Abstract
High-energy radiation from the central T Tauri and protostars plays an impor- tant role in shaping protoplanetary disks and influences their evolution. Such radiation, in particular X-rays and extreme-ultraviolet (EUV) radiation, is predominantly generated in unstable stellar magnetic fields (e.g., the stellar corona), but also in accretion hot spots. Even jets may produce X-ray emission. Cosmic rays, i.e., high-energy particles either from the interstellar space or from the star itself, are of crucial importance. Both high- energy photons and particles ionize disk gas and lead to heating. Ionization and heating subsequently drive chemical networks, and the products of these processes are accessi- ble through observations of molecular line emission. Furthermore, ionization supports the magnetorotational instability and therefore drives disk accretion, while heating of the disk surface layers induces photoevaporative flows. Both processes are crucial for the dispersal of protoplanetary disks and therefore critical for the time scales of planet for- mation. This chapter introduces the basic physics of ionization and heating starting from a quantum mechanical viewpoint, then discusses relevant processes in astrophysical gases and their applications to protoplanetary disks, and finally summarizes some properties of the most important high-energy sources for protoplanetary disks.
Highlights
Protostellar and protoplanetary disks are immersed in various types of high-energy radiation and particle fluxes
Among them figure most prominently extreme-ultraviolet and X-ray radiation from the magnetized stellar corona of the central star and in some cases from collimated jets; hard X-rays and gamma-rays from stellar flares; interstellar X-rays from neighboring stars in stellar clusters or from distributed hot plasma in star-forming regions; and galactic cosmic rays and energetic particles ejected during stellar flares (“stellar cosmic rays”)
We provide some rough estimates for ionization in an astrophysical gas irradiated by an point-like
Summary
Protostellar and protoplanetary disks are immersed in various types of high-energy radiation and particle fluxes. High-energy radiation and cosmic rays modify cold protostellar disk gas primarily due to their ionization power. In the process of ionizing disk gas, the upper layers of the disk are heated to temperatures of up to several thousand K. Both temperature gradients and ionized species drive a plethora of chemical reactions that would be impossible in the absence of high-energy sources. The subsequent sections will discuss applications of X-ray and cosmic-ray irradiation of protoplanetary disks and show results pertaining to the disk ionization degree, the temperature structure, the magnetorotational instability, photoevaporation of disk material, and chemical processing. I will summarize our knowledge about the ionizing sources themselves
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