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J/A+A/291/261 H2O masers in HII regions (Cadella+, 1994)
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The occurrence of H2O masers in HII regions
Codella C., Felli M., Natale V., Palagi F., Palla F.
<Astron. Astrophys. 291, 261 (1994)>
=1994A&A...291..261C (SIMBAD/NED Reference)
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ADC_Keywords: Masers; H II regions
Keywords: maser - stars: formation - HII regions - radio lines: interstellar
Abstract:
The results of a survey of 22.2 GHz H2O maser emission towards a large
sample of H II regions (427) detected in hydrogen recombination lines
(Lockman 1989) are presented. The sample favors classical H II regions
with large excitation parameter. We found 68 water maser sources, 17 of
which are new detections. Adding the detections found in the literature,
the total number of masers associated with the sample is 80. The overall
maser percentage is low (19%), much less than what found in samples of
ultracompact (UC) H II regions. The percentage changes if we divide our
sample into two sub-classes: H II regions known to have ultracompact
components (i.e. where a younger generation of star formation is also
present) and the rest. In the first sub-class the maser percentage is 28%,
in the second 15%. The total number of masers in H II regions where no UC
component has been (so far) reported is 43. This value is greater than that
expected from the ratio between the lifetime of the H II region 10^6 yr and
that of the maser phase, even assuming a maser lifetime as long as 10^5 yr.
This implies that maser excitation (and star formation) occurs several
times in the lifetime of the same star forming complex. From 410 positions
observed, 230 can be associated with a high degree of confidence to an
IRAS Point Source (PS). We expect that most of the H II regions not
associated with IRAS PS are diffuse regions in the last phases of their
evolution. The percentage of water masers in this sub-class is very low:
8%. The number of masers with no counterpart in the Lockman sample (and
above delta = 30 deg) is very large: 334. This number most probably
reflects the extreme high density conditions known to exist in the
earliest phases of an H II region, which strongly reduce its ratio
continuum and line emission. The analysis of a homogeneous sample of
classical H II regions allows to confirm in a quantitative fashion the
well known result that the maser phenomenon occurs during the earliest
phase of the evolution of a massive star and that it as the H II region
evolves into the diffuse stage.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 78 410 Summary of the observed sources
table1.tex 103 662 LaTeX version of table1
table2.dat 111 129 Derived parameters of the detected maser
sources
table2.tex 129 296 LaTeX version of table2
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Right ascension 1950
4- 5 I2 min RAm Right ascension 1950
7- 10 F4.1 s RAs Right ascension 1950
12 A1 --- DE- Declination sign
13- 14 I2 deg DEd Declination 1950
16- 17 I2 arcmin DEm Declination 1950
19- 20 I2 arcsec DEs Declination 1950
26- 32 F7.3 deg GLON Galactic longitude
34- 40 F7.3 deg GLAT Galactic latitude
49- 53 F5.2 kpc d []? Distance
55- 65 A11 --- IRAS IRAS name
66- 69 F4.1 Jy Dl []? Detection limit (3 r.m.s.)
70 A1 --- n_Dl [M] A 'M' indicates that maser is detected
72- 78 A7 --- Rem Remarks:
D : double position
nd: maser known but not detected in this survey
N : new detection
K : kinematic distance
P : photometric distance
S : source associated with Sharpless regions
(see the description of table1 in the paper)
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Right ascension 1950
4- 5 I2 min RAm Right ascension 1950
7- 10 F4.1 s RAs Right ascension 1950
13 A1 --- DE- Declination sign
14- 15 I2 deg DEd Declination 1950
17- 18 I2 arcmin DEm Declination 1950
20- 21 I2 arcsec DEs Declination 1950
23- 30 A8 "DD/MM/YY" Date Observation date
32 A1 --- l_Fpeak limit flag
34- 39 I6 Jy Fpeak Flux of the strongest component
43- 46 I4 Jy rms []? Noise of the spectrum (r.m.s.)
50- 54 F5.3 km/s FWHM []? FWHM of the strongest component
58- 62 F5.1 km/s Vpeak []? Velocity of the strongest component
67- 71 F5.1 km/s Vmin []? Lower extreme of the emission interval
76- 80 F5.1 km/s Vmax []? Upper extreme of the emission interval
84- 88 F5.3 km/s Vres Spectral resolution
96-101 I6 Jy.km/s FH2O []? Integrated flux
105-111 E7.1 solLum LH2O []? Water maser luminosity
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(End) Patricia Bauer [CDS] 16-Jun-1994