Different evolutionary stages in massive star formation. Centimeter
  continuum and H2O maser emission with ATCA
  We present ATCA observations of the H2O maser line and radio continuum at
18.0GHz and 22.8GHz, toward a sample of 192 massive star forming regions
containing several clumps already imaged at 1.2mm. The main aim of this study
is to investigate the water maser and centimeter continuum emission (likely
tracing thermal free-free emission) in sources at different evolutionary
stages, using the evolutionary classifications proposed by Palla et al (1991)
and Molinari et al (2008). We used the recently comissioned CABB backend at
ATCA obtaining images with 20arcsec resolution in the 1.3cm continuum and H2O
maser emission, in all targets. For the evolutionary analysis of the sources we
used the millimeter continuum emission from Beltran et al (2006) and the
infrared emission from the MSX Point Source Catalogue. We detect centimeter
continuum emission in 88% of the observed fields with a typical rms noise level
of 0.45mJy/beam. Most of the fields show a single radio continuum source, while
in 20% of them we identify multiple components. A total of 214 centimeter
continuum sources have been identified, likely tracing optically thin HII
regions, with physical parameters typical of both extended and compact HII
regions. Water maser emission was detected in 41% of the regions, resulting in
a total of 85 distinct components. The low angular (20arcsec) and spectral
(14km/s) resolutions do not allow a proper analysis of the water maser
emission, but suffice to investigate its association with the continuum
sources. We have also studied the detection rate of HII regions in the two
types of IRAS sources defined by Palla et (1991) on the basis of the IRAS
colours: High and Low. No significant differences are found, with large
detection rates (>90%) for both High and Low sources. We classify the
millimeter and infrared sources in our fields in three evolutionary stages
following the scheme presented by ...
