Stellar cosmic rays as an important source of ionisation in protoplanetary disks: a disk mass dependent process

Abstract: We assess the ionising effect of low energy protostellar cosmic rays in protoplanetary disks around a young solar mass star for a wide range of disk parameters. We assume a source of low energy cosmic rays located close to the young star which travel diffusively through the protoplanetary disk. We use observationally inferred values from nearby star-forming regions for the total disk mass and the radial density profile. We investigate the influence of varying the disk mass within the observed scatter for a solar mass star. We find that for a large range of disk masses and density profiles that protoplanetary disks are "optically thin" to low energy ($\sim$3 GeV) cosmic rays. At $R\sim10$au, for all of the disks that we consider ($M_\mathrm{disk}=6.0\times10^{-4} - 2.4\times 10^{-2}M_\odot$), the ionisation rate due to low energy stellar cosmic rays is larger than that expected from unmodulated galactic cosmic rays. This is in contrast to our previous results which assumed a much denser disk which may be appropriate for a more embedded source. At $R\sim70$au, the ionisation rate due to stellar cosmic rays dominates in $\sim$50% of the disks. These are the less massive disks with less steep density profiles. At this radius there is at least an order of magnitude difference in the ionisation rate between the least and most massive disk that we consider. Our results indicate, for a wide range of disk masses, that low energy stellar cosmic rays provide an important source of ionisation at the disk midplane at large radii ($\sim$70au).