Water destruction by X-rays in young stellar objects
Abstract
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 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. 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 ergs s-1 cm-2 (t = 104 yrs) and for FX > 10-4 ergs s-1 cm-2 (t = 105 yrs). At higher temperatures (T > 250 K) and hydrogen densities, water can persist with x(H2O) ~ 10-4 even for high X-ray fluxes. The X-ray and FUV models are applied to envelopes around low-mass Class 0 and I young stellar objects (YSOs). Water is destroyed in both Class 0 and I envelopes on relatively short timescales (t ~ 5000 yrs) 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. The average water abundance in Class I sources for LX > 1027 ergs s-1 is predicted to be x(H2O) < 10-6.
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