Evolutionary study of complex organic molecules in high-mass
  star-forming regions
  We have studied four complex organic molecules (COMs), methyl formate
($CH_3OCHO$), dimethyl ether ($CH_3OCH_3$), formamide ($NH_2CHO$), and ethyl
cyanide ($C_2H_5CN$), towards a large sample of 39 high-mass star-forming
regions representing different evolutionary stages, from early to evolved
phases. We aim to identify potential correlations between the molecules and to
trace their evolutionary sequence through the star formation process. We
analysed spectra obtained at 3, 2, and 0.9 mm with the IRAM-30m telescope. We
derived the main physical parameters for each species by fitting the molecular
lines. We compared them and evaluated their evolution, also taking several
other interstellar environments into account. We report detections in 20
sources, revealing a clear dust absorption effect on column densities. Derived
abundances are ~$10^{-10}-10^{-7}$ for $CH_3OCHO$ and $CH_3OCH_3$,
~$10^{-12}-10^{-10}$ for $NH_2CHO$, and ~$10^{-11}-10^{-9}$ for $C_2H_5CN$. The
abundances of $CH_3OCHO$, $CH_3OCH_3$, and $C_2H_5CN$ are very strongly
correlated (r>0.92) across ~4 orders of magnitude. $CH_3OCHO$ and $CH_3OCH_3$
show the strongest correlations in most parameters, and a nearly constant ratio
(~1) over a remarkable ~9 orders of magnitude in luminosity for a wide variety
of sources: pre-stellar to evolved cores, low- to high-mass objects, shocks,
Galactic clouds, and comets. This indicates that COMs chemistry is likely early
developed and then preserved through evolved phases. Moreover, the molecular
abundances clearly increase with evolution. We consider $CH_3OCHO$ and
$CH_3OCH_3$ to be most likely chemically linked: they could e.g. share a common
precursor, or be formed one from the other. We propose a general scenario for
all COMs, involving a formation in the cold, earliest phases of star formation
and a following increasing desorption with the progressive heating of the
evolving core.
