Speaker
Description
Surveys of protoplanetary disks in star-forming regions of similar age revealed
significant variations in average disk mass between some regions. Disks in the Orion Nebular Cluster (ONC) and Corona Australis (CrA) are on average a factor of a few smaller than disks observed in Lupus, Taurus, Chamaeleon I or Ophiuchus. We aim for an understanding of the physical mechanism behind this spread by testing the influence of cosmic-ray ionization rates on the formation process of protoplanetary disks. We run non-ideal magnetohydrodynamical protostellar collapse simulations assuming different cosmic-ray ionization rates. We compute the resitivities for ambipolar diffusion and Ohmic dissipation with a chemical network. Consistent with previous results, our models demonstrate that a higher cosmic-ray ionization rate leads to stronger magnetic braking, and hence to the formation of smaller disks. Considering recent findings that protostars act as forges of comsic rays, we show that a high average cosmic-ray ionization rate in
star-forming regions like the ONC or CrA can explain the detection of smaller disks in these regions. Our results show that on average a higher cosmic-ray ionization rate leads to the formation of smaller disks. Therefore, smaller disks in regions of similar age can be the consequence of different levels of ionization, and may not exclusively be caused by disk truncation via external photoevaporation. We strongly encourage observations that allow measuring the cosmic-ray ionization degrees in different star-forming regions to test our hypothesis.
