary disks. To achieve this, first we performed smoothed-particle, hydrodynamics, and magneto-hydrodynamics simulations: • The resulting density distributions are post-processed with 3D radiative transfer codes (Mol3D and MC3D) to generate re-emission and scattered light maps of the continuum radiation. Based on these distributions, we studied the influence of various parameters, such as the mass of the stellar components, mass of the disk, and binary separation on observable features in circumbinary disks. We find that the Atacama Large Millimeter/submillimeter Array (ALMA) as well as the Extremely Large Telescope (ELT) are capable of tracing asymmetries in the inner region of circumbinary disks, which are affected most by the binary-disk interaction. Observations at submillimetre/millimetre wavelengths allow the detection of the density waves at the inner rim of the disk and inner cavity. With the ELT one can partially resolve the innermost parts of the disk in the infrared wavelength range, including the disk’s rim, accretion arms, and potentially the expected circumstellar disks around each of the binary components. • We investigated the feasibility to observe Zeeman split spectral lines of circumstellar/circumbinary disks as a potential approach to derive the plane of sky component of the magnetic field. This study was motivated by the general assumption that magnetic fields play a key role in the disk evolution. In particular, we investigate which constraints for magnetic fields in circumstellar disks can be obtained from Zeeman observations of the 113 GHz CN lines. Furthermore, we analyze the requirements to perform these observations and their dependence on selected quantities. We found that Zeeman observations of the 113 GHz CN lines would indeed provide significant insights into the magnetic field of circumstellar disks. However, with the capabilities of recent and upcoming instrument/observatories, even spatially unresolved observations would be challenging. Nevertheless, these observations are feasible for the most massive disks with a strong magnetic field and high abundance of CN/H. The most restrictive quantity is the magnetic field strength, which should be at least in the order of ∼1 mG. In addition, the inclination of the disk should be around 60◦ to preserve the ability to derive the line-of-sight magnetic field strength and to obtain a sufficiently high circularly polarized flux. Finally, we simulate the radiative transfer of a circumbinary disk model based on a magnetohydrodynamic simulation. We find that our analysis of the magnetic field is still applicable. However, owing to their lower circularly polarized emission, Zeeman observations of circumbinary disks with a significant separation between their stellar components (∼ 10 AU) are more challenging if compared to circumstellar disks with a single star. Besides these feasibility studies for future observations, we analyzed the binary T-Tauri system VV Corona Australis, in which both components host disks. Based on high-angular resolution observatons in the mid-infrared and millimeter range as well as near-infrared spectroscopy, we performed an indepth study of the configuration and structure of this enigmatc system. In preparation of the above studies and in parallel to them we performed several related studies which were required to develop and test the numerical tools and analysis techniques.

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