We report sensitive ATCA radio continuum observations toward IRAS 15596 - 5301 and 16272 - 4837 , two luminous objects ( { \cal L } > 2 \times 10 ^ { 4 } L _ { \sun } ) thought to represent massive star forming regions in early stages of evolution ( due to previously undetected radio emission at the 1 \sigma level of 2 mJy per beam ) .
Also reported are 1.2 millimeter continuum and a series of molecular line observations made with the SEST telescope .

The radio continuum observations toward IRAS 15596 - 5301 reveal the presence of three distinct compact sources , with angular sizes of 2.7″ to 8.8″ ( FWHM ) , all located within a region of 30″ in diameter .
Assuming that these are regions of ionized gas , we find that they have diameters of 0.06-0.2 pc , electron densities of 8 \times 10 ^ { 2 } -2 \times 10 ^ { 3 } cm ^ { -3 } , and that they are excited by early B type stars .
The 1.2-mm observations show that the dust emission arises from a region of 42 \arcsec \times 25 \arcsec ( FWHM ) with a total flux of 5.8 Jy , implying a mass of 1.4 \times 10 ^ { 3 } M _ { \sun } .
The line observations indicate that IRAS 15596 - 5301 is associated with a molecular cloud with a FWHM angular size of 37″ ( \sim 0.4 pc radius at the distance of 4.6 kpc ) , a molecular hydrogen density of \sim 4 \times 10 ^ { 5 } cm ^ { -3 } and a rotational temperature of \sim 27 K. We suggest that the massive dense core associated with IRAS 15596 - 5301 contains a cluster of B stars which are exciting compact H ii regions that are in pressure equilibrium with the dense molecular surroundings .

No radio continuum emission was detected from IRAS 16272 - 4837 up to a 3 \sigma limit of 0.2 mJy .
However , the 1.2-mm observations show strong dust emission arising from a region of 41 \arcsec \times 25 \arcsec ( FWHM ) with a total flux of 13.8 Jy , implying a mass of 2.0 \times 10 ^ { 3 } M _ { \sun } .
The line observations indicate the presence of an elongated molecular cloud with FWHM major and minor axes of 61″ and 42″ ( 0.50 \times 0.35 pc in radius at the distance of 3.4 kpc ) , a molecular hydrogen density of \sim 2 \times 10 ^ { 5 } cm ^ { -3 } and a rotational temperature of \sim 27 K. The high luminosity ( 2.4 \times 10 ^ { 4 } L _ { \sun } ) and lack of radio emission from this massive core suggest that it hosts an embedded young massive protostar that is still undergoing an intense accretion phase .
This scenario is supported by the observed characteristics of the line profiles and the presence of a bipolar outflow detected from observations of the SiO emission .
We suggest that IRAS 16272 - 4837 is a bona-fide massive star forming region in a very early evolutionary stage , being the precursor of an ultra compact H ii region .