Ionization and heating by X-rays and cosmic rays

Abstract

Abstract. High-energy radiation from the central T Tauri and protostars plays an important 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 highenergy photons and particles ionize disk gas and lead to heating. Ionization and heating subsequently drive chemical networks, and the products of these processes are accessible 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 formation. 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.

Living With a Red Dwarf: Rotation and X-Ray and Ultraviolet Properties of the Halo Population Kapteyn’s Star*

“…It also does not agree with the rotation period of 143 ± 3 days proposed by Robertson et al (2015), which lead them to question the existence of Kapteyn b. Thus, in an indirect way, our result lends support to the Anglada-Escudé et al (2014a, 2015 findings that Kapteyn's Star hosts at least two possible super-earth size planets. However, additional photometry (and spectroscopy) are still preferred to confirm the star's rotation period.…”

“…see Engle & Guinan 2011). This rotation period (if confirmed by additional observations) does not appear to be related to the orbital periods of the planets proposed by Anglada-Escudé et al (2014a, 2015. It also does not agree with the rotation period of 143 ± 3 days proposed by Robertson et al (2015), which lead them to question the existence of Kapteyn b.…”

“…If the planet's atmosphere survives this strong XUV irradiation (or is replenished later by outgassing and cometary collision), then that planet (at a later time) may have the potential to support the development of life. Of course, as discussed by Güdel (2015) and others, the properties of the planet itself -such as its orbit, instellation, mass, atmosphere, composition, water inventory, albedo, rotation rate, plate tectonics and geomagnetic field also play important roles on the planet's suitability for life.…”

“…Recent statistical analyses of Kepler Mission exoplanet data indicate that a significant fraction (∼15 -25%) of red dwarfs are expected to host terrestrialsize planets within their habitable zones (see: e.g. Dressing & Charbonneau 2013, 2015Bonfils et al 2013). This indicates statistically that ∼35 -50 potentially habitable, terrestrialsize planets are expected to orbit the 240+ red dwarfs within ∼10 pc (∼33 light years) of the Sun.…”

“…As the nearest Pop II red dwarf, it serves as an old age anchor for calibrating Activity/Irradiance-Rotation-Age relations, and an important test bed for stellar dynamos and the resulting X-ray -UV emissions of slowly rotating, near-fully convective red dwarf stars. Adding to the notoriety, Kapteyn's Star has recently been reported to host two super-Earth candidates, one of which (Kapteyn b) is orbiting within the habitable zone (Anglada-Escudé et al 2014a, 2015. However, Robertson et al (2015) questioned the planet's existence since its orbital period may be an artifact of activity, related to the star's rotation period.…”