2000A&A...355..617M

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>10⁴L_☉), 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 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 10⁴L_☉. Using simple greybody fitting to model the observed spectral energy distribution, we derive global properties of the circumstellar dust associated with the detected sources. We find that the dust temperature varies from 24K to 45K, while the exponent of the dust emissivity vs frequency power-law spans a range 1.56<β<2.38, characteristic of silicate dust; total circumstellar masses range up to more than 500M_☉. We present a detailed analysis of the sources associated with millimeter peaks, but without radio emission. In particular, 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 (e.g. optically thick HII regions, dust extinction of Lyman photons, clusters instead of single sources) 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, thus explaining the lack of radio emission in these bright sources. 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.