EDP Sciences
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Volume 440, Number 3, September IV 2005
Page(s) 949 - 966
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20052889

A&A 440, 949-966 (2005)
DOI: 10.1051/0004-6361:20052889

X-ray chemistry in the envelopes around young stellar objects

P. Stäuber1, S. D. Doty2, E. F. van Dishoeck3 and A. O. Benz1

1  Institute of Astronomy, ETH-Zentrum, 8092 Zurich, Switzerland
    e-mail: pascalst@astro.phys.ethz.ch
2  Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
3  Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands

(Received 16 February 2005 / Accepted 6 June 2005)

We present chemical models of the envelope of a young stellar object (YSO) exposed to a central X-ray source. The models are applied to the massive star-forming region AFGL 2591 for different X-ray fluxes. Model results for this region show that the X-ray ionization rate with and without the effects of Compton scattering differs by only a few percent and the influence of Compton scattering on the chemistry is negligible. The total X-ray ionization rate is dominated by the "secondary" ionization rate of H2 resulting from fast electrons. The abundance profiles of several molecular and atomic species are shown to depend on the X-ray luminosity and on the distance from the source. The carbon, sulphur and nitrogen chemistries are discussed. It is found that He+ and H3+ are enhanced and trigger a peculiar chemistry. Several molecular X-ray tracers are found and compared to tracers of the far ultraviolet (FUV) field. Like ultraviolet radiation fields, X-rays enhance simple hydrides, ions and radicals. In contrast to ultraviolet photons, X-rays can penetrate deep into the envelope and affect the chemistry even at large distances from the source. Whereas the FUV enhanced species cover a region of $\approx $200-300 AU, the region enhanced by X-rays is $\gtrsim $1000 AU. We find that N2O, HNO, SO, SO+, HCO+, CO+, OH+, N2H+, SH+ and HSO+ (among others) are more enhanced by X-rays than by FUV photons even for X-ray luminosities as low as $L_{{\rm X}}
\approx 10^{30}$ erg s-1. CO2 abundances are reduced in the gas-phase through X-ray induced FUV photons. For temperatures $T
\lesssim 230$ K, H2O is destroyed by X-rays with luminosities $L_{{\rm X}}
\gtrsim 10^{30}$ erg s-1. Best-fit models for AFGL 2591 predict an X-ray luminosity $L_{{\rm X}} \gtrsim 10^{31}$ erg s-1 with a hard X-ray spectrum $T_{{\rm X}} \gtrsim 3 \times 10^7$ K. This is the first time that the X-ray flux of a highly obscured source has been estimated by its envelope chemistry. Furthermore, we find $L_{{\rm X}}/L_{{\rm bol}}
\approx 10^{-6}$. The chemistry of the bulk of the envelope mass is dominated by cosmic-ray induced reactions rather than by X-ray induced ionization for X-ray luminosities $L_{{\rm X}} \lesssim 10^{33}$ erg s-1. The calculated line intensities of HCO+ and HCS+ show that high-J lines are more affected than lower J lines by the presence of X-rays due to their higher critical densities, and that such differences are detectable even with large aperture single-dish telescopes. Future instruments such as Herschel-HIFI or SOFIA will be able to observe X-ray enhanced hydrides whereas the sensitivity and spatial resolution of ALMA is well-suited to measure the size and geometry of the region affected by X-rays.

Key words: stars: formation -- stars: individual: AFGL 2591 -- ISM: molecules -- X-rays: ISM

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© ESO 2005