A search for precursors of Ultracompact HII regions in a sample of luminous IRAS sources III. Circumstellar dust properties

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 asso ciated with dense gas. In spite of their high luminosities (L greater than or similar to 10(4) L-circle dot), only ten of these sources are also assoc iated with a radio counterpart. Tn 17 cases we could identify a clear peak of millimeter emission associated with the IRAS source, while in 9 sources the millimeter emission was either extended or faint and a clear peak could not be identified; upper limits were found in 4 cases only. The submm/mm observations allow us to make a more accurate estimate of the source luminosities, typically of the order of 10(4) L-circle dot. Using si mple greybody fitting to model the observed spectral energy distribution, w e derive global properties of the circumstellar dust associated with the de tected sources. We find that the dust temperature varies from 24 K to 45 K, while the exponent of the dust emissivity vs frequency power-law spans a r ange 1.56 < beta < 2.38, characteristic of silicate dust; total circumstell ar masses range up to more than 500 M-circle dot. We present a detailed analysis of the sources associated with millimeter pe aks, but without radio emission. In particular, we find that for sources wi th comparable luminosities, the total column densities derived from the dus t masses do not distinguish between sources with and without radio counterp art. We interpret this result as an indication that dust does not play a do minant role in inhibiting the formation of the I-III region. We examine sev eral scenarios for their origin in terms of newborn ZAMS stars and although most of these (e.g. optically thick HII regions, dust extinction of Lyman photons, clusters instead of single sources) fail to explain the observatio ns, we cannot exclude that these sources are young stars already on the ZAM S with modest residual accretion that quenches the expansion of the HII reg ion, thus explaining the lack of radio emission in these bright sources. Fi nally, we consider the possibility that the IRAS sources are high-mass pre- ZAMS (or pre-H-burning) objects deriving most of the emitted luminosity fro m accretion.