EXCLUSION OF COSMIC RAYS IN PROTOPLANETARY DISKS: STELLAR AND MAGNETIC EFFECTS
By L. Cleeves, Fred C. Adams, E. Bergin. 2013
153 citations recorded by the source corpus.
Cosmic rays (CRs) are thought to provide an important source of ionization in the outermost and densest regions of protoplanetary disks; however, it is unknown to what degree they are physically present. As is observed in the solar system, stellar winds can inhibit the propagation of CRs within the circumstellar environment and subsequently into the disk. In this work, we explore the hitherto neglected effects of CR modulation by both stellar winds and magnetic field structures and study how these processes act to reduce disk ionization rates. We construct a two-dimensional protoplanetary disk model of a T-Tauri star system, focusing on ionization from stellar and interstellar FUV, stellar X-ray photons, and CRs. We show that stellar winds can power a heliosphere-like analog, i.e., a “T-Tauriosphere,” diminishing CR ionization rates by several orders of magnitude at low to moderate CR energies (ECR ⩽ 1 GeV). We explore models of both the observed solar wind CR modulation and a highly simplified estimate for “elevated” CR modulation as would be expected from a young T-Tauri star. In the former (solar analog) case, we estimate the ionization rate from galactic CRs to be ζCR ∼ (0.23–1.4) × 10−18 s−1. This range of values, which we consider to be the maximum CR ionization rate for the disk, is more than an order of magnitude lower than what is generally assumed in current models for disk chemistry and physics. In the latter elevated case, i.e., for a “T-Tauriosphere,” the ionization rate by CRs is ζCR ≲ 10−20 s−1, which is 1000 times smaller than the interstellar value. We discuss the implications of a diminished CR ionization rate on the gas physics by estimating the size of the resulting magnetorotational instability dead zones. Indeed, if winds are as efficient at CR modulation as predicted here, short-lived radionuclides (now extinct) would have provided the major source of ionization (ζRN ∼ 7.3 × 10−19 s−1) in the planet-forming zone of the young solar nebula.
Subfields: Astronomy and Astrophysics
View Paper: https://www.semanticscholar.org/paper/3d501090959fda0b44ddf0a88951e851623e3926
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In Astronomy and Astrophysics
Citations in the corpus
- Global Simulations of the Inner Regions of Protoplanetary Disks with Comprehensive Disk Microphysics (2017, background): “16SLR 19 1 Note that the rate is expected to be in the range of ~ ´ -( – )1 10 10 19 s−1 and is higher at smaller radii and early times (Umebayashi & Nakano 2009; Cleeves et al. 2013b).”
- Photodissociation and photoionisation of atoms and molecules of astrophysical interest (2017, background): “A reduction of the ionisation rate likely occurs in dark clouds and protoplanetary disks because of the shielding effects of the surrounding material (Padovani et al. 2009) or stellar winds and magnetic fields (Cleeves et al. 2013, 2015).”
- GLOBAL SIMULATIONS OF PROTOPLANETARY DISKS WITH OHMIC RESISTIVITY AND AMBIPOLAR DIFFUSION (2015, background): “External sources of ionization such as stellar X-rays, FUV photons and galactic cosmic rays are expected to ionize the disk surface layers, providing good coupling there – even though recent results suggest that cosmic rays may be highly attenuated by the stellar wind (Cleeves et al. 2013).”
- WEAK TURBULENCE IN THE HD 163296 PROTOPLANETARY DISK REVEALED BY ALMA CO OBSERVATIONS (2015, background): “Recent observations suggest that the cosmic ray ionization rate, important in the outer disk, may be lower than previously expected (Cleeves et al. 2013), further damping the influence of MRI in the outer disk.”