TY - JOUR

T1 - Water destruction by X-rays in young stellar objects

AU - Stäuber, P.

AU - Jørgensen, J. K.

AU - Van Dishoeck, E. F.

AU - Doty, S. D.

AU - Benz, A. O.

PY - 2006/7/1

Y1 - 2006/7/1

N2 - 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 = 104-109 cm-3 and temperatures between T = 10-1000 K are studied. Results. 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(H 2O) ≈10-4 following ice evaporation to x(H 2O) ≈ 10-6 for FX ≳ 10-3 erg s-1 cm-2 (t = 104 yr) and for FX ≳ 10-4 erg s-1 cm-2 (t = 105 yr). At higher temperatures (T ≳ 250 K) 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 ≈ 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 ≲ 250-300 K for fluxes FX ≳ 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 LX ≳ 1027 erg s -1 is predicted to be x(H2O) ≲ 10-6. Central UV fields have a negligible influence, unless the photons can escape through cavities.

AB - 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 = 104-109 cm-3 and temperatures between T = 10-1000 K are studied. Results. 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(H 2O) ≈10-4 following ice evaporation to x(H 2O) ≈ 10-6 for FX ≳ 10-3 erg s-1 cm-2 (t = 104 yr) and for FX ≳ 10-4 erg s-1 cm-2 (t = 105 yr). At higher temperatures (T ≳ 250 K) 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 ≈ 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 ≲ 250-300 K for fluxes FX ≳ 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 LX ≳ 1027 erg s -1 is predicted to be x(H2O) ≲ 10-6. Central UV fields have a negligible influence, unless the photons can escape through cavities.

KW - Astrochemistry

KW - ISM: Molecules

KW - Stars: Formation

KW - X-rays: ISM

UR - http://www.scopus.com/inward/record.url?scp=33745762373&partnerID=8YFLogxK

U2 - 10.1051/0004-6361:20054263

DO - 10.1051/0004-6361:20054263

M3 - Journal article

AN - SCOPUS:33745762373

VL - 453

SP - 555

EP - 565

JO - Astronomy & Astrophysics

JF - Astronomy & Astrophysics

SN - 0004-6361

IS - 2

ER -