Desorption of Grain Mantles in Quiescent Dark Clouds by h2 Formation

Astronomy and Astrophysics – Astronomy

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Molecular Processes - Ism: Clouds - Cosmic Rays - Dust, Extinction - Ism: Molecules

Scientific paper

We investigate the consequences of a desorption mechanism driven by the energy released when H2 forms on dust. We show that if the process is at least 1 per cent efficient then this mechanism is as effective as others that are (also) initiated by cosmic rays. If the process is sufficiently efficient that about one molecule is released per H2 formation, then substantial gas-phase abundances of molecules can be maintained in cold dark quiescent molecular clouds for many times the freeze-out time. The CO fraction in the grain mantle is shown to depend on the gas-phase H-atom abundance. The CO deposited initially is in an environment of H2O; CO deposited at later times is in an environment of non-polar molecules, mostly CO. The ratio of non-polar CO to polar CO is around unity unless CO is chemically processed to other species, in which case the ratio exceeds 10. The processing of CO in cold dark quiescent clouds is required to be very slow.

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