SiO and CH 3 CCH abundances and dust emission in high - mass star - forming cores ⋆

Aims. The main goal of the present study is to determine the fractional SiO abundance in high-mass star-forming cores, and to investigate its dependence on the physical conditions. In this way we wish to provide constraints on the chemistry models concerning the formation of SiO in the gas phase or via grain mantle evaporation. The work addresses also CH 3 CCH chemistry as the kinetic temperature is determined using this molecule. Methods. We estimate the physical conditions of 15 high-mass star-forming cores and derive the fractional SiO and CH 3 CCH abundances in them by using spectral line and dust continuum observations with the SEST. Results. The kinetic temperatures as derived from CH 3 CCH range from 25 to 39 K, the average being 33 K. The average gas density in the cores is 4.5 · 10 6 cm −3. The SiO emission regions are extended and typically half of the integrated line emission comes from the velocity range traced out by CH 3 CCH emission. The upper limit of SiO abundance in this 'quiescent' gas component is ∼ 10 −10. The average CH 3 CCH abundance is about 7 · 10 −9. It shows a shallow, positive correlation with the temperature, whereas SiO shows the opposite tendency. Conclusions. We suggest that the high CH 3 CCH abundance and its possible increase when the clouds get 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 with the models where neutral reactions with activation barriers dominate the 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.