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J/A+A/336/339 VLA observations of ultracompact HII regions (Molinari+ 1998)
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A search for precursors of ultracompact HII regions in a sample of luminous
IRAS sources. II. VLA observations.
Molinari S., Brand J., Cesaroni R., Palla F., Palumbo G.G.C.
<Astron. Astrophys. 336, 339 (1998)>
=1998A&A...336..339M (SIMBAD/NED BibCode)
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ADC_Keywords: YSOs ; Radio sources ; Infrared sources
Keywords: stars: formation - pre-main sequence stars -
stars: circumstellar matter - ISM: HII regions -
sources as a function of wavelength: radio continuum: ISM
Abstract:
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.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1a.dat 112 45 High sources at 6-cm (obs. data)
table1b.dat 113 101 Low sources at 2 and 6-cm (obs. data)
table2a.dat 75 21 High sources at 6-cm derived physical parameters
table2b.dat 103 39 Low sources at 2 and 6-cm derived phys. parameters
note1a.dat 86 34 Individual notes to table1a.dat
note1b.dat 86 19 Individual notes to table1b.dat
fig.dat 62 49 map/* titles
map/* . 49 2 and 6cm wavelength map of the sources
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See also:
J/A+A/308/573 : Ammonia on YSOs IRAS sources, Paper I. (Molinari+ 1996)
Byte-by-byte Description of file: table1a.dat, table1b.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Mol Molinari number from Paper I
(Cat. <J/A+A/308/573>)
4 A1 --- Ass An * indicates association of radio emission
with the IRAS source
6- 10 A5 --- Com Identification of multiple components within
a source, or multiple sources within a field
12 I1 cm Lam Wavelength of observation
14- 16 A3 --- Run Code for observing run (1)
18- 19 I2 h RAh ? Right ascension (1950) (2)
21- 22 I2 min RAm ? Right ascension (1950)
24- 29 F6.3 s RAs ? Right ascension (1950)
31 A1 --- DE- Sign of declination
32- 33 I2 deg DEd ? Declination (1950)
35- 36 I2 arcmin DEm ? Declination (1950)
38- 42 F5.2 arcsec DEs ? Declination (1950)
44- 46 I3 arcsec Sep ? Angular distance radio- and IRAS sources
48 A1 --- Note1 Note (3)
49- 53 F5.2 arcsec Maj ? Observed major axis of radio peak
55- 58 F4.2 arcsec Min ? Observed minor axis of radio peak
60- 64 F5.1 deg PA [-90/90]? Position angle radio peak (E of N)
66- 70 F5.2 arcsec Majd ? Major axis radio peak, deconvolved by beam
72- 75 F4.2 arcsec Mind ? Minor axis radio peak, deconvolved by beam
77- 81 F5.1 deg PAd [0/180]? Position angle deconvolved
radio peak (E of N)
83- 84 I2 arcsec Ha1 ? Size of halo in one direction (4)
85 A1 --- --- [x]
86- 87 I2 arcsec Ha2 ? Size of halo in other direction
89- 90 A2 --- Mc Morphological class (5)
92 A1 --- l_F [<] To indicate next number is upper limit
93- 97 F5.2 mJy F ? Observed peak flux density (mJy/beam)
99-104 F6.2 mJy S ? Observed integrated flux
106-109 F4.2 mJy rms ? Observed rms (mJy/beam) (6)
110 A1 --- --- [)] See note (3)
112 A1 --- Note Individual notes in note1a.dat or note1b.dat
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Note (1): I: 26 June 1994
II: 4 Oct. 1994
III: 23-24 Jan. 1995
Note (2): When there is no positions, the source has not been detected
Note (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 (4): Halo: extended component above 2 {sigma} level
Note (5): S = spherical/unresolved
CH = Core-Halo
I = irregular/multiply peaked
Note (6): If no value is listed, the theoretical rms (0.15 mJy/beam) is assumed
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Byte-by-byte Description of file: table2a.dat, table2b.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Mol Molinari number from Paper I
(Cat. <J/A+A/308/573>)
4 A1 --- Ass An * indicates association of radio
emission with the IRAS source
6- 10 A5 --- Com Identification of multiple components
within a source, or multiple sources
within a field
12 I1 cm Lam Wavelength of observation
14 A1 --- Note1 Note (1)
15 A1 --- l_DiamP [<] To indicate DiamP is upper limit
16- 21 F6.4 pc DiamP Peak diameter
23- 26 F4.2 pc DiamH ? Mean halo diameter
28- 32 F5.1 K Tb ? Brightness temperature
34- 37 F4.1 --- tau ? Optical depth * 1000.
39 A1 --- l_Ne [>] To indicate ne is lower limit
40- 44 F5.2 10+4cm-3 Ne Electron density
46 A1 --- l_EM [>] To indicate EM is lower limit
47- 52 F6.3 10+6pc/cm6 EM Emission measure
54 A1 --- l_M(Hii) [<] To indicate M(Hii) is upper limit
55- 61 F7.3 10-3solMass M(Hii) Mass of ionized hydrogen
63- 67 F5.2 [s-1] log(Nly) Log10 of Lyman continuum flux
68 A1 --- --- [)] See note (1)
70- 74 F5.2 --- Rat ? Ratio of integrated and peak fluxes
76- 80 F5.2 --- SpI ? Spectral index
82-103 A22 --- Note Comments
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Note (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.
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Byte-by-byte Description of file: note1a.dat, note1b.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Mol Mol number
5- 10 A6 --- Com Identification of multiple components
12 I1 --- Ntot Total number of lines for the comment
14 I1 --- Nline Running line number in range [1,Ntot]
16- 86 A71 --- Comm Comment
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Byte-by-byte Description of file: fig.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- Mol Mol number
5- 15 A11 --- FileName map name, stored in subdirectory map
18- 62 A45 --- Title Title of the map
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Acknowledgements: Sergio Molinari <molinari@ipac.caltech.edu>
Jan Brand <BRAND@astbo1.bo.cnr.it>
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(End) Patricia Bauer [CDS] 22-May-1998