Protostars and Planets VI, Heidelberg, July 15-20, 2013
A CRITICAL LOOK AT THE IONIZATION OF PROTOPLANETARY DISKS: CHEMISTRY IN THE PRESENCE OF AN ANALOGUE HELIOSPHERE
Cleeves, L. Ilsedore (University of Michigan)
Bergin, Edwin (University of Michigan)
Adams, Fred (University of Michigan)
Cosmic rays as ionizing agents have important astrochemical and physical consequences in the dense ISM. However, as evidenced by the existence of our own solar Heliosphere, the degree to which CRs are present in the circumstellar environment is unknown. This issue is especially important for low mass pre-main sequence stars with molecular disks, i.e. T-Tauri systems. In these disks the physical and chemical properties, such as ionization fraction, are key in setting the conditions for future planet formation. Within our own Solar System the solar wind shields the inner ~100 AU from galactic cosmic rays with energies below a 300 MeV. T-Tauri stars with relatively high mass loss rates and magnetic activity can likewise power a ”T Tauriosphere” that could substantially reduce the galactic CR flux incident on their protoplanetary disks. We find that T-Tauri wind modulation of CRs can be so effective that ionization resulting from short-lived radionuclide decay could dominate the outer disk midplane ionization at typical ISM abundances, if the decay products do not freely escape. We examine implications for both the extent of MRI ”dead-zones” as well as for ion chemistry in disks resulting from low CR fluxes. We also provide predictions on how to use observable molecular ions to infer the presence of extrasolar Heliospheres around T-Tauri stars.
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