X-irradiation of molecular gas will produce a complicated sequence of chemical reactions of molecular material at various distances within and around YSOs (e.g. Krolik & Kallman 1983, Maloney et al 1996, Lepp & Dalgarno 1996, Yan & Dalgarno 1997, Aikawa et al 1998). The detailed consequences are quite difficult to calculate in a fully self-consistent model, and they depend on the assumed chemical reaction network. In the warmer regions, a variety of neutral reactions are promoted by X-ray heating of the gas, whereas in cooler regions the X-rays stimulate molecular synthesis by ion-molecule reactions.
Kastner et al (1997) report possible evidence for X-ray induced chemistry in the disk of the nearby ( 70#70 pc) CTT star TW Hya. They attribute the high CN/HCN ratio and HCO+ abundance to X-ray illumination. The isotopes of HCO+ have long served as tracers of electron fractions in molecular clouds; HCO+ itself may be destroyed near the X-ray source by dissociative recombination with electrons, as observed in the molecular cloud close to the ``microquasar'' 1E1740.7-2942 (Yan & Dalgarno 1997). In YSO environs, X-ray ionization can thus be probed with high-resolution millimeter interferometers. Other potentially observable tracers of X-ray induced chemistry are located in the far-IR and include the 14942#42m rotational transition of HeH+, and the [OI]6342#42m, [SII]3542#42m, [FeII]2642#42m and [CII]15842#42m fine-structure lines (Maloney et al 1996). Such lines are being sought with the ISO satellite.
A dust particle subject to an X-ray will absorb most of the secondary electrons, resulting in increased temperature. For example, a small grain with a=5 nm absorbs all photons with energy Ex < 0.1keV, whereas a large grain with a=200 nm absorbs all photons with Ex < 2keV (Dwek & Smith 1996). Very small grains may evaporate completely, which suggests that their ``unidentified infrared'' emission features in the 71#71m band may disappear near X-ray luminous YSOs (Voit 1992). This effect may have been seen in active galactic nuclei, where ISO spectra show the disappearance of the near-infrared PAH features (Genzel et al 1998).
Laboratory accelerator X-ray exposures of various materials relevant to interstellar grains (carbonaceous compounds, silicates, etc.) have been recently conducted to study the structural damage produced by X-irradiation (Gougeon 1998). Radiation doses equivalent to those obtained at 0.1pc from a TTS having 72#72 erg s-1 for a period of 108 yr cause dehydrogenation and breakage of aromatic rings in hydrocarbons but have little effect on silicates. X-irradiation will also cause photodissociation and chemical changes in the ices of dust grain mantles (Cornelison et al 1998). These possible effects of YSO X-rays on dust mainly affect line profiles, and their detection must await high-resolution spectroscopic study with future instruments, such as those planned for the SIRTF satellite.