ATOMS: ALMA Three-millimeter Observations of Massive Star-forming
  regions -- VIII. A search for hot cores by using C$_2$H$_5$CN, CH$_3$OCHO and
  CH$_3$OH lines
  Hot cores characterized by rich lines of complex organic molecules are
considered as ideal sites for investigating the physical and chemical
environments of massive star formation. We present a search for hot cores by
using typical nitrogen- and oxygen-bearing complex organic molecules
(C$_2$H$_5$CN, CH$_3$OCHO and CH$_3$OH), based on ALMA Three-millimeter
Observations of Massive Star-forming regions (ATOMS). The angular resolutions
and line sensitivities of the ALMA observations are better than 2 arcsec and 10
mJy/beam, respectively. A total of 60 hot cores are identified with 45 being
newly detected, in which the complex organic molecules have high gas
temperatures ($>$ 100 K) and small source sizes ($<$ 0.1 pc). So far this is
the largest sample of hot cores observed with similar angular resolution and
spectral coverage. The observations have also shown nitrogen and oxygen
differentiation in both line emission and gas distribution in 29 hot cores.
Column densities of CH$_3$OH and CH$_3$OCHO increase as rotation temperatures
rise. The column density of CH$_3$OCHO correlates tightly with that of
CH$_3$OH. The pathways for production of different species are discussed. Based
on the spatial position difference between hot cores and UC~H{\sc ii} regions,
we conclude that 24 hot cores are externally heated while the other hot cores
are internally heated. The observations presented here will potentially help
establish a hot core template for studying massive star formation and
astrochemistry.
