  We aim to identify and characterize cores in the high-mass proto-cluster W49,
determine their evolutionary stages and measure the associated lifetimes. We
built a catalog of 129 cores extracted from an ALMA 1.3 mm continuum image at
0.26" (2900 au) angular resolution. The association between cores and
Hyper/Ultra Compact HII (H/UC HII) regions was established from the analysis of
VLA 3.3 cm continuum and H30$\alpha$ line observations. We also looked for
emission of hot molecular cores (HMCs) using the methyl formate doublet at
218.29 GHz. We identified 40 cores associated with an H/UC HII region and 19
HMCs over the ALMA mosaic. The 52 cores with an H/UC HII region and/or a HMC
are assumed to be high-mass protostellar cores, while the rest of the core
population likely consists in prestellar cores and low-mass protostellar cores.
We found a good agreement between the two tracers of ionized gas, with 23
common detections and only four cores detected at 3.3 cm and not in
H30$\alpha$. The spectral indexes from 3.3 cm to 1.3 mm range from 1, for the
youngest cores with partially optically thick free-free emission, to about
-0.1, that is optically thin free-free emission obtained for cores likely more
evolved. Using as a reference the H/UC HII regions, we found the statistical
lifetimes of the HMC and massive protostellar phases in W49N to be about
$6\times10^4$ yr and $1.4\times10^5$ yr, respectively. We also showed that HMC
can co-exist with H/UC HII regions during a short fraction of the core
lifetime, about $2\times10^4$ yr. This indicates a rapid dispersal of the inner
molecule envelope once the HC HII is formed.
