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A search for precursors of ultracompact HII regions in a sample of luminous IRAS sources
II. VLA observations *
S. Molinari 1, 2, 3,
J. Brand 1,
R. Cesaroni 4,
F. Palla 4 and
G.G.C. Palumbo 2, 5
Received 1 September 1997 / Accepted 16 March 1998
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 with H2O masers. The observed sample is divided into two groups of sources: High , with IRAS spectral energy distributions resembling those of ultracompact HII regions, and Low , for which previously collected evidence suggests that they may contain a higher fraction of protostellar objects than the High group; such objects might not have started hydrogen burning yet.
Radio continuum emission was detected towards 37 sources (55%), although only in 22 cases an association with the IRAS source is established. Of the latter, 9 (24%) objects belong to the Low type 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 , corresponding to central stars of spectral type between B2 and O7, the lack of radio emission is interpreted as being due to the action of accreting matter that chokes off the expansion of the ionised gas. We show that this requires only moderate mass accretion rates, below yr-1. Alternatively, dust absorption can also effectively absorb UV photons and the gas column density implied by our observations indicates values in excess of cm-2.
The physical properties of IRAS sources with associated radio counterpart derived from the present observations do not distinguish between High and Low sources. These sources are likely to be ZAMS stars with variable amounts of dust within the ionised region which acts as UV field absorber. The large majority of detected sources (75%) have spherical or unresolved morphology, while 15% are irregular or multiply peaked and only 10% have a core-halo structure. These results agree with the known properties of ultracompact HII regions, even though the average luminosity of the present sample is an order of magnitude lower than that in previous studies.
Key words: stars: formation stars: pre-main sequence stars: circumstellar matter ISM: Hii regions sources as a function of wavelength: radio continuum: ISM
Present address: IPAC/Caltech, Mail Stop 100-22, Pasadena CA 91125, USA
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Online publication: July 7, 1998