In terms of physical processes, ionization of the disk is important for the operation of the magnetocentrifugal instability (MRI; Balbus and Hawley 1991) thought to be the main driver of accretion in young stellar objects. Although cosmic rays have long been suspected to be an effective disk ionization source (Gammie, 1996), the high-level, hard coronal emission and frequent stellar flares may be more effective ionizing sources (Glassgold et al., 1997; Feigelson et al., 2002b). This is even more so as young solar analogs in the T Tau stage drive very strong winds that are very likely magnetized; such winds effectively shield the inner disk from cosmic rays, as does the present-day solar wind, at least for cosmic rays with energies 100 MeV.
The distance to which stellar X-ray ionization dominates over that produced by galactic cosmic rays can be estimated to be (Glassgold et al., 1997; Montmerle, 2001)
where E is the photon energy in the range of 1 – 20 keV (for other energies, the first term in the parentheses must be generalized to (kTX) / (1 keV)), is the cosmic ray ionization rate, and J0 is an attenuation factor (J0 0.13 for optical depth of unity for a 1 keV photon). The values given in the parentheses are characteristic for our situation, with the cosmic-ray ionization rate referring to a UV shielded molecular core. Stellar X-ray ionization therefore dominates cosmic ray ionization out to about 1000 AU, i.e., across most of the radius of a typical circumstellar disk. Taking into account frequent, strong flares, significant portions of molecular cores may predominantly be ionized by the central star rather than by cosmic rays.
Glassgold et al. (1997) and Igea and Glassgold (1999) modeled ionization and heating of circumstellar disks by stellar coronal X-ray sources. The incoming X-ray photons are subject to Compton scattering and photoelectric absorption as they propagate through the disk. X-ray photons may interact with molecules or atoms by ejecting a fast (primary) photoelectron. This photoelectron collisionally produces on average 27 secondary electrons and ions (for a photon energy of 1 keV). Harder photons on average penetrate deeper and thus ionize layers of the disk closer to the equatorial plane, while softer X-rays ionize closer to the disk surface. The disk ionization fraction is then determined when an equilibrium between ionization and recombination has been reached. Electron fractions of 10–15 – 10–10 are obtained at vertical disk column densities of NH = 1027 – 1021 cm–2 (as measured from infinity) for distances of 0.1 – 10 AU from the central star. The precise results depend somewhat on the hardness of the X-ray spectrum (a modest LX = 1029 erg s–1 has been assumed).
The important points here are: 1) that the ionization fraction at the top of the disk is orders of magnitude higher than the ionization fraction that would result from standard cosmic-ray irradiation, and 2) that at vertical column densities of NH = 1024 – 1025 cm–2 and less, the disk is sufficiently ionized to become unstable against MRI. The disk surface will thus couple to the magnetic field and accrete to the star. In contrast, the deeper layers remain decoupled and therefore “quiescent”, at least within 5 AU (Figure 35, Igea and Glassgold 1999). These are the likely sites of planet formation (Glassgold et al., 1997). Modifications of these calculations by introducing trace heavy metals and diffusion have been discussed by Fromang et al. (2002) and Ilgner and Nelson (2006).
Disk irradiation and photoionization by stellar UV photons is further responsible for photoevaporation of gaseous disks (Hollenbach et al., 1994; Clarke et al., 2001; Alexander et al., 2006a,b), and therefore the long-term accretion history of the star-disk system. Additional X-ray irradiation is, however, of secondary importance only (Alexander et al., 2004).
Apart from disk ionization, X-ray irradiation also leads to disk heating (Igea and Glassgold, 1999; Glassgold et al., 2004). While dust disks are heated by the central star’s optical and UV light to a few 100 K at distances up to a few AU, the gas component may thermally decouple in particular in the upper layers where the density is small. A model calculation based on accretion viscosity heating combined with X-ray heating due to the central star shows that the upper layers of the gaseous disk (NH 1021 cm–2) can be heated up to 5000 K (Figure 36). This holds even for low viscous heating efficiency where the X-ray heating contribution entirely dominates (Glassgold et al., 2004). At the same time, the strong temperature gradients in the temperature inversion region lead to the production of large amounts of “warm” CO. Similar calculations by Gorti and Hollenbach (2004) support the above picture of X-rays dominating gas heating at the disk surface.
The elevated ultraviolet and X-ray activity level of young low-mass stars leads to significant irradiation of circumstellar accretion disks. Interactions between high-energy photons and disk matter is evident from X-ray photoabsorption in star-disk systems seen edge-on, but also from reprocessed starlight: Spatially unresolved FUV fluorescence lines of H2 have been detected from several CTTS (Brown et al., 1981; Valenti et al., 2000; Ardila et al., 2002; Herczeg et al., 2002, 2006), but usually not from WTTS (Valenti et al., 2000); this emission is reprocessed stellar Ly emission most intensely radiated from the accretion spots. At least in cases where the line radial velocities are coincident with stellar radial velocities, an origin of the fluorescence in a hydrogenic surface layer of the inner accretion disk at temperatures of 2000 – 3500 K is likely (Herczeg et al., 2002, 2004) although significantly blueshifted H2 emission points to outflow-related fluorescence in some systems (Brown et al., 1981; Ardila et al., 2002; Walter et al., 2003; Saucedo et al., 2003; Herczeg et al., 2006). Fluorescent emission is also generated by reprocessing of X-ray photons; X-ray fluorescence is seen in particular in the 6.4 keV line of cold iron (Imanishi et al., 2001; Tsujimoto et al., 2005; Favata et al., 2005).
Simple energy considerations are revealing: Herczeg et al. (2004) estimate the rate of energy deposited in the environment of the CTTS TW Hya as a result of Ly photoexcitation and subsequent far-ultraviolet fluorescence of H2 to be 1.4 × 1029 erg s–1. The total soft X-ray luminosity of 1.4 × 1030 erg s–1 will at least partially heat the disk surface layer further (Igea and Glassgold, 1999).
Warm H2 has also been detected through infrared 2.12 m ro-vibrational emission from several T Tauri stars (Weintraub et al., 2000; Bary et al., 2003). This emission is thought to be excited by collisions between H2 molecules and X-ray induced non-thermal electrons, or by an UV radiation field. High temperatures, of order 1000 – 2000 K, are required, but such temperatures are predicted from disk irradiation by X-rays out to several AU (Glassgold et al., 2004), or by UV radiation from the central star out to 10 AU if the star shows an UV excess (Nomura and Millar, 2005).
Glassgold et al. (2007) proposed forbidden [Ne ii and Ne iii] infrared line emission at 12.81 m and 15.55 m, respectively, to be indicative of X-ray irradiation. The high first ionization potential of Ne (21.6 eV) indeed requires Ly continuum or X-ray photons for ionization (or cosmic rays, which are unlikely to be abundant in the inner disk region). The transitions are collisionally excited in warm gas, requiring temperatures of a few 1000 K, attained in disk surface layers out to about 20 AU for X-ray irradiated disks (Glassgold et al., 2004). The [Ne ii] 12.81 m transition has indeed been detected in several CTTS (Pascucci et al., 2007; Lahuis et al., 2007; Ratzka et al., 2007).
As a further consequence, specific chemical reactions may be induced. For example, Ly itself can dissociate molecules like H2 and H2O and can ionize Si and C (Herczeg et al., 2004). Ly radiation photodissociates HCN (but not CN), which leads to an enhancement of CN relative to HCN (Bergin et al., 2003).
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