2:30 PM - 2:45 PM
[PPS06-04] The amount of cosmic rays in protoplanetary disks and their effects on ionization degrees
Keywords:Cosmic rays, Protoplanetary Disks, Ionization fraction, Magnetic fields
Protoplanetary disk gas is weakly ionized by various ionization sources, such as cosmic rays, X-rays, ultraviolet, and decay of radionuclides. Ionization fraction of the gas is essential information to study the magnetohydrodynamic evolution of the disk, because it determines the coupling between the gas and magnetic fields. Cosmic rays are also known to play an important role in chemical evolution by triggering reactions in the disk. Thus, we need to understand how deep inside of the disk cosmic rays can penetrate.
Previous studies adopted interstellar cosmic-ray ionization rate at the surface of the disk and the attenuation of cosmic rays are considered based on the vertical column of the gas from the point of interest to the surface of the disk/infinity. However, the actual path of cosmic ray particles should be affected by magnetic fields, and thus so will the column density swept by them. Cosmic rays are focused towards a protoplanetary disk from a larger area by magnetic fields, but some of them are reflected due to magnetic mirrors; in total, we found that the flux arriving at a protoplanetary disk is reduced by a factor of 4. We also found that low energy cosmic rays are captured by azimuthal component of the fields in the surface area of the disk and lose their energy before entering to the inside. Thus, the cosmic-ray ionization rate is expected to be smaller than previously thought.
Previous studies adopted interstellar cosmic-ray ionization rate at the surface of the disk and the attenuation of cosmic rays are considered based on the vertical column of the gas from the point of interest to the surface of the disk/infinity. However, the actual path of cosmic ray particles should be affected by magnetic fields, and thus so will the column density swept by them. Cosmic rays are focused towards a protoplanetary disk from a larger area by magnetic fields, but some of them are reflected due to magnetic mirrors; in total, we found that the flux arriving at a protoplanetary disk is reduced by a factor of 4. We also found that low energy cosmic rays are captured by azimuthal component of the fields in the surface area of the disk and lose their energy before entering to the inside. Thus, the cosmic-ray ionization rate is expected to be smaller than previously thought.