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Non-equilibrium chemistry and destruction of CO by X-ray flares

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Aharonian,  Felix
Division Prof. Dr. Werner Hofmann, MPI for Nuclear Physics, Max Planck Society;

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Citation

Mackey, J., Walch, S., Seifried, D., Glover, S. C. O., Wuensch, R., & Aharonian, F. (2019). Non-equilibrium chemistry and destruction of CO by X-ray flares. Monthly Notices of the Royal Astronomical Society, 486(1), 1094-1122. doi:10.1093/mnras/stz902.


Cite as: https://hdl.handle.net/21.11116/0000-0005-4D7B-C
Abstract
Sources of X-rays such as active galactic nuclei and X-ray binaries are often variable by orders of magnitude in luminosity over time-scales of years. During and after these flares the surrounding gas is out of chemical and thermal equilibrium. We introduce a new implementation of X-ray radiative transfer coupled to a time-dependent chemical network for use in 3D magnetohydrodynamical simulations. A static fractal molecular cloud is irradiated with X-rays of different intensity, and the chemical and thermal evolution of the cloud are studied. For a simulated 10(5) M-circle dot fractal cloud, an X-ray flux <0.01 erg cm(-2) s(-1) allows the cloud to remain molecular, whereas most of the CO and H-2 are destroyed for a flux of >= 1 erg cm(-2) s(-1). The effects of an X-ray flare, which suddenly increases the X-ray flux by 10(5)x, are then studied. A cloud exposed to a bright flare has 99 per cent of its CO destroyed in 10-20 yr, whereas it takes >10(3) yr for 99 per cent of the H-2 to be destroyed. CO is primarily destroyed by locally generated far-UVemission from collisions between non-thermal electrons and H-2; He+ only becomes an important destruction agent when the CO abundance is already very small. After the flare is over, CO re-forms and approaches its equilibrium abundance after 10(3)-10(5) yr. This implies that molecular clouds close to Sgr A(star) in the Galactic Centre may still be out of chemical equilibrium, and we predict the existence of clouds near flaring X-ray sources in which CO has been mostly destroyed but H is fully molecular.