Water destruction by X-rays in young stellar objects
Institute of Astronomy, ETH Zürich, 8092 Zürich, Switzerland e-mail: email@example.com
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
4 Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
Accepted: 27 January 2006
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 –109 cm-3 and temperatures between –1000 K are studied.
Results.Three different regimes are found: for 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 K, H2O is reduced from initial following ice evaporation to for erg s-1 cm-2 ( yr) and for erg s-1 cm-2 ( yr). At higher temperatures ( K) and hydrogen densities, water can persist with even for high X-ray fluxes. Water is destroyed in both Class 0 and I envelopes on relatively short timescales ( 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 –300 K for fluxes –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 erg s-1 is predicted to be . Central UV fields have a negligible influence, unless the photons can escape through cavities.
Key words: stars: formation / ISM: molecules / X-rays: ISM / astrochemistry
© ESO, 2006