2006A&A...460..721M

We determine the fractional SiO abundance in high-mass star-forming cores, and investigate its dependence on physical conditions, to provide constraints on the chemistry models of the formation of SiO in the gas phase or via grain mantle evaporation. The work addresses also CH₃CCH chemistry, as the kinetic temperature is determined using this molecule. We estimate the physical conditions of 15 high-mass star-forming cores and derive the fractional SiO and CH₃CCH abundances using spectral line and dust continuum observations with the SEST. The kinetic temperatures as derived from CH₃CCH range from 25 to 39K, the average being 33K. The average gas density in the cores is 4.5x10⁶cm^–3. The SiO emission regions are extended and typically half of the integrated line emission comes from the velocity range traced out by CH₃CCH emission. The upper limit of SiO abundance in this ``quiescent'' gas component is ∼10^–10. The average CH₃CCH abundance is about 7x10^–9. It shows a shallow, positive correlation with the temperature, whereas SiO shows the opposite tendency. We suggest that the high CH₃CCH abundance and its possible increase when the clouds become warmer is related to the intensified desorption of the chemical precursors of the molecule from grain surfaces. In contrast, the observed tendency of SiO does not support the idea that the evaporation of Si-containing species from the grain mantles would be important, and it contradicts models where neutral reactions with activation barriers dominate SiO production. A possible explanation for the decrease is that warmer cores represent more evolved stages of core evolution with fewer high-velocity shocks and thus less efficient SiO replenishment.