Water destruction by X-rays in young stellar objects

Aims. 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). Methods. 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 nH = 104–109 cm−3 and temperatures between T = 10–1000 K are studied. Results. Three different regimes are found: for T < 100 K, 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 100 K T 250 K, H2O is reduced from initial x(H2O) ≈ 10−4 following ice evaporation to x(H2O) ≈ 10−6 for FX 10−3 erg s−1 cm−2 (t = 104 yr) and for FX 10−4 erg s−1 cm−2 (t = 105 yr). At higher temperatures (T 250 K) 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 ≈ 5000 yr) 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. Conclusions. X-rays reduce the water abundances especially in regions where the gas temperature is T 250–300 K for fluxes FX 10−5–10−4 erg s−1 cm−2. The affected regions can be envelopes, disks or outflow hot spots. The average water abundance in Class I sources for LX 1027 erg s−1 is predicted to be x(H2O) 10−6. Central UV fields have a negligible influence, unless the photons can escape through cavities.