The TOPG\"ot high-mass star-forming sample. I. Methyl cyanide emission
  as tracer of early phases of star formation
  The TOPG\"ot 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.
The sources have been observed with the IRAM 30m 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 TOPG\"ot sources to
derive physical parameters. We use the MADCUBA software to analyze the emission
of methyl cyanide (CH$_3$CN), a well-known tracer of high-mass star formation.
The emission of the $\rm{CH_3CN(5_{K}-4_{K})}$ K-transitions has been detected
towards 73 sources (85% of the sample), with 12 non-detections and one source
not observed in the frequency range of $\rm{CH_3CN(5_{K}-4_{K})}$. The emission
of CH$_3$CN has been detected towards 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 $X_{\rm CH_3CN}<4.0\times10^{-11}$,
and a range in abundance of $4.0\times10^{-11}<X_{\rm
CH_3CN}<7.0\times10^{-11}$ 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 $\rm{CH_3CN}$ can thus be
used to identify high-mass star-forming regions in early phases of
star-formation.
