X-ray chemistry in the envelopes around young stellar objects

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

Abstract

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 H 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  AU, the region enhanced by X-rays is 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  erg s^-1. CO₂ abundances are reduced in the gas-phase through X-ray induced FUV photons. For temperatures  K, H₂O is destroyed by X-rays with luminosities  erg s^-1. Best-fit models for AFGL 2591 predict an X-ray luminosity  erg s^-1 with a hard X-ray spectrum  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 . 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  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.