Methanol is formed via surface reactions on icy dust grains. Methanol is alsodetected in the gas-phase at temperatures below its thermal desorptiontemperature and at levels higher than can be explained by pure gas-phasechemistry. The process that controls the transition from solid state togas-phase methanol in cold environments is not understood. The goal of thiswork is to investigate whether thermal CO desorption provides an indirectpathway for methanol to co-desorb at low temperatures. MixedCH$_{3}$OH:CO/CH$_{4}$ ices were heated under UHV (ultra-high vacuum)conditions and ice contents are traced using RAIRS (reflection absorption IRspectroscopy), while desorbing species were detected mass spectrometrically. Anupdated gas-grain chemical network was used to test the impact of the resultsof these experiments. The physical model used is applicable for TW Hya, aprotoplanetary disk in which cold gas-phase methanol has recently beendetected. Methanol release together with thermal CO desorption is found to bean ineffective process in the experiments, resulting in an upper limit of$\leq$ 7.3 $\times$ 10$^{-7}$ CH3OH molecules per CO molecule over all icemixtures considered. Chemical modelling based on the upper limits shows thatco-desorption rates as low as 10$^{-6}$ CH$_{3}$OH molecules per CO moleculeare high enough to release substantial amounts of methanol to the gas-phase atand around the location of the CO thermal desorption front in a protoplanetarydisk. The impact of thermal co-desorption of CH3OH with CO as a grain-gasbridge mechanism is compared with that of UV induced photodesorption andchemisorption.
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