Determining the Ice Desorption Mechanism in Cold Molecular Clouds

Interstellar ices are readily formed from accretion of atoms and molecules onto cold dust grains, and in dense cores the gas would be depleted in allheavy molecules were it not for the operation of some nonthermal desorption process. Methanol molecules are only formed efficiently from hydrogenation of COmolecules accreted onto grains, but is still widely observed in cold dense clouds. This attests to the operation of desorption processes not related to the heating up of the gas from a nearby star. Proposed mechanisms for injecting the ice mantle molecules into the gasphase includes both continuous processes, such as cosmicray impacts, photodesorption, or ejection upon formation, and more transient ones such as clumpclump collisions, and effects of embedded protostars influencing the chemistry through shocks or dissipation of magnetohydrodynamic (MHD) waves. The presence of distinct peaks of methanol emission at positions significantly offset from protostars, however, implies that the desorption process is indeed transient, and likely to also disrupt a major part of the ice mantles. This would lead to very high water abundances at these peaks, clearly distinguishable from what is expected from photodesorption or steadystate gasphase chemistry.In order to constrain the active ice desorption mechanisms in cold molecular clouds, we thus propose to use HIFI to observe the ground state transition of ortho water at three different methanol peaks: two at different distances from the Class I protostar Barnard 5 IRS1, and one in L1512, a region free from protostellar activity. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]