J/A+A/336/339 J/A+A/336/339 VLA observations of ultracompact HII regions S Molinari J Brand R Cesaroni F Palla G G C Palumbo Astron. Astrophys. 336 339 1998 1998A&A...336..339M J/A+A/308/573 : Ammonia on YSOs IRAS sources, Paper I. (Molinari+ 1996) Infrared sources Radio sources YSOs circumstellar matter HII regions radio continuum: ISM stars: formation stars: pre-main sequence 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 H_2_O 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 ~10^4^L_{sun}_, 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 require s only moderate mass accretion rates, below ~10^-4^M_{sun}_/yr. Alternatively, dust absorption can also effectively absorb UV photons and the gas column density implied by our observations indicates values in excess of 10^22^cm^-2^. The physical properties of IRAS sources with associated radio counterpa rt 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.

Mol Molinari number from Paper I (Cat. <J/A+A/308/573>) --- Ass An * indicates association of radio emission with the IRAS source --- Com Identification of multiple components within a source, or multiple sources within a field --- Lam Wavelength of observation cm Run Code for observing run number=1 I: 26 June 1994 II: 4 Oct. 1994 III: 23-24 Jan. 1995 --- RAh Right ascension (1950) number=2 When there is no positions, the source has not been detected h RAm Right ascension (1950) min RAs Right ascension (1950) s DE- Sign of declination --- DEd Declination (1950) deg DEm Declination (1950) arcmin DEs Declination (1950) arcsec Sep Angular distance radio- and IRAS sources arcsec Note1 Note number=3 A "(" in this column, and a ")" in column 110 indicate that the values of all parameters in between refer to the image that has been degraded to the resolution of the other (lower resolution) image. --- Maj Observed major axis of radio peak arcsec Min Observed minor axis of radio peak arcsec PA Position angle radio peak (E of N) deg Majd Major axis radio peak, deconvolved by beam arcsec Mind Minor axis radio peak, deconvolved by beam arcsec PAd Position angle deconvolved radio peak (E of N) deg Ha1 Size of halo in one direction number=4 Halo: extended component above 2 {sigma} level arcsec --- --- Ha2 Size of halo in other direction arcsec Mc Morphological class number=5 S = spherical/unresolved CH = Core-Halo I = irregular/multiply peaked --- l_F To indicate next number is upper limit --- F Observed peak flux density (mJy/beam) mJy S Observed integrated flux mJy rms Observed rms (mJy/beam) number=6 If no value is listed, the theoretical rms (0.15 mJy/beam) is assumed mJy --- See note number=3 A "(" in this column, and a ")" in column 110 indicate that the values of all parameters in between refer to the image that has been degraded to the resolution of the other (lower resolution) image. --- Note Individual notes in note1a.dat or note1b.dat --- Mol Molinari number from Paper I (Cat. <J/A+A/308/573>) --- Ass An * indicates association of radio emission with the IRAS source --- Com Identification of multiple components within a source, or multiple sources within a field --- Lam Wavelength of observation cm Note1 Note number=1 A "(" in this column, and a ")" in column 68 indicate that the values of all parameters in between refer to the image that has been degraded to the resolution of the other (lower resolution) image. --- l_DiamP To indicate DiamP is upper limit --- DiamP Peak diameter pc DiamH Mean halo diameter pc Tb Brightness temperature K tau Optical depth * 1000. --- l_Ne To indicate ne is lower limit --- Ne Electron density 10+4cm-3 l_EM To indicate EM is lower limit --- EM Emission measure 10+6pc/cm6 l_M(Hii) To indicate M(Hii) is upper limit --- M(Hii) Mass of ionized hydrogen 10-3solMass log(Nly) Log10 of Lyman continuum flux [s-1] --- See note number=1 A "(" in this column, and a ")" in column 68 indicate that the values of all parameters in between refer to the image that has been degraded to the resolution of the other (lower resolution) image. --- Rat Ratio of integrated and peak fluxes --- SpI Spectral index --- Note Comments --- Mol Mol number --- Com Identification of multiple components --- Ntot Total number of lines for the comment --- Nline Running line number in range [1,Ntot] --- Comm Comment --- Mol Mol number --- FileName map name, stored in subdirectory map --- Title Title of the map --- Patricia Bauer CDS 1998May22Sergio Molinari <molinari@ipac.caltech.edu> Jan Brand <BRAND@astbo1.bo.cnr.it> J_A+A_336_339.xml