  The James Clerk Maxwell Telescope has been used to obtain submillimeter and
millimeter continuum photometry of a sample of 30 IRAS sources previously
studied in molecular lines and centimeter radio continuum. All the sources have
IRAS colours typical of very young stellar objects (YSOs) and are associated
with dense gas. In spite of their high luminosities (L>10000 solar units), only
ten of these sources are also associated with a radio counterpart. In 17 cases
we could identify a clear peak of millimeter emission associated with the IRAS
source, while in 9 sources the millimeter emission was extended or faint and a
clear peak could not be identified; upper limits were found in 4 cases only.
  Using simple greybody fitting model to the observed SED, we derive global
properties of the circumstellar dust. The dust temperature varies from 24 K to
45 K, while the exponent of the dust emissivity vs frequency power-law spans a
range 1.56<beta<2.38, characteristic of silicate dust; total circumstellar
masses range up to more than 500 solar masses.
  We find that for sources with comparable luminosities, the total column
densities derived from the dust masses do not distinguish between sources with
and without radio counterpart. We interpret this result as an indication that
dust does not play a dominant role in inhibiting the formation of the HII
region. We examine several scenarios for their origin in terms of newborn ZAMS
stars and although most of these fail to explain the observations, we cannot
exclude that these sources are young stars already on the ZAMS with modest
residual accretion that quenches the expansion of the HII region. Finally, we
consider the possibility that the IRAS sources are high-mass pre-ZAMS (or
pre-H-burning) objects deriving most of the emitted luminosity from accretion.
