It has been proposed that some hot molecular cores (HMCs) harbor a young embedded massive star, which heats an infalling envelope and accretes mass at a rate high enough to ``choke off'' an incipient HII region. This class of HMCs would mark the youngest phase known of massive star formation. In order to test this hypothesis, we model this type of object calculating the radiative transfer through a spherically symmetric dusty envelope infalling onto a central OB star, with accretion rates from 0.0006 to 0.001 solar masses per year. The dust thermal spectrum from infrared to radio wavelengths is derived and is compared with the observed fluxes of several hot cores which may be internally heated. We find that the data are best fitted using an envelope with the density distribution resulting from the collapse of a singular logatropic sphere, instead of that of a singular isothermal sphere. We conclude that several of these sources may be undergoing an intense accretion phase and find in all the cases that the accretion luminosity exceeds the stellar luminosity. We discuss the implications of this phase on the formation of massive stars.

26 pages, 6 figures