2005A&A...440..949S

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 AFGL2591 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 ≃200-300AU, the region enhanced by X-rays is >1000AU. 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_X≃10³⁰erg/s. CO₂ abundances are reduced in the gas-phase through X-ray induced FUV photons. For temperatures T≲230K, H₂O is destroyed by X-rays with luminosities L_X>10³⁰erg/s. Best-fit models for AFGL2591 predict an X-ray luminosity L_X>10³¹erg/s with a hard X-ray spectrum T_X>3x10⁷K. 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_X/L_bol≃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_X≲10³³erg/s. 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.