A SEARCH FOR PRECURSORS OF ULTRACOMPACT HII-REGIONS IN A SAMPLE OF LUMINOUS IRAS SOURCES - II - VLA OBSERVATIONS

We have used the Very Large Array (VLA) to search for radio continuum emission towards a sample of 67 IRAS sources selected from a previous study. All observed sources are associated with high density molecular gas, exhibit an infrared spectral energy distribution characteristic of very cold young stellar objects and many of them are associated wit h H2O masers. The observed sample is divided into two groups of source s: High, with IRAS spectral energy distributions resembling those of u ltracompact HII regions, and Low, for which previously collected evide nce suggests that they may contain a higher fraction of protostellar o bjects than the High group; such objects might not have started hydrog en burning yet. Radio continuum emission was detected towards 37 sourc es (55%), although only in 22 cases an association with the IRAS sourc e is established. Of the latter, 9 (24%) objects belong to the Low typ e and 13 (43%) to the High type. Thus, we find that 76% of Low and 57% of High sources are not associated with a radio counterpart. Because the majority of the sources have luminosities above N 10(4) L., corres ponding to central stars of spectral type between B2 and O7, the lack of radio emission is interpreted as being due to the action of accreti ng matter that chokes off the expansion of the ionised gas. We show th at this requires only moderate mass accretion rates, below similar to 10(-4) M. yr(-1) Alternatively, dust absorption can also effectively a bsorb W photons and the gas column density implied by our observations indicates values in excess of 10(22) cm(-2) The physical properties o f IRAS sources with associated radio counterpart derived from the pres ent observations do not distinguish between High and Low sources. Thes e sources are likely to be ZAMS stars with variable amounts of dust wi thin the ionised region which acts as UV field absorber. The large maj ority of detected sources (75%) have spherical or unresolved morpholog y, while 15% are irregular or multiply peaked and only 10% have a core -halo structure. These results agree with the known properties of ultr acompact HII regions, even though the average luminosity of the presen t sample is an order of magnitude lower than that in previous studies.