Stellar cosmic rays as an important source of ionization in protoplanetary discs: a disc mass-dependent process
Rodgers-Lee, DonnaORCID: 0000-0002-0100-1297, Taylor, A. M., Downes, TurloughORCID: 0000-0002-7639-5446 and Ray, TomORCID: 0000-0002-2110-1068
(2019)
Stellar cosmic rays as an important source of ionization in protoplanetary discs: a disc mass-dependent process.
Monthly Notices Of The Royal Astronomical Society, 491
(4).
pp. 4742-4751.
ISSN 0035-8711
We assess the ionizing effect of low-energy protostellar cosmic rays in protoplanetary discs
around a young solar mass star for a wide range of disc parameters. We assume a source of
low-energy cosmic rays located close to the young star that travels diffusively through the
protoplanetary disc. We use observationally inferred values from nearby star-forming regions
for the total disc mass and the radial density profile. We investigate the influence of varying
the disc mass within the observed scatter for a solar mass star. We find that for a large range
of disc masses and density profiles that protoplanetary discs are ‘optically thin’ to low-energy
(∼3 GeV) cosmic rays. At R ∼ 10 au, for all of the discs that we consider (Mdisc = 6.0 × 10−4–
2.4 × 10−2M), the ionization 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
that assumed a much denser disc that may be appropriate for a more embedded source. At R
∼ 70 au, the ionization rate due to stellar cosmic rays dominates in ∼50 per cent of the discs.
These are the less massive discs with less steep density profiles. At this radius, there is at least
an order of magnitude difference in the ionization rate between the least and most massive
disc that we consider. Our results indicate, for a wide range of disc masses, that low-energy
stellar cosmic rays provide an important source of ionization at the disc mid-plane at large
radii (∼70 au).