2006A&A...453..555S

We study the H₂O chemistry in star-forming environments under the influence of a central X-ray source and a central far ultraviolet (FUV) radiation field. The X-ray models are applied to envelopes around low-mass Class 0 and I young stellar objects (YSOs). The gas-phase water chemistry is modeled as a function of time, hydrogen density and X-ray flux. To cover a wide range of physical environments, densities between n_H=10⁴-10⁹cm^–3 and temperatures between T=10-1000K are studied. Three different regimes are found: for T<100K, the water abundance is of order 10^–7-10^–6 and can be somewhat enhanced or reduced due to X-rays, depending on time and density. For 100K≲T≲250K, H₂O is reduced from initial x(H₂O)≃10^–4 following ice evaporation to x(H₂O)≃10^–6 for F_X>10^–3erg/s/cm2 (t=10⁴yr) and for F_X>10^–4erg/s/cm2 (t=10⁵yr). At higher temperatures (T>250K) and hydrogen densities, water can persist with x(H₂O)≃10^–4 even for high X-ray fluxes. Water is destroyed in both Class 0 and I envelopes on relatively short timescales (t≃5000yr) for realistic X-ray fluxes, although the effect is less prominent in Class 0 envelopes due to the higher X-ray absorbing densities there. FUV photons from the central source are not effective in destroying water. X-rays reduce the water abundances especially in regions where the gas temperature is T≲250-300K for fluxes F_X>10^–5-10^–4erg/s/cm2. The affected regions can be envelopes, disks or outflow hot spots. The average water abundance in Class I sources for L_X>10²⁷erg/s is predicted to be x(H₂O)≲10^–6. Central UV fields have a negligible influence, unless the photons can escape through cavities.