X-ray ionization of a fraction of the primarily molecular gas within and around YSOs is particularly important because of its role in coupling gas and magnetic fields. Unlike high-mass YSOs emitting copious ultraviolet photons, which create a fully ionized HII region terminated by a thin transition region, a low-mass YSO emitting X-rays produces an extended region of low ionization and X-ray heating. The effects of the X-rays gradually become negligible due to absorption by intervening material and geometric dilution. Research on these X-ray dissociation regions began many years ago (e.g. Dalgarno & McCray 1972, Halpern & Grindley 1980, Lepp & McCray 1983, Krolik & Kallman 1983) and has recently witnessed a vigorous renewal.
The most important X-ray ionization process of cold material is photoionization of the inner K and L shells of heavy elements: a 56#56 keV photoelectron is generated, which produces a cascade of secondary electrons that are responsible most subsequent ionizations in the medium. The mean energy to create an ion pair in a cosmic abundance gas is approximately 35 eV, so that approximately 30 secondary electrons are created by an initial 1 keV photoelectron. The atomic structure of the heavy atom rearranges itself with the ejection of a few Auger electrons and fluorescent photons (Kaastra & Mewe 1993). In addition, X-rays with Ex > 2 keV can ionize when they scatter through a large angle (Halpern & Grindlay 1980), an effect known as Compton ionization. The energy transfer is much smaller than in the photoelectric effect, especially for the heavy atoms responsible for most of the absorption cross section, and Compton energy losses do not become competitive until 57#57keV.
The total photoionization cross-section decreases rapidly with photon energy, roughly as 58#58. It also depends on the atomic weight of the material as 59#59. In a gas with cosmic abundances, most of the ionizations occur in light atoms (H and He); but metals are responsible for the absorption of higher-energy X-rays. For 1 keV photons, the optical depth is unity for a hydrogen column density of 60#60 cm-2 which, for a normal interstellar dust-to-gas ratio, is equivalent to 61#61 or 62#62 (Ryter 1996). Therefore, 5 keV photons, which are detected in YSOs with the ASCA satellite, are able to penetrate to 63#63 and thus have a potential effect even in deeply embedded environments.
The principal competing sources of ionizing radiation in the vicinity of YSOs are ambient ultraviolet starlight, which dominates the outer 64#64 regions of star forming clouds, and galactic cosmic rays, which produce 65#65 ionizations s-1(McKee 1989). The cosmic ray ionization rate, which corresponds to ionization fractions 66#66 throughout cloud interiors, is particularly uncertain; for instance, low-energy cosmic rays may be excluded from dense cloud cores by magnetic scattering (Lepp 1992). For a typical YSO with Lx = 1029 erg s-1and photon energies approximately 1 keV, the total X-ray ionization may (depending on intervening absorption) dominate cosmic ray ionization out to distances 67#67 erg s-1) pc 68#68 AU (Krolik & Kallman 1983, Glassgold et al 1999).
The X-ray ionization effects in the molecular cloud environment may be considerably greater than this estimate because the recombination timescale is of order 101 yr, considerably longer than the < 10-1 yr timescale of flare recurrence in YSOs. For cloud ionization, the episodic YSO flares flare appear essentially as a continuous high-intensity process. Thus, a single YSO with flare luminosities around 1030-1031 erg s-1 may be the dominant source of ionization for a 0.1 pc cloud core (Glassgold et al 1999). Note that the low ionization fractions involved are sufficient to couple the neutral matter with any magnetic fields and require ambipolar diffusion for gas passage through the field (e.g. Ciolek & Mouschovias 1995).
On smaller spatial scales, the implications of YSO X-ray ionization for circumstellar disks has recently been calculated by Glassgold et al (1997) and Igea & Glassgold (1999). They consider a model disk similar to the solar nebula illuminated by an X-ray source elevated a few stellar radii above the corotation radius (Figure 3 top). Figure 8 shows the ionization of X-rays at various energies in terms of the column density of the disk measured perpendicularly from its surface, 69#69. These particular curves were calculated at a radial distance of 1 AU from the star. YSO X-rays penetrate to some distance towards the midplane in the inner disk, whereas they reach all disk material in the outer disk. The inner disk thus has a midplane neutral ``dead zone'' surrounded by an ionized zone (Gammie 1996).
Although the resulting disk ionization level is always low, the YSO X-rays impact a far larger volume in the disk than is ionized by cosmic rays. The X-ray ionization will couple the disk material to magnetic fields and MHD processes. In particular, this weakly ionized differentially rotating disk is thought to stimulate the Balbus-Hawley magnetorotational instability (Balbus & Hawley 1991), which will induce a MHD turbulent viscosity and promote flow towards the inner boundary of the disk (Gammie 1996). By this means, YSO X-rays may regulate the supply of material accreting onto the protostar and for ejection in high-velocity winds, Herbig-Haro jets, or FU Orionis outbursts.
In an analogous fashion, X-ray ionization may be important for the coupling between disk and outflow required for outflow acceleration. Models of highly collimated Herbig-Haro jets and weakly collimated molecular bipolar flows often require that the rotational energy of the disk be converted into a radial acceleration by some kind of magneto-centrifugal process (Königl & Ruden 1993, Pudritz et al 1999). No calculations of the X-ray effects on outflow physics have been made to date.