Water destruction by X-rays in young stellar objects

¹ Institute of Astronomy, ETH Zürich, 8092 Zürich, Switzerland e-mail: pascalst@astro.phys.ethz.ch
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

Received: 28 September 2005
Accepted: 27 January 2006

Abstract

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.