Title: Stellar energetic particle ionization in protoplanetary disks around T Tauri stars,
Abstract: Anomalies in the abundance measurements of short lived radionuclides in
meteorites indicate that the protosolar nebulae was irradiated by a high amount
of energetic particles (E$\gtrsim$10 MeV). The particle flux of the
contemporary Sun cannot explain these anomalies. However, similar to T Tauri
stars the young Sun was more active and probably produced enough high energy
particles to explain those anomalies. We want to study the interaction of
stellar energetic particles with the gas component of the disk and identify
possible observational tracers of this interaction. We use a 2D radiation
thermo-chemical protoplanetary disk code to model a disk representative for T
Tauri stars. We use a particle energy distribution derived from solar flare
observations and an enhanced stellar particle flux proposed for T Tauri stars.
For this particle spectrum we calculate the stellar particle ionization rate
throughout the disk with an accurate particle transport model. We study the
impact of stellar particles for models with varying X-ray and cosmic-ray
ionization rates. We find that stellar particle ionization has a significant
impact on the abundances of the common disk ionization tracers HCO$^+$ and
N$_2$H$^+$, especially in models with low cosmic-ray ionization rates. In
contrast to cosmic rays and X-rays, stellar particles cannot reach the midplane
of the disk. Therefore molecular ions residing in the disk surface layers are
more affected by stellar particle ionization than molecular ions tracing the
cold layers/midplane of the disk. Spatially resolved observations of molecular
ions tracing different vertical layers of the disk allow to disentangle the
contribution of stellar particle ionization from other competing ionization
sources. Modeling such observations with a model like the one presented here
allows to constrain the stellar particle flux in disks around T Tauri stars.