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 = 10(4) - 10(9) cm(-3) and temperatures between T = 10-1000 K are studied. Results. 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 100 K less than or similar to T similar to 250K, H2O is reduced from initial x(H2O) approximate to 10(-4) following ice evaporation to x(H2O) approximate to 10(-6) for F-X greater than or similar to 10(-3) erg s(-1) cm(-2) (t = 10(4) yr) and for F-X similar to 10(-4) erg s(-1) cm(-2) ( t = 10(5) yr). At higher temperatures ( T greater than or similar to 250 K) and hydrogen densities, water can persist with x(H2O) approximate to 10(-4) even for high X-ray fluxes. Water is destroyed in both Class 0 and I envelopes on relatively short timescales ( t approximate to 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 less than or similar to 250 - 300 K for fluxes FX greater than or similar to 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 L-X greater than or similar to 10(27) erg s(-1) is predicted to be x(H2O) less than or similar to 10(-6). Central UV fields have a negligible influence, unless the photons can escape through cavities.