Astronomy and Astrophysics, volume 453, 555-565 (2006/7-2)
Water destruction by X-rays in young stellar objects.
STAEUBER P., JORGENSEN J.K., VAN DISHOECK E.F., DOTY S.D. and BENZ A.O.
Abstract (from CDS):
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 nH=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 FX>10–3erg/s/cm2 (t=104yr) and for FX>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 FX>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 LX>1027erg/s is predicted to be x(H2O)≲10–6. Central UV fields have a negligible influence, unless the photons can escape through cavities.