A survey of 27 galactic star-forming regions in the 6 K −5 K , 5 K − 4 K , and 8 K − 7 K CH 3 CN lines at 110, 92, and 147 GHz, respectively, was made. Twenty-five sources were detected at 110 GHz, nineteen at 92 GHz, and three at 147 GHz. The strongest CH 3 CN emission arise in hot cores in the regions of massive star formation. CH 3 CN abundance in these objects is larger than 10 −9 due to grain mantle evaporation. Weaker CH 3 CN lines were found in a number of sources. They may arise either in warm (30-50 K) dense (10 5 -10 7 cm −3 ) clouds, or in hot regions accompanied by colder gas.

Abstract. The results of a survey of 63 galactic star-forming regions in the 6K − 5K and 5K − 4K methyl acetylene lines at 102 and 85 GHz, respectively, are presented. Fourty-three sources were detected at 102 GHz, and twentyfive at 85 GHz. Emission was detected towards molecular clouds with kinetic temperatures 20-60 K (so-called "warm clouds"). The CH3CCH abundances in these clouds appeared to be about several units × 10 −9 . Five mapped sources were analyzed using the maximum entropy method. The sizes of the mapped clouds fall within the range between 0.1 and 1.7 pc, virial masses -between 90 -6200 M , and densities -between 6 × 10 4 and 6 × 10 5 cm −3 . The CH3CCH sources spatially coincide with the CO and CS sources. Chemical evolution simulations showed that the typical methyl acetylene abundance in the observed clouds corresponds to an age of ≈ 6 × 10 4 years.

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