X-ray ionization rates in protoplanetary discs

Low-mass young stellar objects are powerful emitters of X-rays that can ionize and heat the discs and the young planets they harbour. The X-rays produce molecular ions that affect the chemistry of the disc atmospheres and their spectroscopic signatures. Deeper down, X-rays are the main ionization source and influence the operation of the magnetorotational instability, believed to be the main driver for the angular momentum redistribution crucial for the accretion and formation of these pre-main-sequence stars. X-ray ionization also affects the character of the dead zones around the disc mid-plane where terrestrial planets are likely to form. To obtain the physical and chemical effects of the stellar X-rays, their propagation through the disc has to be calculated taking into account both absorption and scattering. To date the only calculation of this type was done almost 15 years ago, and here we present new three-dimensional radiative transfer calculations of X-ray ionization rates in protoplanetary discs. Our study confirms the results from previous work for the same physical parameters. It also updates them by including a more detailed treatment of the radiative transfer and by using ionizing spectra and elemental abundance more appropriate for what is currently known about protoplanetary discs and their host stars. The new calculations for a typical ionizing spectrum yield, respectively, lower and higher ionization rates at high and low column densities at a given radius in a disc. The differences can be up to an order of magnitude near 1 au, depending on the abundances used.