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Surveys of protoplanetary disks in star-forming regions of similar agerevealed significant variations in average disk mass between some regions. Forinstance, disks in the Orion Nebular Cluster (ONC) and Corona Australis (CrA)are on average smaller than disks observed in Lupus, Taurus, Chamaeleon I orOphiuchus. In contrast to previous models that study truncation of disks at alate stage of their evolution, we investigate whether disks may already be bornwith systematically smaller disk sizes in more massive star-forming regions asa consequence of enhanced ionization rates. Assuming various cosmic-rayionization rates, we compute the resistivities for ambipolar diffusion andOhmic dissipation with a chemical network, and perform 2D non-idealmagnetohydrodynamical protostellar collapse simulations. A higher ionizationrate leads to stronger magnetic braking, and hence to the formation of smallerdisks. Accounting for recent findings that protostars act as forges of cosmicrays and considering only mild attenuation during the collapse phase, we showthat a high average cosmic-ray ionization rate in star-forming regions like theONC or CrA can explain the detection of smaller disks in these regions. Ourresults show that on average a higher ionization rate leads to the formation ofsmaller disks. Therefore, smaller disks in regions of similar age can be theconsequence of different levels of ionization, and may not exclusively becaused by disk truncation via external photoevaporation. We strongly encourageobservations that allow measuring the cosmic-ray ionization degrees indifferent star-forming regions to test our hypothesis.

Ionization: a possible explanation for the difference of mean disk sizes in star-forming regions