The TOPGöt high-mass star-forming sample. I. Methyl cyanide emission as tracer of early phases of star formation

Aims. The TOPGot project studies a sample of 86 high-mass star-forming regions in different evolutionary stages from starless cores to ultra compact HII regions. The aim of the survey is to analyze different molecular species in a statistically significant sample to study the chemical evolution in high-mass star-forming regions, and identify chemical tracers of the different phases.Methods. The sources have been observed with the IRAM 30 m telescope in different spectral windows at 1, 2, and 3 mm. In this first paper, we present the sample and analyze the spectral energy distributions (SEDs) of the TOPGot sources to derive physical parameters such as the dust temperature, T-dust, the total column density, N-H2, the mass, M, the luminosity, L, and the luminosity-to-mass ratio, L/M, which is an indicator of the evolutionary stage of the sources. We use the MADCUBA software to analyze the emission of methyl cyanide (CH3CN), a well-known tracer of high-mass star formation.Results. We built the spectral energy distributions for similar to 80% of the sample and derived T-dust and N-H2 values which range between 9-36 K and similar to 3 x 10(21)-7 x 10(23) cm(-2), respectively. The luminosity of the sources spans over four orders of magnitude from 30 to 3 x 10(5) L-circle dot, masses vary between similar to 30 and 8 x 10(3) M-circle dot, and the luminosity-to-mass ratio L/M covers three orders of magnitude from 6 x 10(-2) to 3 x 10(2) L-circle dot/M-circle dot. The emission of the CH3CN(5(K)-4(K)) K-transitions has been detected toward 73 sources (85% of the sample), with 12 nondetections and one source not observed in the frequency range of CH3CN(5(K)-4(K)). The emission of CH3CN has been detected toward all evolutionary stages, with the mean abundances showing a clear increase of an order of magnitude from high-mass starless cores to later evolutionary stages. We found a conservative abundance upper limit for high-mass starless cores of XCH3CN < 4.0 x 10(-11), and a range in abundance of 4.0 x 10(-11) < XCH3CN < 7.0 x 10(-11) for those sources that are likely high-mass starless cores or very early high-mass protostellar objects. In fact, in this range of abundance we have identified five sources previously not classified as being in a very early evolutionary stage. The abundance of CH3CN can thus be used to identify high-mass star-forming regions in early phases of star-formation.