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

Mackey, Jonathan and Walch, Stefanie and Seifried, Daniel and Glover, Simon C.O. and Wünsch, Richard and Aharonian, Felix (2019) Non-equilibrium chemistry and destruction of CO by X-ray flares. Monthly Notices of the Royal Astronomical Society, 486 (1). pp. 1094-1122.

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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_sun 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 H2 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 10^5x, 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 H2 to be destroyed. CO is primarily destroyed by locally generated far-UV emission from collisions between non-thermal electrons and H2; 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* 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.

Item Type: Article
Uncontrolled Keywords: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2019 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Divisions: School of Cosmics Physics > Astronomy and Astrophysics
Date Deposited: 29 Apr 2019 15:15
Last Modified: 15 Dec 2022 02:28

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