X-ray chemistry in the envelopes around young stellar objects

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 H 2 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 H$_3^+$ 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 N 2 O, HNO, SO, SO + , HCO + , CO + , OH + , N 2 H + , 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 . CO 2 abundances are reduced in the gas-phase through X-ray induced FUV photons. For temperatures $T \lesssim 230$ K, H 2 O 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.